Understanding a split-mode power supply.

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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.

Re: Understanding a split-mode power supply.
On Sun, 04 Jan 2009 13:32:44 +1100, Sylvia Else

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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

Re: Understanding a split-mode power supply.
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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.

Re: Understanding a split-mode power supply.
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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.

Re: Understanding a split-mode power supply.

"Sylvia Else"

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**  In an earlier post you state the PCB had been subjected to water ingress
and insect attack -  so almost anything is possible.

Odds are, the switching transistor was forced hard on by the above, taking
out the 8.2 ohms and Q1 in quick succession.

Relace whatever parts you find are damaged, clean the PCB carefully with
detergent and a brush (as you would washing glass ware etc in the sink )
rinse with clean water and dry thoroughly with lotsa hot air ( ie a hair
dryer).

Then cross you fingers and try it again.

If all is well, coat the PCB is clear acrylic lacquer to help it survive.



.....  Phil






Re: Understanding a split-mode power supply.
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It's true that I mentioned brushing away a dead spider.

I wish I'd noted exactly where the spider was. It conceivably was around
the 200K resistor, which could explain both the failures, and the demise
of the spider.

Anyone know the resistance of a small spider before it dies from shock?

Sylvia.

Re: Understanding a split-mode power supply.

"Sylvia Else"
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**  If you see one with three red stripes across its abdomen

-   means it is 2.2 kohms,   +/- 20 % .............




.....  Phil




Re: Understanding a split-mode power supply.

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Power rating?

And what's the pinout?

Dave.



Re: Understanding a split-mode power supply.
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The power rating is certainly an issue. At 2.2 kohms, it would be
dissipating about 50W. I would expect to find it splattered around PS
cover, an effect which is noticeably absent.

At higher resistances, significantly more power would be dissipated in
it than in the 8.2 ohm resistor. It's hard to see how the resistor can
burn out without there being obvious signs of arachnid distress.

Sylvia.

Re: Understanding a split-mode power supply.

"David L. Jerkoff"


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**  Depends on the size of the damn spider  -  fuckwit.


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**  Mobile octal  -  of course.




....   Phil






Re: Understanding a split-mode power supply.

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    You might find it on the web ...



Re: Understanding a split-mode power supply.

"Bob Parker"

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**  Cobblers  ......




........    Phil





Re: Understanding a split-mode power supply.
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If you find the spider there you should'a looked before you sat.


Re: Understanding a split-mode power supply.

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To me it looks like a safety circuit that is designed that requires you
to  pull the plug , so that Q1 being a thyristor in my case of thinking
will clamp down on the circuit from an over voltage, and remain that way
  until you pull the plug and wait. The 8.2R could just be burnt out or
  it maybe located in a thermo area intentionally to burn out (thermo Fuse)
   in which case, Q1 would also clamp.

  Either scenario will prevent the coil to be energized, if that is what
  T1 is?

  Since you haven't supplied any component part numbers, it's hard to say
really, if that is what's going on.

--
"I'd rather have a bottle in front of me than a frontal lobotomy"

http://webpages.charter.net/jamie_5 "


Re: Understanding a split-mode power supply.
On Sun, 04 Jan 2009 16:05:58 +1100, Sylvia Else

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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

Re: Understanding a split-mode power supply.
On Sun, 04 Jan 2009 14:41:04 +1100, Sylvia Else

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You seem to have access to some tools and a fair understanding of
basic electronics. There's nothing preventing you from undertaking a
repair by component substitution, for your own interest's sake,
providing you are aware of, and use, use safe procedures around
powered circuitry.

This assembly was designed to fail in a safe manner - no fire,
explosion or shock hazard was intended to result from a single fault
resulting in the fusing of the emitter resistor. However, this does
not mean that the repaired unit will meet the same standards. The
events that occurred may have affected the safety isolation of
magnetic isolators in such a way that a second similar event may not
be as benign.

If the main switch is not damaged (a big if), damage to the magnetic
component's internal insulation is also unlikely. With the
introduction of foreign particles, anything is possible. It should at
least be subjected to a basic hipot stress test procedure, after the
repaired and cleaned assembly is burned in and still warm.

RL

Re: Understanding a split-mode power supply.

"Sylvia Else"
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** Looks well cleaned up.

Perfectly OK to use in any device that has only  FUNCTIONAL insulation
requirements.

( As opposed to "double insulation " or Class 2 safety insulation
requirements )



.....   Phil



Re: Understanding a split-mode power supply.
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system.


Someone else , last month, asked a similar question here.I have now found
the text book that I found useful for understanding SMPSs

Simplified design of Switching Power Supplies
by John D Lenk, 1995


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
We've slightly trimmed the long signature. Click to see the full one.
Re: Understanding a split-mode power supply.
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Thanks for the reference.

Sylvia.

Re: Understanding a split-mode power supply.
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I obtained a second hand ex-library copy for a few dollars plus postage.
Interesting reading.

Judging by the book's condition - mint - I'd have to wonder whether it
was ever even looked at.

Sylvia.

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