Jamie wrote:
Sylvia Else wrote:

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.
Ok, now we're getting some where.
Just replace the 8.2R and transistor..

The tranny went because it's only rated for 50 Vceo, also
the the Vbe was taking a little hard this way with that
200KR driving it through the 560R.

when the 8.2R opened, it allowed the voltage to get
high and simply shorted it.

That's my story and I'm sticking to it!

Semi-plausible, though the small transistor should have been able to
sink enough current through the 200K to prevent its Vce rising outside
its limits.

Sylvia.