🚀 go-pugleaf

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