🚀 go-pugleaf

"Trevor Wilson" <trevor@_SPAMBLOCK_rageaudio.com.au> wrote in message 
news:6scl8sF5b1hpU1@mid.individual.net...
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> "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.
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>> 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.
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> **What are the type numbers of the two transistors?

**Scratch that. You already told me.


-- 
Trevor Wilson
www.rageaudio.com.au