Jamie wrote:
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.
How about Q1 being a thyristor as a crow bar on the bias of that second
transistor?

In which case, your symbol is incorrect. And would show the reason
why you're getting low ohm reading is my guess on what you call the
base-emitter and Collector being opened which is actually the M1
terminal etc..

That's just a guess of course.

The small transistor is a C1815 - an NPN, with an annoyingly difficult
to match pin-out.

Sylvia.