🚀 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.
>
> 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


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