🚀 go-pugleaf

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.