Arfa Daily wrote:
"legg" wrote in message
...
On Mon, 05 Jan 2009 09:21:54 +1100, 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.
Looking at the package's flat surface, with leads pointing down, the
pin-out of 2SC1815 is ECB.

RL

i.e. absolutely standard pinning for just about any and all Japanese TO92
transistors starting 2SA, B, C, or D ...

Well, to avoid P&P costs, I need to use what I already have, or what I
can buy from DSE or Jaycar.

FWIW, the '1815 is about the commonest general purpose small signal NPN
Jappo transistor to be found anywhere, and anything similarly general
purpose will sub for it. It's generally not hard to rearrange the leads of a
differently pinned device, with a bit of sleeving on one or two legs to stop
them touching.

Yes, I'd already concluded that given its position in the circuit, any
vaguely similar type will suffice, and that rearranging the leads is
practical, if not very elegant.

I've obtained a suitable resistor (not fuisible, but OK to test the
solution) and will give that a go later. Shame it's getting so hot outside.

Sylvia.