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.

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

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.

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.

"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.


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

Phil Allison wrote:
"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.

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.


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.

"Sylvia Else"
Phil Allison wrote:

** 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?



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

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




...... Phil

"Phil Allison" wrote in message
...

"Sylvia Else"
Phil Allison wrote:

** 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?



** 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.

David L. Jones wrote:
"Phil Allison" wrote in message
...
"Sylvia Else"
Phil Allison wrote:
** 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?


** 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?


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.

"David L. Jerkoff"


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

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

Power rating?


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


And what's the pinout?


** Mobile octal - of course.




..... Phil

Sylvia Else wrote in message
u...
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


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N_Cook wrote:
Sylvia Else wrote in message
u...
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

Thanks for the reference.

Sylvia.

On Sun, 04 Jan 2009 14:41:04 +1100, 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.
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

"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 ****ing real you asinine ****ing WOG ****** !!

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

Internal insulation is functional ONLY !!!

So fuuuuuuccckk ooofffffff

IMBECILE




...... Phil

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.


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

Sylvia Else wrote:

Phil Allison wrote:

"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.

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.


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.

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

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

"Sylvia Else" wrote in message
u...
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
www.rageaudio.com.au

"Trevor Wilson" wrote in message
...



"Sylvia Else" wrote in message
u...
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?

**Scratch that. You already told me.


--
Trevor Wilson
www.rageaudio.com.au

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

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.

Phil Allison wrote:
"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 ****ing real you asinine ****ing WOG ****** !!

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

Leaving aside the abuse, I confess this was my sentiment as well. And
not only that, neither the outside nor inside units even get touched in
the normal way of things.

The entire board could go up in flames, if it were inflammable, and
still cause nothing more than a bad smell. Since it's outside even the
resulting fumes wouldn't be a threat.

Sylvia.

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

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!

The larger transistor is most likely ok.

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

"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 ...

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.

Arfa

On Mon, 05 Jan 2009 09:21:54 +1100, Sylvia Else
wrote:

The small transistor is a C1815 - an NPN, with an annoyingly difficult
to match pin-out.

Why stress? Replace with the same part# - WES have them in Sydney.

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.

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.

On Mon, 05 Jan 2009 09:28:54 +1100, Sylvia Else
wrote:

Phil Allison wrote:
"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 ****ing real you asinine ****ing WOG ****** !!

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

Leaving aside the abuse, I confess this was my sentiment as well. And
not only that, neither the outside nor inside units even get touched in
the normal way of things.

The entire board could go up in flames, if it were inflammable, and
still cause nothing more than a bad smell. Since it's outside even the
resulting fumes wouldn't be a threat.

Sylvia.

I see that specific poster's dialog in your response, only. His actual
postings get filtered autiomatically here.

The unit is unlikely to go up in flames, for good reasons.

As a technician, you'd be advised to follow industry standards in
repair of any line-isolating assembly. One doesn't always know the
n'th reason why something was done.

A hipot is not impossible to arrange, locally, if you don't have the
equipment yourself.

RL

legg wrote:
On Mon, 05 Jan 2009 09:28:54 +1100, Sylvia Else
wrote:

Phil Allison wrote:
"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 ****ing real you asinine ****ing WOG ****** !!

The WHOLE damn AC unit is metal encased, installed & MAINS earthed
!!!!
Leaving aside the abuse, I confess this was my sentiment as well. And
not only that, neither the outside nor inside units even get touched in
the normal way of things.

The entire board could go up in flames, if it were inflammable, and
still cause nothing more than a bad smell. Since it's outside even the
resulting fumes wouldn't be a threat.

Sylvia.

I see that specific poster's dialog in your response, only. His actual
postings get filtered autiomatically here.

The unit is unlikely to go up in flames, for good reasons.

As a technician, you'd be advised to follow industry standards in
repair of any line-isolating assembly. One doesn't always know the
n'th reason why something was done.

A hipot is not impossible to arrange, locally, if you don't have the
equipment yourself.

RL

My test repair with a non-fuisible resistor and an NPN transistor from
my stock seems to have worked (rather to my surprise), so I'll have to
source a fusible resistor now.

I'll take your comment about a hi-pot on board, though in fact I've only
worked on the high voltage side of the system - the transformer hasn't
been touched. Still, I suppose insulation breakdown in the transformer
could be implicated in the original failure.

Sylvia.

"Sylvia Else"
"No legg to stand on "

** Jesus Christ -

get ****ing real you asinine ****ing WOG ****** !!

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


My test repair with a non-fusible resistor and an NPN transistor from my
stock seems to have worked (rather to my surprise), so I'll have to source
a fusible resistor now.

** WES Components in Ashfield have them.

Code: " 1F8.2 " for 24 cents each.


I'll take your comment about a hi-pot on board, though in fact I've only
worked on the high voltage side of the system - the transformer hasn't
been touched. Still, I suppose insulation breakdown in the transformer
could be implicated in the original failure.


** Ignore "legg" - he is an ignorant ****.

There is no such industry practice as he alludes to in the repair of SMPSs
incorporated inside products.

Where the PSU is a module and plenty of stock is available at LOW cost to a
repair organisation - of course they prefer to just replace it, no matter
what the fault. Any excuse in the book is then dragged out to justify that.

Where such replacement is NOT possible or would be uneconomic - it get
repaired just like you have done.

BTW:

What does "legg" have to say about the way your SMPS was installed in the
outside unit of an air-con with no protection against water ingress,
condensation or invasion by any and all insect and rodent life ?

What does that do for the " safety isolation " angle - eh ???

Big joke.




...... Phil

Phil Allison wrote:
"Sylvia Else"
"No legg to stand on "
** Jesus Christ -

get ****ing real you asinine ****ing WOG ****** !!

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

My test repair with a non-fusible resistor and an NPN transistor from my
stock seems to have worked (rather to my surprise), so I'll have to source
a fusible resistor now.

** WES Components in Ashfield have them.

Code: " 1F8.2 " for 24 cents each.


I'll take your comment about a hi-pot on board, though in fact I've only
worked on the high voltage side of the system - the transformer hasn't
been touched. Still, I suppose insulation breakdown in the transformer
could be implicated in the original failure.


** Ignore "legg" - he is an ignorant ****.

There is no such industry practice as he alludes to in the repair of SMPSs
incorporated inside products.

Where the PSU is a module and plenty of stock is available at LOW cost to a
repair organisation - of course they prefer to just replace it, no matter
what the fault. Any excuse in the book is then dragged out to justify that.

Where such replacement is NOT possible or would be uneconomic - it get
repaired just like you have done.

BTW:

What does "legg" have to say about the way your SMPS was installed in the
outside unit of an air-con with no protection against water ingress,
condensation or invasion by any and all insect and rodent life ?

What does that do for the " safety isolation " angle - eh ???

He might be unaware, since I posted that in a separate thread to
different groups. That thread was basically an opportunity for me to
winge about the quality of the circuit board, and give a heads up to
anyone considering buying an A/C unit.

I only posted this thread here when I had found something definitely
wrong with the PS, other than that it simply didn't work.

Legg FYI, that other thread referred to the fact that the board had
obviously suffered somewhat from being inadequately protected from the
environment and small creatures. I found a dead spider, some of its web,
and a good deal of dirt on the board.

http://members.optusnet.com.au/sylviae/cbCrop.jpg

Sylvia.

"Sylvia Else"
"Sylvia Else"
"No legg to stand on "


Legg FYI, that other thread referred to the fact that the board had
obviously suffered somewhat from being inadequately protected from the
environment and small creatures. I found a dead spider, some of its web,
and a good deal of dirt on the board.

http://members.optusnet.com.au/sylviae/cbCrop.jpg



** 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

On 2009-01-04, Sylvia Else wrote:

The small transistor is a C1815 - an NPN, with an annoyingly difficult
to match pin-out.

2SC1815

http://www.toshiba.com/taec/componen...nc/50/6455.pdf

Ok, as I mentioned elsewhere in this thread, I managed to get the A/C
working again by replacing Q1 and the 8.2 ohm resistor.

Actually, I'd misread it, and when I looked more carefully, it realised
it was 6.2 ohm, as is the other one on the board with the same markings
(i.e., a not blown one). Nice standard value that.

So now I have to replace the resistor with a fusible, but sourcing a 6.2
ohm fusible is problematic.

The situation is not made any easier because if the switching transistor
fails by shorting out, then after it blows the fusible will have 340V
across it.

From the suppliers I now know about (thanks, Trevor), I can get a 0.5
watt 340V 4.7 ohm fusible. I propose to put it in series with a 1.5 ohm
1 watt non-fusible. My reasoning is that the 1.5 ohm resistor will have
only 1/3 the power dissipation, and will handle twice as much. Therefore
the 4.7 ohm will go open circuit before the 1.5 ohm could get hot enough
to be a problem.

Does this stand up?

Sylvia.

On Mon, 05 Jan 2009 14:34:36 +1100, Sylvia Else
wrote:

Phil Allison wrote:
"Sylvia Else"
"No legg to stand on "
** Jesus Christ -

get ****ing real you asinine ****ing WOG ****** !!

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

My test repair with a non-fusible resistor and an NPN transistor from my
stock seems to have worked (rather to my surprise), so I'll have to source
a fusible resistor now.

** WES Components in Ashfield have them.

Code: " 1F8.2 " for 24 cents each.


I'll take your comment about a hi-pot on board, though in fact I've only
worked on the high voltage side of the system - the transformer hasn't
been touched. Still, I suppose insulation breakdown in the transformer
could be implicated in the original failure.


** Ignore "legg" - he is an ignorant ****.

There is no such industry practice as he alludes to in the repair of SMPSs
incorporated inside products.

Where the PSU is a module and plenty of stock is available at LOW cost to a
repair organisation - of course they prefer to just replace it, no matter
what the fault. Any excuse in the book is then dragged out to justify that.

Where such replacement is NOT possible or would be uneconomic - it get
repaired just like you have done.

BTW:

What does "legg" have to say about the way your SMPS was installed in the
outside unit of an air-con with no protection against water ingress,
condensation or invasion by any and all insect and rodent life ?

What does that do for the " safety isolation " angle - eh ???

He might be unaware, since I posted that in a separate thread to
different groups. That thread was basically an opportunity for me to
winge about the quality of the circuit board, and give a heads up to
anyone considering buying an A/C unit.

I only posted this thread here when I had found something definitely
wrong with the PS, other than that it simply didn't work.

Legg FYI, that other thread referred to the fact that the board had
obviously suffered somewhat from being inadequately protected from the
environment and small creatures. I found a dead spider, some of its web,
and a good deal of dirt on the board.

http://members.optusnet.com.au/sylviae/cbCrop.jpg

Sylvia.

Unless it was x-posted to s.e.d. I won't see it. It's also -10C here,
with not much call for AC at the moment. I clean my central AC heat
exchanger each fall - I assume that discrete units might benefit from
similar treatment.

I think you mentioned that the unit was 10 years old; can't have been
too unsuitable an installation, if you're only having trouble with it
now. Funny how people religiously clean their computer cpu heatsinks
and fans, but expect AC units to clean themselves.

Glad to hear you've had success reviving the unit.......fusible or
flame-resistant resistors are not hard to get, with sources like
DigiKey, on-line.

By the way, what kind of scope were you using, that had Huntron-like
branch stimulation capabilities?

RL

legg wrote:


By the way, what kind of scope were you using, that had Huntron-like
branch stimulation capabilities?

It was a low end Hameg 20Mhz that I bought in France about 25 years ago.

Sylvia.

"Sylvia Else" wrote in message
u...
Ok, as I mentioned elsewhere in this thread, I managed to get the A/C
working again by replacing Q1 and the 8.2 ohm resistor.

Actually, I'd misread it, and when I looked more carefully, it realised it
was 6.2 ohm, as is the other one on the board with the same markings
(i.e., a not blown one). Nice standard value that.

So now I have to replace the resistor with a fusible, but sourcing a 6.2
ohm fusible is problematic.

The situation is not made any easier because if the switching transistor
fails by shorting out, then after it blows the fusible will have 340V
across it.

From the suppliers I now know about (thanks, Trevor), I can get a 0.5 watt
340V 4.7 ohm fusible. I propose to put it in series with a 1.5 ohm 1 watt
non-fusible. My reasoning is that the 1.5 ohm resistor will have only 1/3
the power dissipation, and will handle twice as much. Therefore the 4.7
ohm will go open circuit before the 1.5 ohm could get hot enough to be a
problem.

Does this stand up?

Sylvia.


Are you absolutely sure that it is definitely 6.2 ohms ? That is a *very*
odd value, particularly for a fusible type ... FWIW, I really don't think
that the circuit would give a damn if you replaced it with a 6.8 ohm, which
is a standard value. Did you actually measure the one that's ok with an
accurate low ohms meter, and get a reading of 6.2 ? Seems to me that a grey
band and a blue band might easily be misread one for the other, with some of
the banding paints I've seen used over the years. I find that it is often
very easy to misread red for orange or brown, especially if the resistor
runs warm in normal use.

Otherwise, if it definitely is 6.2 ohms, and you really want to replace it
with exactly that value, your reasoning with making such a value in the way
that you suggest, would be quite valid.

Arfa

Arfa Daily wrote:
"Sylvia Else" wrote in message
u...
Ok, as I mentioned elsewhere in this thread, I managed to get the A/C
working again by replacing Q1 and the 8.2 ohm resistor.

Actually, I'd misread it, and when I looked more carefully, it realised it
was 6.2 ohm, as is the other one on the board with the same markings
(i.e., a not blown one). Nice standard value that.

So now I have to replace the resistor with a fusible, but sourcing a 6.2
ohm fusible is problematic.

The situation is not made any easier because if the switching transistor
fails by shorting out, then after it blows the fusible will have 340V
across it.

From the suppliers I now know about (thanks, Trevor), I can get a 0.5 watt
340V 4.7 ohm fusible. I propose to put it in series with a 1.5 ohm 1 watt
non-fusible. My reasoning is that the 1.5 ohm resistor will have only 1/3
the power dissipation, and will handle twice as much. Therefore the 4.7
ohm will go open circuit before the 1.5 ohm could get hot enough to be a
problem.

Does this stand up?

Sylvia.


Are you absolutely sure that it is definitely 6.2 ohms ? That is a *very*
odd value, particularly for a fusible type ... FWIW, I really don't think
that the circuit would give a damn if you replaced it with a 6.8 ohm, which
is a standard value. Did you actually measure the one that's ok with an
accurate low ohms meter, and get a reading of 6.2 ? Seems to me that a grey
band and a blue band might easily be misread one for the other, with some of
the banding paints I've seen used over the years. I find that it is often
very easy to misread red for orange or brown, especially if the resistor
runs warm in normal use.

Blue Red Gold Gold = 6.2 ohms, 5%. It is in the E24 series, but I've
certainly never seen one before.

When I made up an equivalent, my meter gave the same reading (allowing
for tolerance) for the equivalent as it does for the identical resistor
on the board.

So, yes, I'm pretty sure.

As for whether I could substitute a 6.8, maybe I could, though I'd have
trouble getting one that has a 350 volt rating. The only supplier I know
of that purports to have them doesn't give the rating. The suppliers
that give ratings don't carry that value.

If I understood the circuit better, I'd be more comfortable about
changing the value. The mere fact that an unusual value has been used
gives me pause - maybe it's the value that's required there. Even if
another value worked, I'd not know the ramifications.

Sylvia.

"Sylvia Else"



As for whether I could substitute a 6.8, maybe I could, though I'd have
trouble getting one that has a 350 volt rating.


** Huh ??????????

No need for that whatsoever.

In operation, the resistor is not subjected to more than a few volts.




..... Phil

Phil Allison wrote:
"Sylvia Else"


As for whether I could substitute a 6.8, maybe I could, though I'd have
trouble getting one that has a 350 volt rating.


** Huh ??????????

No need for that whatsoever.

In operation, the resistor is not subjected to more than a few volts.

As I commented earlier, if the switching transistor fails by shorting
out, then the resistor will blow. After it's blown it'll have 340V
across it. There's not much point in having a fuisible there if it
remains conductive through insulation break down after it has fused.

Sylvia.