🚀 go-pugleaf

N_Cook wrote:
> Sylvia Else <sylvia@not.at.this.address> wrote in message
> news:49601fcc$0$26314$afc38c87@news.optusnet.com.au...
>> 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.
> 
> 
> Someone else , last month, asked a similar question here.I have now found
> the text book that I found useful for understanding SMPSs
> 
> Simplified design of Switching Power Supplies
> by John D Lenk, 1995

Thanks for the reference.

Sylvia.