On Sun, 04 Jan 2009 16:05:58 +1100, Sylvia Else
wrote:

Sylvia Else wrote:
legg wrote:
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

The main switching transistor appears to be intact. I haven't removed it
from the circuit, but the voltage/current plot given by a component
tester feature on my oscilloscope gives the expected traces for the base
collector and base emitter junctions, allowing for the presence of the
diode across the base emitter junction.

Sylvia.

Hmm....

I further surmise that the failure of Q1 doesn't explain the failure of
the 8.2 Ohm resistor, because even with its specified maximum beta of
40, the switching transistor wouldn't pass enough current to burn out
the resistor.

The implication is that some other event has done for the both of them,
but surprisingly not destroyed the switching transistor.

sigh I thought I'd understood the sequence of events, but clearly not

Sylvia.

In the self-oscillatin flyback, base current is supplied
regeneratively by the feedback winding, to provide over-drive for
saturated switching. Beta is assumed to be somewhere between 4 and 12.

Note that the drive connection is drawn across the EB jn, so fusing of
the resistor would not turn the switch off - the voltage on the
emitter and base terinals could then be quite large - popping locally
connected parts like Q1.

Some re-examination of the actual board connections and further
examination of the feedback section (usually not very elaborate) may
offer further insight.

RL