🚀 go-pugleaf

On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else
<sylvia@not.at.this.address> wrote:

>After my airconditioner failed the other day, and being reluctant to pay 
>a technician to come and fix it, I've been taking a look at its 
>electronics board.
>
>It's clear that its power supply circuit has failed. From the board 
>itself I've inferred this partial circuit:
>
>http://members.optusnet.com.au/sylviae/smps.jpg
>
>I am pretty sure there are no other components connected to the 
>transistor labelled Q1, and it is this transistor that has failed. The 
>failure mode is a short (a few ohms, polarity insensitive) from base to 
>emitter. The collector is open circuit. The transistor is thus unable to 
>sink enough current to prevent the switching transistor from turning on, 
>and as a result the 8.2 Ohm fusible resistor has also failed.
>
>It seems moderately likely that by replacing these two components I can 
>get the board working again.
>
>The circuit nevertheless puzzles me. The function of Q1 appears to be to 
>  bias the switching transistor. But this seems to rely somewhat on the 
>characteristics of the two transistors, which I would have thought was 
>asking for trouble. In particular, it looks to me as if Q1 could simply 
>prevent the switching transistor from ever conducting, and nothing would 
>happen.
>
>Is this an accepted technique? Or have I misunderstood the purpose of Q1?
>
>BTW, this is from the external unit of a nine year old Daikin split system.
>
>Sylvia.

As drawn, Q1 is intended to limit current in the 8R2 emitter resistor,
when the main switch is functional. The event that killed the main
switch and fused the emitter resistor, could have fried Q1 in the
process. I think you'll find that Q1 is intentionally inverted
(collector and emitter swapped). When it functions, it resembles a
temperature-dependant voltage reference of ~ 1V4.

This is one of the simpler self-oscillating flyback circuits popular
below 25W (in switch-mode power supplies) since about 1970.

Without the appropriate tools, training and test procedures, you
cannot ensure that this unit, even restored to an apparently
functional condition, is safe to use.

If you know the supply's output requirements, you would probably be
better off replacing the whole assembly.

RL

legg wrote:
> On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else
> <sylvia@not.at.this.address> wrote:
> 
>> After my airconditioner failed the other day, and being reluctant to pay 
>> a technician to come and fix it, I've been taking a look at its 
>> electronics board.
>>
>> It's clear that its power supply circuit has failed. From the board 
>> itself I've inferred this partial circuit:
>>
>> http://members.optusnet.com.au/sylviae/smps.jpg
>>
>> I am pretty sure there are no other components connected to the 
>> transistor labelled Q1, and it is this transistor that has failed. The 
>> failure mode is a short (a few ohms, polarity insensitive) from base to 
>> emitter. The collector is open circuit. The transistor is thus unable to 
>> sink enough current to prevent the switching transistor from turning on, 
>> and as a result the 8.2 Ohm fusible resistor has also failed.
>>
>> It seems moderately likely that by replacing these two components I can 
>> get the board working again.
>>
>> The circuit nevertheless puzzles me. The function of Q1 appears to be to 
>>  bias the switching transistor. But this seems to rely somewhat on the 
>> characteristics of the two transistors, which I would have thought was 
>> asking for trouble. In particular, it looks to me as if Q1 could simply 
>> prevent the switching transistor from ever conducting, and nothing would 
>> happen.
>>
>> Is this an accepted technique? Or have I misunderstood the purpose of Q1?
>>
>> BTW, this is from the external unit of a nine year old Daikin split system.
>>
>> Sylvia.
> 
> As drawn, Q1 is intended to limit current in the 8R2 emitter resistor,
> when the main switch is functional. The event that killed the main
> switch and fused the emitter resistor, could have fried Q1 in the
> process. I think you'll find that Q1 is intentionally inverted
> (collector and emitter swapped). When it functions, it resembles a
> temperature-dependant voltage reference of ~ 1V4.
> 
> This is one of the simpler self-oscillating flyback circuits popular
> below 25W (in switch-mode power supplies) since about 1970.
> 
> Without the appropriate tools, training and test procedures, you
> cannot ensure that this unit, even restored to an apparently
> functional condition, is safe to use.
> 
> If you know the supply's output requirements, you would probably be
> better off replacing the whole assembly.
> 
> RL

The main switching transistor appears to be intact. I haven't removed it 
from the circuit, but the voltage/current plot given by a component 
tester feature on my oscilloscope gives the expected traces for the base 
collector and base emitter junctions, allowing for the presence of the 
diode across the base emitter junction.

Sylvia.

Sylvia Else wrote:
> legg wrote:
>> On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else
>> <sylvia@not.at.this.address> wrote:
>>
>>> After my airconditioner failed the other day, and being reluctant to 
>>> pay a technician to come and fix it, I've been taking a look at its 
>>> electronics board.
>>>
>>> It's clear that its power supply circuit has failed. From the board 
>>> itself I've inferred this partial circuit:
>>>
>>> http://members.optusnet.com.au/sylviae/smps.jpg
>>>
>>> I am pretty sure there are no other components connected to the 
>>> transistor labelled Q1, and it is this transistor that has failed. 
>>> The failure mode is a short (a few ohms, polarity insensitive) from 
>>> base to emitter. The collector is open circuit. The transistor is 
>>> thus unable to sink enough current to prevent the switching 
>>> transistor from turning on, and as a result the 8.2 Ohm fusible 
>>> resistor has also failed.
>>>
>>> It seems moderately likely that by replacing these two components I 
>>> can get the board working again.
>>>
>>> The circuit nevertheless puzzles me. The function of Q1 appears to be 
>>> to  bias the switching transistor. But this seems to rely somewhat on 
>>> the characteristics of the two transistors, which I would have 
>>> thought was asking for trouble. In particular, it looks to me as if 
>>> Q1 could simply prevent the switching transistor from ever 
>>> conducting, and nothing would happen.
>>>
>>> Is this an accepted technique? Or have I misunderstood the purpose of 
>>> Q1?
>>>
>>> BTW, this is from the external unit of a nine year old Daikin split 
>>> system.
>>>
>>> Sylvia.
>>
>> As drawn, Q1 is intended to limit current in the 8R2 emitter resistor,
>> when the main switch is functional. The event that killed the main
>> switch and fused the emitter resistor, could have fried Q1 in the
>> process. I think you'll find that Q1 is intentionally inverted
>> (collector and emitter swapped). When it functions, it resembles a
>> temperature-dependant voltage reference of ~ 1V4.
>>
>> This is one of the simpler self-oscillating flyback circuits popular
>> below 25W (in switch-mode power supplies) since about 1970.
>>
>> Without the appropriate tools, training and test procedures, you
>> cannot ensure that this unit, even restored to an apparently
>> functional condition, is safe to use.
>>
>> If you know the supply's output requirements, you would probably be
>> better off replacing the whole assembly.
>>
>> RL
> 
> The main switching transistor appears to be intact. I haven't removed it 
> from the circuit, but the voltage/current plot given by a component 
> tester feature on my oscilloscope gives the expected traces for the base 
> collector and base emitter junctions, allowing for the presence of the 
> diode across the base emitter junction.
> 
> Sylvia.

Hmm....

I further surmise that the failure of Q1 doesn't explain the failure of 
the 8.2 Ohm resistor, because even with its specified maximum beta of 
40, the switching transistor wouldn't pass enough current to burn out 
the resistor.

The implication is that some other event has done for the both of them, 
but surprisingly not destroyed the switching transistor.

<sigh> I thought I'd understood the sequence of events, but clearly not :(

Sylvia.

"Sylvia Else"

>> The main switching transistor appears to be intact. I haven't removed it 
>> from the circuit, but the voltage/current plot given by a component 
>> tester feature on my oscilloscope gives the expected traces for the base 
>> collector and base emitter junctions, allowing for the presence of the 
>> diode across the base emitter junction.
>>
>
> I further surmise that the failure of Q1 doesn't explain the failure of 
> the 8.2 Ohm resistor, because even with its specified maximum beta of 40, 
> the switching transistor wouldn't pass enough current to burn out the 
> resistor.
>
> The implication is that some other event has done for the both of them, 
> but surprisingly not destroyed the switching transistor.
>
> <sigh> I thought I'd understood the sequence of events, but clearly not :(


**  In an earlier post you state the PCB had been subjected to water ingress 
and insect attack -  so almost anything is possible.

Odds are, the switching transistor was forced hard on by the above, taking 
out the 8.2 ohms and Q1 in quick succession.

Relace whatever parts you find are damaged, clean the PCB carefully with 
detergent and a brush (as you would washing glass ware etc in the sink ) 
rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair 
dryer).

Then cross you fingers and try it again.

If all is well, coat the PCB is clear acrylic lacquer to help it survive.



.....  Phil

Phil Allison wrote:
> "Sylvia Else"
> 
>>> The main switching transistor appears to be intact. I haven't removed it 
>>> from the circuit, but the voltage/current plot given by a component 
>>> tester feature on my oscilloscope gives the expected traces for the base 
>>> collector and base emitter junctions, allowing for the presence of the 
>>> diode across the base emitter junction.
>>>
>> I further surmise that the failure of Q1 doesn't explain the failure of 
>> the 8.2 Ohm resistor, because even with its specified maximum beta of 40, 
>> the switching transistor wouldn't pass enough current to burn out the 
>> resistor.
>>
>> The implication is that some other event has done for the both of them, 
>> but surprisingly not destroyed the switching transistor.
>>
>> <sigh> I thought I'd understood the sequence of events, but clearly not :(
> 
> 
> **  In an earlier post you state the PCB had been subjected to water ingress 
> and insect attack -  so almost anything is possible.
> 
> Odds are, the switching transistor was forced hard on by the above, taking 
> out the 8.2 ohms and Q1 in quick succession.
> 
> Relace whatever parts you find are damaged, clean the PCB carefully with 
> detergent and a brush (as you would washing glass ware etc in the sink ) 
> rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair 
> dryer).
> 
> Then cross you fingers and try it again.
> 
> If all is well, coat the PCB is clear acrylic lacquer to help it survive.
> 

It's true that I mentioned brushing away a dead spider.

I wish I'd noted exactly where the spider was. It conceivably was around 
the 200K resistor, which could explain both the failures, and the demise 
of the spider.

Anyone know the resistance of a small spider before it dies from shock?

Sylvia.

"Sylvia Else"
> Phil Allison wrote:

>> **  In an earlier post you state the PCB had been subjected to water 
>> ingress and insect attack -  so almost anything is possible.
>>
>> Odds are, the switching transistor was forced hard on by the above, 
>> taking out the 8.2 ohms and Q1 in quick succession.
>>
>> Relace whatever parts you find are damaged, clean the PCB carefully with 
>> detergent and a brush (as you would washing glass ware etc in the sink ) 
>> rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair 
>> dryer).
>>
>> Then cross you fingers and try it again.
>>
>> If all is well, coat the PCB is clear acrylic lacquer to help it survive.
>>
>
> It's true that I mentioned brushing away a dead spider.
>
> I wish I'd noted exactly where the spider was. It conceivably was around 
> the 200K resistor, which could explain both the failures, and the demise 
> of the spider.
>
> Anyone know the resistance of a small spider before it dies from shock?



**  If you see one with three red stripes across its abdomen

-   means it is 2.2 kohms,   +/- 20 % .............




.....  Phil

"Phil Allison" <philallison@tpg.com.au> wrote in message 
news:6sb46vF53m30U1@mid.individual.net...
>
> "Sylvia Else"
>> Phil Allison wrote:
>
>>> **  In an earlier post you state the PCB had been subjected to water 
>>> ingress and insect attack -  so almost anything is possible.
>>>
>>> Odds are, the switching transistor was forced hard on by the above, 
>>> taking out the 8.2 ohms and Q1 in quick succession.
>>>
>>> Relace whatever parts you find are damaged, clean the PCB carefully with 
>>> detergent and a brush (as you would washing glass ware etc in the sink ) 
>>> rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair 
>>> dryer).
>>>
>>> Then cross you fingers and try it again.
>>>
>>> If all is well, coat the PCB is clear acrylic lacquer to help it 
>>> survive.
>>>
>>
>> It's true that I mentioned brushing away a dead spider.
>>
>> I wish I'd noted exactly where the spider was. It conceivably was around 
>> the 200K resistor, which could explain both the failures, and the demise 
>> of the spider.
>>
>> Anyone know the resistance of a small spider before it dies from shock?
>
>
>
> **  If you see one with three red stripes across its abdomen
>
> -   means it is 2.2 kohms,   +/- 20 % .............

Power rating?

And what's the pinout?

Dave. 

David L. Jones wrote:
> "Phil Allison" <philallison@tpg.com.au> wrote in message 
> news:6sb46vF53m30U1@mid.individual.net...
>> "Sylvia Else"
>>> Phil Allison wrote:
>>>> **  In an earlier post you state the PCB had been subjected to water 
>>>> ingress and insect attack -  so almost anything is possible.
>>>>
>>>> Odds are, the switching transistor was forced hard on by the above, 
>>>> taking out the 8.2 ohms and Q1 in quick succession.
>>>>
>>>> Relace whatever parts you find are damaged, clean the PCB carefully with 
>>>> detergent and a brush (as you would washing glass ware etc in the sink ) 
>>>> rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair 
>>>> dryer).
>>>>
>>>> Then cross you fingers and try it again.
>>>>
>>>> If all is well, coat the PCB is clear acrylic lacquer to help it 
>>>> survive.
>>>>
>>> It's true that I mentioned brushing away a dead spider.
>>>
>>> I wish I'd noted exactly where the spider was. It conceivably was around 
>>> the 200K resistor, which could explain both the failures, and the demise 
>>> of the spider.
>>>
>>> Anyone know the resistance of a small spider before it dies from shock?
>>
>>
>> **  If you see one with three red stripes across its abdomen
>>
>> -   means it is 2.2 kohms,   +/- 20 % .............
> 
> Power rating?
> 
> And what's the pinout?
> 

The power rating is certainly an issue. At 2.2 kohms, it would be 
dissipating about 50W. I would expect to find it splattered around PS 
cover, an effect which is noticeably absent.

At higher resistances, significantly more power would be dissipated in 
it than in the 8.2 ohm resistor. It's hard to see how the resistor can 
burn out without there being obvious signs of arachnid distress.

Sylvia.

"David L. Jerkoff"


>> **  If you see one with three red stripes across its abdomen
>>
>> -   means it is 2.2 kohms,   +/- 20 % .............
>
> Power rating?


**  Depends on the size of the damn spider  -  fuckwit.


> And what's the pinout?


**  Mobile octal  -  of course.




....   Phil

Sylvia Else <sylvia@not.at.this.address> wrote in message
news:49601fcc$0$26314$afc38c87@news.optusnet.com.au...
> After my airconditioner failed the other day, and being reluctant to pay
> a technician to come and fix it, I've been taking a look at its
> electronics board.
>
> It's clear that its power supply circuit has failed. From the board
> itself I've inferred this partial circuit:
>
> http://members.optusnet.com.au/sylviae/smps.jpg
>
> I am pretty sure there are no other components connected to the
> transistor labelled Q1, and it is this transistor that has failed. The
> failure mode is a short (a few ohms, polarity insensitive) from base to
> emitter. The collector is open circuit. The transistor is thus unable to
> sink enough current to prevent the switching transistor from turning on,
> and as a result the 8.2 Ohm fusible resistor has also failed.
>
> It seems moderately likely that by replacing these two components I can
> get the board working again.
>
> The circuit nevertheless puzzles me. The function of Q1 appears to be to
>   bias the switching transistor. But this seems to rely somewhat on the
> characteristics of the two transistors, which I would have thought was
> asking for trouble. In particular, it looks to me as if Q1 could simply
> prevent the switching transistor from ever conducting, and nothing would
> happen.
>
> Is this an accepted technique? Or have I misunderstood the purpose of Q1?
>
> BTW, this is from the external unit of a nine year old Daikin split
system.
>
> Sylvia.


Someone else , last month, asked a similar question here.I have now found
the text book that I found useful for understanding SMPSs

Simplified design of Switching Power Supplies
by John D Lenk, 1995


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N_Cook wrote:
> Sylvia Else <sylvia@not.at.this.address> wrote in message
> news:49601fcc$0$26314$afc38c87@news.optusnet.com.au...
>> After my airconditioner failed the other day, and being reluctant to pay
>> a technician to come and fix it, I've been taking a look at its
>> electronics board.
>>
>> It's clear that its power supply circuit has failed. From the board
>> itself I've inferred this partial circuit:
>>
>> http://members.optusnet.com.au/sylviae/smps.jpg
>>
>> I am pretty sure there are no other components connected to the
>> transistor labelled Q1, and it is this transistor that has failed. The
>> failure mode is a short (a few ohms, polarity insensitive) from base to
>> emitter. The collector is open circuit. The transistor is thus unable to
>> sink enough current to prevent the switching transistor from turning on,
>> and as a result the 8.2 Ohm fusible resistor has also failed.
>>
>> It seems moderately likely that by replacing these two components I can
>> get the board working again.
>>
>> The circuit nevertheless puzzles me. The function of Q1 appears to be to
>>   bias the switching transistor. But this seems to rely somewhat on the
>> characteristics of the two transistors, which I would have thought was
>> asking for trouble. In particular, it looks to me as if Q1 could simply
>> prevent the switching transistor from ever conducting, and nothing would
>> happen.
>>
>> Is this an accepted technique? Or have I misunderstood the purpose of Q1?
>>
>> BTW, this is from the external unit of a nine year old Daikin split
> system.
>> Sylvia.
> 
> 
> Someone else , last month, asked a similar question here.I have now found
> the text book that I found useful for understanding SMPSs
> 
> Simplified design of Switching Power Supplies
> by John D Lenk, 1995

Thanks for the reference.

Sylvia.

On Sun, 04 Jan 2009 14:41:04 +1100, Sylvia Else
<sylvia@not.at.this.address> wrote:

>legg wrote:
>> On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else
>> <sylvia@not.at.this.address> wrote:
>> 
>>> After my airconditioner failed the other day, and being reluctant to pay 
>>> a technician to come and fix it, I've been taking a look at its 
>>> electronics board.
>>>
>>> It's clear that its power supply circuit has failed. From the board 
>>> itself I've inferred this partial circuit:
>>>
>>> http://members.optusnet.com.au/sylviae/smps.jpg
>>>
>>> I am pretty sure there are no other components connected to the 
>>> transistor labelled Q1, and it is this transistor that has failed. The 
>>> failure mode is a short (a few ohms, polarity insensitive) from base to 
>>> emitter. The collector is open circuit. The transistor is thus unable to 
>>> sink enough current to prevent the switching transistor from turning on, 
>>> and as a result the 8.2 Ohm fusible resistor has also failed.
>>>
>>> It seems moderately likely that by replacing these two components I can 
>>> get the board working again.
>>>
>>> The circuit nevertheless puzzles me. The function of Q1 appears to be to 
>>>  bias the switching transistor. But this seems to rely somewhat on the 
>>> characteristics of the two transistors, which I would have thought was 
>>> asking for trouble. In particular, it looks to me as if Q1 could simply 
>>> prevent the switching transistor from ever conducting, and nothing would 
>>> happen.
>>>
>>> Is this an accepted technique? Or have I misunderstood the purpose of Q1?
>>>
>>> BTW, this is from the external unit of a nine year old Daikin split system.
>>>
>>> Sylvia.
>> 
>> As drawn, Q1 is intended to limit current in the 8R2 emitter resistor,
>> when the main switch is functional. The event that killed the main
>> switch and fused the emitter resistor, could have fried Q1 in the
>> process. I think you'll find that Q1 is intentionally inverted
>> (collector and emitter swapped). When it functions, it resembles a
>> temperature-dependant voltage reference of ~ 1V4.
>> 
>> This is one of the simpler self-oscillating flyback circuits popular
>> below 25W (in switch-mode power supplies) since about 1970.
>> 
>> Without the appropriate tools, training and test procedures, you
>> cannot ensure that this unit, even restored to an apparently
>> functional condition, is safe to use.
>> 
>> If you know the supply's output requirements, you would probably be
>> better off replacing the whole assembly.
>> 
>> RL
>
>The main switching transistor appears to be intact. I haven't removed it 
>from the circuit, but the voltage/current plot given by a component 
>tester feature on my oscilloscope gives the expected traces for the base 
>collector and base emitter junctions, allowing for the presence of the 
>diode across the base emitter junction.
>
>Sylvia.
You seem to have access to some tools and a fair understanding of
basic electronics. There's nothing preventing you from undertaking a
repair by component substitution, for your own interest's sake,
providing you are aware of, and use, use safe procedures around
powered circuitry.

This assembly was designed to fail in a safe manner - no fire,
explosion or shock hazard was intended to result from a single fault
resulting in the fusing of the emitter resistor. However, this does
not mean that the repaired unit will meet the same standards. The
events that occurred may have affected the safety isolation of
magnetic isolators in such a way that a second similar event may not
be as benign.

If the main switch is not damaged (a big if), damage to the magnetic
component's internal insulation is also unlikely. With the
introduction of foreign particles, anything is possible. It should at
least be subjected to a basic hipot stress test procedure, after the
repaired and cleaned assembly is burned in and still warm.

RL

"No legg to stand on "

>
> This assembly was designed to fail in a safe manner - no fire,
> explosion or shock hazard was intended to result from a single fault
> resulting in the fusing of the emitter resistor. However, this does
> not mean that the repaired unit will meet the same standards. The
> events that occurred may have affected the safety isolation of
> magnetic isolators in such a way that a second similar event may not
> be as benign.



 **  Jesus Christ  -

get fucking real you asinine fucking WOG  WANKER !!

The  WHOLE  damn AC unit is metal encased,  installed &  MAINS  earthed 
!!!!

Internal insulation is functional  ONLY !!!

So   fuuuuuuccckk   ooofffffff

IMBECILE




.....  Phil

Sylvia Else wrote:

> After my airconditioner failed the other day, and being reluctant to pay 
> a technician to come and fix it, I've been taking a look at its 
> electronics board.
> 
> It's clear that its power supply circuit has failed. From the board 
> itself I've inferred this partial circuit:
> 
> http://members.optusnet.com.au/sylviae/smps.jpg
> 
> I am pretty sure there are no other components connected to the 
> transistor labelled Q1, and it is this transistor that has failed. The 
> failure mode is a short (a few ohms, polarity insensitive) from base to 
> emitter. The collector is open circuit. The transistor is thus unable to 
> sink enough current to prevent the switching transistor from turning on, 
> and as a result the 8.2 Ohm fusible resistor has also failed.
> 
> It seems moderately likely that by replacing these two components I can 
> get the board working again.
> 
> The circuit nevertheless puzzles me. The function of Q1 appears to be to 
>  bias the switching transistor. But this seems to rely somewhat on the 
> characteristics of the two transistors, which I would have thought was 
> asking for trouble. In particular, it looks to me as if Q1 could simply 
> prevent the switching transistor from ever conducting, and nothing would 
> happen.
> 
> Is this an accepted technique? Or have I misunderstood the purpose of Q1?
> 
> BTW, this is from the external unit of a nine year old Daikin split system.
> 
> Sylvia.
How about Q1 being a thyristor as a crow bar on the bias of that second 
transistor?

    In which case, your symbol is incorrect. And would show the reason
why you're getting low ohm reading is my guess on what you call the
   base-emitter and Collector being opened which is actually the M1
  terminal etc..

    That's just a guess of course.


http://webpages.charter.net/jamie_5"

Sylvia Else wrote:

> Phil Allison wrote:
> 
>> "Sylvia Else"
>>
>>>> The main switching transistor appears to be intact. I haven't 
>>>> removed it from the circuit, but the voltage/current plot given by a 
>>>> component tester feature on my oscilloscope gives the expected 
>>>> traces for the base collector and base emitter junctions, allowing 
>>>> for the presence of the diode across the base emitter junction.
>>>>
>>> I further surmise that the failure of Q1 doesn't explain the failure 
>>> of the 8.2 Ohm resistor, because even with its specified maximum beta 
>>> of 40, the switching transistor wouldn't pass enough current to burn 
>>> out the resistor.
>>>
>>> The implication is that some other event has done for the both of 
>>> them, but surprisingly not destroyed the switching transistor.
>>>
>>> <sigh> I thought I'd understood the sequence of events, but clearly 
>>> not :(
>>
>>
>>
>> **  In an earlier post you state the PCB had been subjected to water 
>> ingress and insect attack -  so almost anything is possible.
>>
>> Odds are, the switching transistor was forced hard on by the above, 
>> taking out the 8.2 ohms and Q1 in quick succession.
>>
>> Relace whatever parts you find are damaged, clean the PCB carefully 
>> with detergent and a brush (as you would washing glass ware etc in the 
>> sink ) rinse with clean water and dry thoroughly with lotsa hot air ( 
>> ie a hair dryer).
>>
>> Then cross you fingers and try it again.
>>
>> If all is well, coat the PCB is clear acrylic lacquer to help it survive.
>>
> 
> It's true that I mentioned brushing away a dead spider.
> 
> I wish I'd noted exactly where the spider was. It conceivably was around 
> the 200K resistor, which could explain both the failures, and the demise 
> of the spider.
> 
> Anyone know the resistance of a small spider before it dies from shock?
> 
> Sylvia.
To me it looks like a safety circuit that is designed that requires you 
to  pull the plug , so that Q1 being a thyristor in my case of thinking 
will clamp down on the circuit from an over voltage, and remain that way
  until you pull the plug and wait. The 8.2R could just be burnt out or
  it maybe located in a thermo area intentionally to burn out (thermo Fuse)
   in which case, Q1 would also clamp.

  Either scenario will prevent the coil to be energized, if that is what
  T1 is?

  Since you haven't supplied any component part numbers, it's hard to say
really, if that is what's going on.

-- 
"I'd rather have a bottle in front of me than a frontal lobotomy"

http://webpages.charter.net/jamie_5"

"Sylvia Else" <sylvia@not.at.this.address> wrote in message 
news:49601fcc$0$26314$afc38c87@news.optusnet.com.au...
> After my airconditioner failed the other day, and being reluctant to pay a 
> technician to come and fix it, I've been taking a look at its electronics 
> board.
>
> It's clear that its power supply circuit has failed. From the board itself 
> I've inferred this partial circuit:
>
> http://members.optusnet.com.au/sylviae/smps.jpg
>
> I am pretty sure there are no other components connected to the transistor 
> labelled Q1, and it is this transistor that has failed. The failure mode 
> is a short (a few ohms, polarity insensitive) from base to emitter. The 
> collector is open circuit. The transistor is thus unable to sink enough 
> current to prevent the switching transistor from turning on, and as a 
> result the 8.2 Ohm fusible resistor has also failed.
>
> It seems moderately likely that by replacing these two components I can 
> get the board working again.
>
> The circuit nevertheless puzzles me. The function of Q1 appears to be to 
> bias the switching transistor. But this seems to rely somewhat on the 
> characteristics of the two transistors, which I would have thought was 
> asking for trouble. In particular, it looks to me as if Q1 could simply 
> prevent the switching transistor from ever conducting, and nothing would 
> happen.
>
> Is this an accepted technique? Or have I misunderstood the purpose of Q1?
>
> BTW, this is from the external unit of a nine year old Daikin split 
> system.

**What are the type numbers of the two transistors?


-- 
Trevor Wilson
www.rageaudio.com.au 

"Trevor Wilson" <trevor@_SPAMBLOCK_rageaudio.com.au> wrote in message 
news:6scl8sF5b1hpU1@mid.individual.net...
>
>
>
> "Sylvia Else" <sylvia@not.at.this.address> wrote in message 
> news:49601fcc$0$26314$afc38c87@news.optusnet.com.au...
>> After my airconditioner failed the other day, and being reluctant to pay 
>> a technician to come and fix it, I've been taking a look at its 
>> electronics board.
>>
>> It's clear that its power supply circuit has failed. From the board 
>> itself I've inferred this partial circuit:
>>
>> http://members.optusnet.com.au/sylviae/smps.jpg
>>
>> I am pretty sure there are no other components connected to the 
>> transistor labelled Q1, and it is this transistor that has failed. The 
>> failure mode is a short (a few ohms, polarity insensitive) from base to 
>> emitter. The collector is open circuit. The transistor is thus unable to 
>> sink enough current to prevent the switching transistor from turning on, 
>> and as a result the 8.2 Ohm fusible resistor has also failed.
>>
>> It seems moderately likely that by replacing these two components I can 
>> get the board working again.
>>
>> The circuit nevertheless puzzles me. The function of Q1 appears to be to 
>> bias the switching transistor. But this seems to rely somewhat on the 
>> characteristics of the two transistors, which I would have thought was 
>> asking for trouble. In particular, it looks to me as if Q1 could simply 
>> prevent the switching transistor from ever conducting, and nothing would 
>> happen.
>>
>> Is this an accepted technique? Or have I misunderstood the purpose of Q1?
>>
>> BTW, this is from the external unit of a nine year old Daikin split 
>> system.
>
> **What are the type numbers of the two transistors?

**Scratch that. You already told me.


-- 
Trevor Wilson
www.rageaudio.com.au 

On Sun, 04 Jan 2009 16:05:58 +1100, Sylvia Else
<sylvia@not.at.this.address> wrote:

>Sylvia Else wrote:
>> legg wrote:
>>> On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else
>>> <sylvia@not.at.this.address> wrote:
>>>
>>>> After my airconditioner failed the other day, and being reluctant to 
>>>> pay a technician to come and fix it, I've been taking a look at its 
>>>> electronics board.
>>>>
>>>> It's clear that its power supply circuit has failed. From the board 
>>>> itself I've inferred this partial circuit:
>>>>
>>>> http://members.optusnet.com.au/sylviae/smps.jpg
>>>>
>>>> I am pretty sure there are no other components connected to the 
>>>> transistor labelled Q1, and it is this transistor that has failed. 
>>>> The failure mode is a short (a few ohms, polarity insensitive) from 
>>>> base to emitter. The collector is open circuit. The transistor is 
>>>> thus unable to sink enough current to prevent the switching 
>>>> transistor from turning on, and as a result the 8.2 Ohm fusible 
>>>> resistor has also failed.
>>>>
>>>> It seems moderately likely that by replacing these two components I 
>>>> can get the board working again.
>>>>
>>>> The circuit nevertheless puzzles me. The function of Q1 appears to be 
>>>> to  bias the switching transistor. But this seems to rely somewhat on 
>>>> the characteristics of the two transistors, which I would have 
>>>> thought was asking for trouble. In particular, it looks to me as if 
>>>> Q1 could simply prevent the switching transistor from ever 
>>>> conducting, and nothing would happen.
>>>>
>>>> Is this an accepted technique? Or have I misunderstood the purpose of 
>>>> Q1?
>>>>
>>>> BTW, this is from the external unit of a nine year old Daikin split 
>>>> system.
>>>>
>>>> Sylvia.
>>>
>>> As drawn, Q1 is intended to limit current in the 8R2 emitter resistor,
>>> when the main switch is functional. The event that killed the main
>>> switch and fused the emitter resistor, could have fried Q1 in the
>>> process. I think you'll find that Q1 is intentionally inverted
>>> (collector and emitter swapped). When it functions, it resembles a
>>> temperature-dependant voltage reference of ~ 1V4.
>>>
>>> This is one of the simpler self-oscillating flyback circuits popular
>>> below 25W (in switch-mode power supplies) since about 1970.
>>>
>>> Without the appropriate tools, training and test procedures, you
>>> cannot ensure that this unit, even restored to an apparently
>>> functional condition, is safe to use.
>>>
>>> If you know the supply's output requirements, you would probably be
>>> better off replacing the whole assembly.
>>>
>>> RL
>> 
>> The main switching transistor appears to be intact. I haven't removed it 
>> from the circuit, but the voltage/current plot given by a component 
>> tester feature on my oscilloscope gives the expected traces for the base 
>> collector and base emitter junctions, allowing for the presence of the 
>> diode across the base emitter junction.
>> 
>> Sylvia.
>
>Hmm....
>
>I further surmise that the failure of Q1 doesn't explain the failure of 
>the 8.2 Ohm resistor, because even with its specified maximum beta of 
>40, the switching transistor wouldn't pass enough current to burn out 
>the resistor.
>
>The implication is that some other event has done for the both of them, 
>but surprisingly not destroyed the switching transistor.
>
><sigh> I thought I'd understood the sequence of events, but clearly not :(
>
>Sylvia.

In the self-oscillatin flyback, base current is supplied
regeneratively by the feedback winding, to provide over-drive for
saturated switching. Beta is assumed to be somewhere between 4 and 12.

Note that the drive connection is drawn across the EB jn, so fusing of
the resistor would not turn the switch off - the voltage on the
emitter and base terinals could then be quite large - popping locally
connected parts like Q1.

Some re-examination of the actual board connections and further
examination of the feedback section (usually not very elaborate) may
offer further insight.

RL

Phil Allison wrote:
>  "Sylvia Else"
>> Phil Allison wrote:
>>> "Sylvia Else"
>>>>>> As for whether I could substitute a 6.8, maybe I could, though I'd 
>>>>>> have trouble getting one that has a 350 volt rating.
>>>>>  **   Huh   ??????????
>>>>>
>>>>> No need for that whatsoever.
>>>>>
>>>>> In operation, the resistor is not subjected to more than a few volts.
>>>> As I commented earlier, if the switching transistor fails by shorting 
>>>> out, then the resistor will blow. After it's blown it'll have 340V 
>>>> across it.
>>> ** Irrelevant.
>>>
>>> Once it has fused, the resistor  will be OPEN circuit.
>> Which just means it won't conduct unless the voltage across it exceeds its 
>> insulating properties.
> 
> 
> ** No,  it  WILL  be open circuit to the voltage that caused it to fuse.
> 
> Cos the fusing behaviour will not cease until the part becomes open circuit 
> in the given situation.

That just begs the question of whether it will fuse in its designed way, 
or whether the excess voltage will cause it to fuse in an unintended, 
and possibly damaging, way.
> 
> 
>>> The voltage across the break can be thousands of volts.
>> So you say, but where's the evidence?
> 
> 
> ** You came here for the advice of folk with experience of electronics,  cos 
> you have none.
> 
> That advice is based on many decades of ACTUAL experience, in my case.
> 
> If you simply replace the fusible resistor in the PSU with a similar part, 
> then it will work as well as the original one did.

How do you know the original wasn't rated to 350V? The issue is not 
whether it will work, but whether it will fail safe.

Sylvia.

 "Stupider than Anyone Else"

>
>>>>
>>>>> As for whether I could substitute a 6.8, maybe I could, though I'd 
>>>>> have trouble getting one that has a 350 volt rating.
>>>>
>>>>  **   Huh   ??????????
>>>>
>>>> No need for that whatsoever.
>>>>
>>>> In operation, the resistor is not subjected to more than a few volts.
>>> As I commented earlier, if the switching transistor fails by shorting 
>>> out, then the resistor will blow. After it's blown it'll have 340V 
>>> across it.
>>
>> ** Irrelevant.
>>
>> Once it has fused, the resistor  will be OPEN circuit.
>
> Which just means it won't conduct unless the voltage across it exceeds its 
> insulating properties.


** No,  it  WILL  be open circuit to the voltage that caused it to fuse.

Cos the fusing behaviour will not cease until the part becomes open circuit
in the given situation.


>> The voltage across the break can be thousands of volts.
>
> So you say, but where's the evidence?


** You came here for the advice of folk with experience of electronics,  cos
you have none.

That advice is based on many decades of ACTUAL experience, in my case.

If you simply replace the fusible resistor in the PSU with a similar part,
then it will work as well as the original one did.

BTW:

The maker's max voltage rating relates to a functional resistor -  not a
blown one.

There is a HUGE amount of electronics YOU have no clue about.



.....   Phil

Phil Allison wrote:
>  "Stupider than Anyone Else"
> 
>>>>>> As for whether I could substitute a 6.8, maybe I could, though I'd 
>>>>>> have trouble getting one that has a 350 volt rating.
>>>>>  **   Huh   ??????????
>>>>>
>>>>> No need for that whatsoever.
>>>>>
>>>>> In operation, the resistor is not subjected to more than a few volts.
>>>> As I commented earlier, if the switching transistor fails by shorting 
>>>> out, then the resistor will blow. After it's blown it'll have 340V 
>>>> across it.
>>> ** Irrelevant.
>>>
>>> Once it has fused, the resistor  will be OPEN circuit.
>> Which just means it won't conduct unless the voltage across it exceeds its 
>> insulating properties.
> 
> 
> ** No,  it  WILL  be open circuit to the voltage that caused it to fuse.
> 
> Cos the fusing behaviour will not cease until the part becomes open circuit
> in the given situation.
> 
> 
>>> The voltage across the break can be thousands of volts.
>> So you say, but where's the evidence?
> 
> 
> ** You came here for the advice of folk with experience of electronics,  cos
> you have none.
> 
> That advice is based on many decades of ACTUAL experience, in my case.
> 
> If you simply replace the fusible resistor in the PSU with a similar part,
> then it will work as well as the original one did.
> 
> BTW:
> 
> The maker's max voltage rating relates to a functional resistor -  not a
> blown one.
> 
> There is a HUGE amount of electronics YOU have no clue about.
> 

You seem to have posted the same reply twice.

Sylvia.

"Sylvia Else" <sylvia@not.at.this.address> wrote in message 
news:49630537$0$4046$afc38c87@news.optusnet.com.au...
> Phil Allison wrote:
>>  "Stupider than Anyone Else"
>>
>>>>>>> As for whether I could substitute a 6.8, maybe I could, though I'd 
>>>>>>> have trouble getting one that has a 350 volt rating.
>>>>>>  **   Huh   ??????????
>>>>>>
>>>>>> No need for that whatsoever.
>>>>>>
>>>>>> In operation, the resistor is not subjected to more than a few volts.
>>>>> As I commented earlier, if the switching transistor fails by shorting 
>>>>> out, then the resistor will blow. After it's blown it'll have 340V 
>>>>> across it.
>>>> ** Irrelevant.
>>>>
>>>> Once it has fused, the resistor  will be OPEN circuit.
>>> Which just means it won't conduct unless the voltage across it exceeds 
>>> its insulating properties.
>>
>>
>> ** No,  it  WILL  be open circuit to the voltage that caused it to fuse.
>>
>> Cos the fusing behaviour will not cease until the part becomes open 
>> circuit
>> in the given situation.
>>
>>
>>>> The voltage across the break can be thousands of volts.
>>> So you say, but where's the evidence?
>>
>>
>> ** You came here for the advice of folk with experience of electronics, 
>> cos
>> you have none.
>>
>> That advice is based on many decades of ACTUAL experience, in my case.
>>
>> If you simply replace the fusible resistor in the PSU with a similar 
>> part,
>> then it will work as well as the original one did.
>>
>> BTW:
>>
>> The maker's max voltage rating relates to a functional resistor -  not a
>> blown one.
>>
>> There is a HUGE amount of electronics YOU have no clue about.
>>
>
> You seem to have posted the same reply twice.
>
> Sylvia.

I must say that in this case, I agree with Phil. It's sometimes possible to 
get too pedantic about these things. I wouldn't think for one moment that 
the value of 6.2 ohms is in any way critical. If it were, then they wouldn't 
be using a bog standard 5% tolerance part, which could be up at over 6.5 
ohms and still be in spec. Far more likely that either the manufacturing 
department had a shedload of that value in stock, or were being offered them 
for virtually nothing, or the designer had some bee in his bonnet about 
calculating some 'correct' value for some aspect of his design that he 
thought important enough to warrant it. If it really really did need to be 
*exactly* that value, then I'm afraid that I would rate it as a poorly 
designed circuit, with insufficient tolerance of component spread and aging.

Like Phil, based on decades of service work, with switch mode power supplies 
figuring in most of that, my experience of fusible resistors is that they 
often fail open for no apparent reason - which is maybe what happened in 
your case - but if they do fail for a 'real' reason, that failure is usually 
catastrophic enough to blow the resistance material off the substrate. Even 
where these resistors have failed either benignly or catastrophically in the 
high voltage side of switchers, I can't ever recall seeing any secondary 
problem caused by continued arcing or leakage within the failed part.

If you are really that bothered that the exact characteristics of the 
original failed device should be preserved, then you should not really be 
contemplating making up the value from two series connected resistors with 
totally *different* characteristics.

FWIW, I just looked at a couple of types in one of my catalogues, and one 
was rated at 250v ac, and the other at 350v ac, so either would be fine at 
340v dc. If the ones that you find listed don't have a voltage rating in the 
catalogue, check them out on their manufacturers' websites, where they 
surely will.

Arfa 

Arfa Daily wrote:

> I must say that in this case, I agree with Phil. It's sometimes possible to 
> get too pedantic about these things. I wouldn't think for one moment that 
> the value of 6.2 ohms is in any way critical. If it were, then they wouldn't 
> be using a bog standard 5% tolerance part, which could be up at over 6.5 
> ohms and still be in spec. 

Phil was only on about the voltage rating issue.

> Far more likely that either the manufacturing 
> department had a shedload of that value in stock, or were being offered them 
> for virtually nothing, or the designer had some bee in his bonnet about 
> calculating some 'correct' value for some aspect of his design that he 
> thought important enough to warrant it. If it really really did need to be 
> *exactly* that value, then I'm afraid that I would rate it as a poorly 
> designed circuit, with insufficient tolerance of component spread and aging.

It was a 5% tolerance component. Maybe at 5% and with allowance for 
aging it will stay within the limits required by the design. But all 
bets would be off if I stuck in a 5% component of some other value.

If this had been the 200K component, (also a non-standard value), I'd 
happily have stuck in a 220K of 180K, and thought nothing of it. But 
it's a low value component connected to the feedback system. Who's to 
say what a 10% change to 5.6 ohms, let alone a 30% change to 4.7 ohms 
would do. Maybe nothing, but it's not worth the possible trouble to find 
out.

> 
> Like Phil, based on decades of service work, with switch mode power supplies 
> figuring in most of that, my experience of fusible resistors is that they 
> often fail open for no apparent reason - which is maybe what happened in 
> your case

It's credible, given that there was no sign of distress, but it's odd 
that Q1 was taken out as well. I still can't see a mechanism for that, 
which is why I was somewhat surprised that the repair worked.

- but if they do fail for a 'real' reason, that failure is usually
> catastrophic enough to blow the resistance material off the substrate. Even 
> where these resistors have failed either benignly or catastrophically in the 
> high voltage side of switchers, I can't ever recall seeing any secondary 
> problem caused by continued arcing or leakage within the failed part.

Well, you wouldn't if they've been correctly specified. Phil was 
essentially arguing that I could substitute a component with a lower 
voltage rating.

As part of my research for this reply, I found a data sheet for a 0.22 
ohm 0.25 W fusible, which can be reached from this page

http://tinyurl.com/8jy2zx

It quotes a number of different voltage levels. In particular, it quotes 
  a specific "Maximum withstand voltage after fusing." To my mind, in 
the particular circuit in question, that value would have to be 340V or 
greater.

> 
> If you are really that bothered that the exact characteristics of the 
> original failed device should be preserved, then you should not really be 
> contemplating making up the value from two series connected resistors with 
> totally *different* characteristics.

It's not really the case that I was trying to match the exact 
characteristics. Indeed, I haven't, because a 4.7 ohm fusible requires 
more current to fail than a 6.2 ohm fusible of the same power rating.

Instead, my aim was to reproduce the original resistance, within the 
tolerance chosen by the designer, to minimise the chance of 
destabilising the circuit, while preserving a safe failure more should 
the transistor fail shorted. In this context, by safe I mean a failure 
mode that would have a fair chance of protecting the upstream 
components, thus leaving the board in a repairable state.

Sylvia.

"Sylvia Else" <sylvia@not.at.this.address> wrote in message 
news:496343d1$0$26491$afc38c87@news.optusnet.com.au...
> Arfa Daily wrote:
>
>> I must say that in this case, I agree with Phil. It's sometimes possible 
>> to get too pedantic about these things. I wouldn't think for one moment 
>> that the value of 6.2 ohms is in any way critical. If it were, then they 
>> wouldn't be using a bog standard 5% tolerance part, which could be up at 
>> over 6.5 ohms and still be in spec.
>
> Phil was only on about the voltage rating issue.
>
>> Far more likely that either the manufacturing department had a shedload 
>> of that value in stock, or were being offered them for virtually nothing, 
>> or the designer had some bee in his bonnet about calculating some 
>> 'correct' value for some aspect of his design that he thought important 
>> enough to warrant it. If it really really did need to be *exactly* that 
>> value, then I'm afraid that I would rate it as a poorly designed circuit, 
>> with insufficient tolerance of component spread and aging.
>
> It was a 5% tolerance component. Maybe at 5% and with allowance for aging 
> it will stay within the limits required by the design. But all bets would 
> be off if I stuck in a 5% component of some other value.
>
> If this had been the 200K component, (also a non-standard value), I'd 
> happily have stuck in a 220K of 180K, and thought nothing of it. But it's 
> a low value component connected to the feedback system. Who's to say what 
> a 10% change to 5.6 ohms, let alone a 30% change to 4.7 ohms would do. 
> Maybe nothing, but it's not worth the possible trouble to find out.
>
>>
>> Like Phil, based on decades of service work, with switch mode power 
>> supplies figuring in most of that, my experience of fusible resistors is 
>> that they often fail open for no apparent reason - which is maybe what 
>> happened in your case
>
> It's credible, given that there was no sign of distress, but it's odd that 
> Q1 was taken out as well. I still can't see a mechanism for that, which is 
> why I was somewhat surprised that the repair worked.
>
> - but if they do fail for a 'real' reason, that failure is usually
>> catastrophic enough to blow the resistance material off the substrate. 
>> Even where these resistors have failed either benignly or 
>> catastrophically in the high voltage side of switchers, I can't ever 
>> recall seeing any secondary problem caused by continued arcing or leakage 
>> within the failed part.
>
> Well, you wouldn't if they've been correctly specified. Phil was 
> essentially arguing that I could substitute a component with a lower 
> voltage rating.
>
> As part of my research for this reply, I found a data sheet for a 0.22 ohm 
> 0.25 W fusible, which can be reached from this page
>
> http://tinyurl.com/8jy2zx
>
> It quotes a number of different voltage levels. In particular, it quotes a 
> specific "Maximum withstand voltage after fusing." To my mind, in the 
> particular circuit in question, that value would have to be 340V or 
> greater.
>
>>
>> If you are really that bothered that the exact characteristics of the 
>> original failed device should be preserved, then you should not really be 
>> contemplating making up the value from two series connected resistors 
>> with totally *different* characteristics.
>
> It's not really the case that I was trying to match the exact 
> characteristics. Indeed, I haven't, because a 4.7 ohm fusible requires 
> more current to fail than a 6.2 ohm fusible of the same power rating.
>
> Instead, my aim was to reproduce the original resistance, within the 
> tolerance chosen by the designer, to minimise the chance of destabilising 
> the circuit, while preserving a safe failure more should the transistor 
> fail shorted. In this context, by safe I mean a failure mode that would 
> have a fair chance of protecting the upstream components, thus leaving the 
> board in a repairable state.
>
> Sylvia.

Well, I hear what you're saying, and you could debate this in detail for 
ever, but again, I would say to you, based on many years experience of 
domestic and commercial repair work right down there at component level, if 
the circuit is that critical of an emitter resistor value and type, then 
it's poorly designed. If you believe this to be the case, and you see it as 
an ongoing safety issue, then you should only replace it with an exact same 
device. If you were able to look at a schematic for the board, and it showed 
this resistor as a 'designated safety component', then I would not hesitate 
to say that it should not be substituted. But in the event that you can't 
confirm one way or the other, then it has to be your judgement call alone, 
and no amount of advice from any of us, can change that.

Arfa 

 "Stupider than Anyone Else"

> Phil was only on about the voltage rating issue.

**  Massively wrong presumption.


> It's credible, given that there was no sign of distress, but it's odd that 
> Q1 was taken out as well. I still can't see a mechanism for that, which is 
> why I was somewhat surprised that the repair worked.


** Ignorance is neither a virtue nor ever a basis to argue from.

 Learn that and you will be an entirely better person.

 Shame how no-body can teach old bitches new tricks.


> Well, you wouldn't if they've been correctly specified. Phil was 
> essentially arguing that I could substitute a component with a lower 
> voltage rating.


**  Massively wrong presumption.


> Instead, my aim was to reproduce the original resistance, within the 
> tolerance chosen by the designer, to minimise the chance of destabilising 
> the circuit, while preserving a safe failure more should the transistor 
> fail shorted. In this context, by safe I mean a failure mode that would 
> have a fair chance of protecting the upstream components, thus leaving the 
> board in a repairable state.


**  The fusible resistor in question is there  ONLY  to prevent the PCB 
material catching fire  -  as might occur if a non fusible type were used  - 
cos they can act like an incendiary.

A more accurate name for fusibles is " flame proof " types  -  as described 
in your link.

Protecting delicate parts like semis is not part of their job description - 
at all.

BTW:

You came here for the advice of folk with experience of electronics,  cos
you have absolutely none.

The advice I gave is based on many decades of ACTUAL experience.

If you simply replace the fusible resistor in the PSU with a similar part,
then it will perform just as well as the original one did.

There is a HUGE amount of electronics YOU have no clue about.

Nor any other fucking thing either.





.....   Phil

Phil Allison wrote:
>  "Stupider than Anyone Else"
> 
>> Phil was only on about the voltage rating issue.
> 
> **  Massively wrong presumption.

Phil, it's a matter of record. Just go back and look.

> 
> 
>> It's credible, given that there was no sign of distress, but it's odd that 
>> Q1 was taken out as well. I still can't see a mechanism for that, which is 
>> why I was somewhat surprised that the repair worked.
> 
> 
> ** Ignorance is neither a virtue nor ever a basis to argue from.

You could try construing the point as being an invitation to explain how 
a failure of the resistor could take out Q1, or vice versa.

>> Well, you wouldn't if they've been correctly specified. Phil was 
>> essentially arguing that I could substitute a component with a lower 
>> voltage rating.
> 
> 
> **  Massively wrong presumption.

Another matter of record, Phil.

I note you've snipped out the stuff about the data sheet that directly 
contradicts your claim that the potential across the fused resistor 
could be several thousand volts.

> 
> BTW:
> 
> You came here for the advice of folk with experience of electronics,  cos
> you have absolutely none.

I come here for comment, to help drive my thinking about a problem. Like 
any other reader of NGs, I'd be foolish to accept anything said here as 
gospel, because I have no way of judging the motives of other posters.

> 
> The advice I gave is based on many decades of ACTUAL experience.

So you say, but how am I to know?

Sylvia.

 "Stupider than Anyone Else"
>
>>> Phil was only on about the voltage rating issue.
>>
>> **  Massively wrong presumption.
>
> Phil, it's a matter of record.


** Then go read what was written properly.

 I know you will never do that as you are completely autistic.



>>> It's credible, given that there was no sign of distress, but it's odd 
>>> that Q1 was taken out as well. I still can't see a mechanism for that, 
>>> which is why I was somewhat surprised that the repair worked.
>>
>>
>> ** Ignorance is neither a virtue nor ever a basis to argue from.
>
> You could try construing the point as being an invitation to explain how a 
> failure of the resistor could take out Q1, or vice versa.


** Been explained already -  the likely scenario is that the high base 
current that drove the switching transistor hard on passed to over to Q1 
after the fusible resistor opened.

Q1 is a relatively fragile, low voltage device  -  so it failed.



>>> Well, you wouldn't if they've been correctly specified. Phil was 
>>> essentially arguing that I could substitute a component with a lower 
>>> voltage rating.
>>
>>
>> **  Massively wrong presumption.
>
> Another matter of record, Phil.


** Then go post under those words.

I know you never will do that since they do not exist.



> I note you've snipped out the stuff about the data sheet that directly 
> contradicts your claim that the potential across the fused resistor could 
> be several thousand volts.

** There is no such thing in the data sheet.

Correctly interpreting component maker's data is a whole science in 
self   -  that also takes long experience to become expert at.

Counts YOU out completely.



>> BTW:
>>
>> You came here for the advice of folk with experience of electronics,  cos
>> you have absolutely none.
>
> I come here for comment, to help drive my thinking about a problem.


** Your totally bizarre thinking is not my problem.


>> The advice I gave is based on many decades of ACTUAL experience.
>
> So you say, but how am I to know?


** I was explaining that the basis for the advice given was experience  - 
shame I cannot supply a URL for that.

 Now piss off -

you trolling  PITA  idiot.




.....   Phil

This PS circuit seems rathed old fashioned. I opened a larged A/C unit 
from the same manufacturer, and which was installed at the same time. 
It's circuit board is only about half the size.

Although this is a switch mode PS, it doesn't appear to use switch mode 
regulation. As far as I can see, there is no feedback from the secondary 
side back to the primary feedback circuit. Instead, there is a linear 
regulator in the secondary side to set the voltage for the electronics.

For what it does, the PS primary circuit seems rather complicated.

Sylvia.

"Sylvia Else" <sylvia@not.at.this.address> wrote in message 
news:4964169c$0$25646$afc38c87@news.optusnet.com.au...
> This PS circuit seems rathed old fashioned. I opened a larged A/C unit 
> from the same manufacturer, and which was installed at the same time. It's 
> circuit board is only about half the size.
>
> Although this is a switch mode PS, it doesn't appear to use switch mode 
> regulation. As far as I can see, there is no feedback from the secondary 
> side back to the primary feedback circuit. Instead, there is a linear 
> regulator in the secondary side to set the voltage for the electronics.
>
> For what it does, the PS primary circuit seems rather complicated.
>
> Sylvia.

It's not at all uncommon to see switchers that are secondary-side 
linear-regulated only. I worked on a mixer / PA just a couple of weeks ago 
which had a full blown switcher in it. The main rails for the output stages 
had no regulation at all, and were completely dependant on the input line 
voltage. The low rails for the preamps had only simple linear regulators. At 
first, I thought this was a little odd, but of course, it is actually quite 
common for the main 'big' rails not to be regulated in this sort of kit, 
when it uses a conventional transformer-based supply. So what was the 
advantage of complicating things by using a switcher ? I can only assume 
that it's primarily because it's a portable item of band equipment, so it 
makes it much lighter to carry around. A lot of other kit uses switchers 
these days because of the energy efficiency, particularly when they are put 
into a 'sleep' mode, but an A/C unit ? It's beyond me why it would be worth 
the added complication of a switcher over a linear, given the energy-gulping 
nature of the appliance in the first place, and the potentially hostile 
environment that it has got to work in ...

Arfa 

Arfa Daily wrote:
> It's beyond me why it would be worth 
> the added complication of a switcher over a linear, given the energy-gulping 
> nature of the appliance in the first place, and the potentially hostile 
> environment that it has got to work in ...

Though the PS and basic electronics are running 24/7, not just when the 
A/C is running.

Maybe a hypothesis of yours applies here - they got a good deal on 
transformers suitable for use in a switch mode PS.

Of if I were being cynical, which of course, I never am, I might suspect 
that it was to ensure that the boards were uneconomic to repair (except 
DIY), thus increasing the market for replacement boards.

Although I don't know how much a replacement costs, because 3 days after 
I asked, Daikin still haven't replied to my email.

Sylvia.

On Tue, 06 Jan 2009 13:11:10 +1100, Sylvia Else
<sylvia@not.at.this.address> wrote:

>Arfa Daily wrote:
>> "Sylvia Else" <sylvia@not.at.this.address> wrote in message 
>> news:4961f0fa$0$25649$afc38c87@news.optusnet.com.au...
>>> Ok, as I mentioned elsewhere in this thread, I managed to get the A/C 
>>> working again by replacing Q1 and the 8.2 ohm resistor.
>>>
>>> Actually, I'd misread it, and when I looked more carefully, it realised it 
>>> was 6.2 ohm, as is the other one on the board with the same markings 
>>> (i.e., a not blown one). Nice standard value that.
>>>
>>> So now I have to replace the resistor with a fusible, but sourcing a 6.2 
>>> ohm fusible is problematic.
>>>
>>> The situation is not made any easier because if the switching transistor 
>>> fails by shorting out, then after it blows the fusible will have 340V 
>>> across it.
>>>
>>> From the suppliers I now know about (thanks, Trevor), I can get a 0.5 watt 
>>> 340V 4.7 ohm fusible. I propose to put it in series with a 1.5 ohm 1 watt 
>>> non-fusible. My reasoning is that the 1.5 ohm resistor will have only 1/3 
>>> the power dissipation, and will handle twice as much. Therefore the 4.7 
>>> ohm will go open circuit before the 1.5 ohm could get hot enough to be a 
>>> problem.
>>>
>>> Does this stand up?
>>>
>>> Sylvia.
>>>
>>>
>> Are you absolutely sure that it is definitely 6.2 ohms ? That is a *very* 
>> odd value, particularly for a fusible type ... FWIW, I really don't think 
>> that the circuit would give a damn if you replaced it with a 6.8 ohm, which 
>> is a standard value. Did you actually measure the one that's ok with an 
>> accurate low ohms meter, and get a reading of 6.2 ? Seems to me that a grey 
>> band and a blue band might easily be misread one for the other, with some of 
>> the banding paints I've seen used over the years. I find that it is often 
>> very easy to misread red for orange or brown, especially if the resistor 
>> runs warm in normal use.
>
>Blue Red Gold Gold = 6.2 ohms, 5%. It is in the E24 series, but I've 
>certainly never seen one before.
>
>When I made up an equivalent, my meter gave the same reading (allowing 
>for tolerance) for the equivalent as it does for the identical resistor 
>on the board.
>
>So, yes, I'm pretty sure.
>
>As for whether I could substitute a 6.8, maybe I could, though I'd have 
>trouble getting one that has a 350 volt rating. The only supplier I know 
>of that purports to have them doesn't give the rating. The suppliers 
>that give ratings don't carry that value.
>
>If I understood the circuit better, I'd be more comfortable about 
>changing the value. The mere fact that an unusual value has been used 
>gives me pause - maybe it's the value that's required there. Even if 
>another value worked, I'd not know the ramifications.
>
>Sylvia.

It's possible that the original is just a flame-proof part - not
advertised as 'fusible' by the part mfr. You'll easily find these as
6R2 in 5% and 2% ranges.

The fact is that metal film resistor values below ~ 18R in 1/4 watt
and 10R in 1/2 watt are demonstrably fusible, under a certain range of
conditions, providing the coating is flame resistant. This
characteristic is often used to establish predictable single fault
abnormal behavior, where the overstress is predictably high under the
expected fault. It doesn't work as well for slower, lower voltage
fault surges.

If you want to know if the part you've selected is suitable, run tests
with a few samples by attaching them to the end of a line cord and
plugging them into the applicable line voltage, making sure that they
are located in a suitable container of some sort. The result may
suprise you - often the fusing event may be hardly audible for some
constructions.

This is only normally practical if you have access to parts in volume,
and control over the purchasing specification.

RL

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:eK_8l.140589$m53.91179@newsfe04.ams2...
>So what was the
> advantage of complicating things by using a switcher ? I can only assume
> that it's primarily because it's a portable item of band equipment, so it
> makes it much lighter to carry around.

That would be most likely for that purpose.

>A lot of other kit uses switchers
> these days because of the energy efficiency, particularly when they are
put
> into a 'sleep' mode, but an A/C unit ?

Or the reason a lot of gear uses SMP these days, easier to supply various
countries differing mains voltages.
(Another major factor of less shipping weight would apply to
air-conditioners)

MrT.

"Mr.T" <MrT@home> wrote in message
news:49655ad8$0$32004$afc38c87@news.optusnet.com.au...
> (Another major factor of less shipping weight would apply to
> air-conditioners)

Oops, that should be "wouldn't".

MrT.