🚀 go-pugleaf

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.


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