🚀 go-pugleaf

On Mon, 05 Jan 2009 09:21:54 +1100, Sylvia Else
<sylvia@not.at.this.address> 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.

Looking at the package's flat surface, with leads pointing down, the
pin-out of 2SC1815 is ECB.

RL