🚀 go-pugleaf

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! :)

  The larger transistor is most likely ok.

http://webpages.charter.net/jamie_5"