🚀 go-pugleaf

Jamie wrote:
> Sylvia Else wrote:
> 
>> Jamie wrote:
>>
>>> 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.
>>
>>
>> The small transistor is a C1815 - an NPN, with an annoyingly difficult 
>> to match pin-out.
>>
>> Sylvia.
> Ok, now we're getting some where.
>    Just replace the  8.2R and transistor..
> 
>  The tranny went because it's only rated for  50 Vceo, also
> the the Vbe was taking a little hard this way with that
>  200KR driving it through the 560R.
> 
>    when the 8.2R opened, it allowed the voltage to get
>  high and simply shorted it.
> 
>     That's my story and I'm sticking to it! :)

Semi-plausible, though the small transistor should have been able to 
sink enough current through the 200K to prevent its Vce rising outside 
its limits.

Sylvia.