🚀 go-pugleaf

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.