Understanding a split-mode power supply.

[Sylvia Else]

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.

 

[legg]

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

 

[Sylvia Else]

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.

 

[Sylvia Else]

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.

 

[Phil Allison]

"Sylvia Else"

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

** 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

 

[Sylvia Else]

"Sylvia Else"



** 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.

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.

 

[Phil Allison]

"Sylvia Else"

Phil Allison wrote:

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?

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

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




...... Phil

 

[David L. Jones]

"Sylvia Else"



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

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

Power rating?

And what's the pinout?

Dave.

 

[Sylvia Else]

Power rating?

And what's the pinout?

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.

 

[Bob Parker]

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

You might find it on the web ...

 

[Phil Allison]

"David L. Jerkoff"

Power rating?

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

And what's the pinout?

** Mobile octal - of course.




..... Phil

 

[Phil Allison]

"Bob Parker"

You might find it on the web ...

** Cobblers ......




......... Phil

 

[N_Cook]

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.

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

 

[Sylvia Else]

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

Thanks for the reference.

Sylvia.

 

[legg]

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.

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

 

[Phil Allison]

"No legg to stand on "

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.

** Jesus Christ -

get fucking real you asinine fucking WOG WANKER !!

The WHOLE damn AC unit is metal encased, installed & MAINS earthed
!!!!

Internal insulation is functional ONLY !!!

So fuuuuuuccckk ooofffffff

IMBECILE




...... Phil

 

[Jamie]

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.


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

 

[Jamie]

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.

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.

 

[Trevor Wilson]

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.

**What are the type numbers of the two transistors?

 

[Trevor Wilson]

**What are the type numbers of the two transistors?

**Scratch that. You already told me.