🚀 go-pugleaf

"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.

**What are the type numbers of the two transistors?


-- 
Trevor Wilson
www.rageaudio.com.au