On Sun, 04 Jan 2009 13:32:44 +1100, 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.

As drawn, Q1 is intended to limit current in the 8R2 emitter resistor,
when the main switch is functional. The event that killed the main
switch and fused the emitter resistor, could have fried Q1 in the
process. I think you'll find that Q1 is intentionally inverted
(collector and emitter swapped). When it functions, it resembles a
temperature-dependant voltage reference of ~ 1V4.

This is one of the simpler self-oscillating flyback circuits popular
below 25W (in switch-mode power supplies) since about 1970.

Without the appropriate tools, training and test procedures, you
cannot ensure that this unit, even restored to an apparently
functional condition, is safe to use.

If you know the supply's output requirements, you would probably be
better off replacing the whole assembly.

RL