Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default SMPS troubleshooting

I have an automotive battery charger like this one:

http://www.carstel.com/index/product...fo/id/100.html

I have not been able to find schematics, and the manufacturer ignores
me.

There are two boards, one looks like the power supply section, while
the other one contains buttons and LED display. The SMPS board looks
like an ordinary flyback arrangement, with a transformer and opto
isolator for feedback.

The symptom is that the 5 A input fuse blows as soon as power is
connected. The fuse blows with a magnificent blue flash and produces
an audible pop, so the overcurrent is considerable.

The SMPS is based on an ST 3845B, driving a 9N90C MOSFET. Across the
MOSFET's D/S, there is a 470 pF / 1 kV capacitor, located very close
to the MOSFET. The capacitor is split open and has spewed its guts
onto the MOSFET. The capacitor measures open circuit with an ohmmeter.
I do not have a megger available, so I cannot check the capacitor at
high voltage.

I have unsoldered the MOSFET and the capacitor. Now the fuse does not
blow. Between the MOSFET's G and S pads on the PCB, I can see a nice
27 kHz square wave with declining amplitude for 7 ms, and then nothing
for 17 ms, before another burst of 27 kHz, so it looks like the
controller is trying to start from its bootstrap supply.

The MOSFET tests OK on one of those cheap ATMEGA-based component
testers.

What is the most likely fault scenario?

What should I check next?

Could the blown capacitor be the only problem, or is it only a result
of the actual fault?
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The MOSFet will set you back a princely $4, including shipping.
The cap will set you back, perhaps, a whole dollar ($1).
The controller - perhaps one more whole dollar ($1).

Replace them all. But only *after* you have checked any and all diodes for open or short conditions.

Strange that these devices do not like surges of any nature, and yet they style themselves as jump-starters. Go figure.

Peter Wieck
Melrose Park, PA
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On Monday, July 8, 2019 at 9:52:36 AM UTC-4, HW wrote:
I have an automotive battery charger like this one:

http://www.carstel.com/index/product...fo/id/100.html

I have not been able to find schematics, and the manufacturer ignores
me.

There are two boards, one looks like the power supply section, while
the other one contains buttons and LED display. The SMPS board looks
like an ordinary flyback arrangement, with a transformer and opto
isolator for feedback.

The symptom is that the 5 A input fuse blows as soon as power is
connected. The fuse blows with a magnificent blue flash and produces
an audible pop, so the overcurrent is considerable.

The SMPS is based on an ST 3845B, driving a 9N90C MOSFET. Across the
MOSFET's D/S, there is a 470 pF / 1 kV capacitor, located very close
to the MOSFET. The capacitor is split open and has spewed its guts
onto the MOSFET. The capacitor measures open circuit with an ohmmeter.
I do not have a megger available, so I cannot check the capacitor at
high voltage.

I have unsoldered the MOSFET and the capacitor. Now the fuse does not
blow. Between the MOSFET's G and S pads on the PCB, I can see a nice
27 kHz square wave with declining amplitude for 7 ms, and then nothing
for 17 ms, before another burst of 27 kHz, so it looks like the
controller is trying to start from its bootstrap supply.

The MOSFET tests OK on one of those cheap ATMEGA-based component
testers.

What is the most likely fault scenario?

What should I check next?

Could the blown capacitor be the only problem, or is it only a result
of the actual fault?



I would have bet almost anything the mosfet is shorted. Put it on an ohmmeter and check for continuity across all the pins.

Those tiny blue disc 1kv caps are known to both split and blow the device they're across. I stock the 331, 471, 681, 102, and 222 in 2kV


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Default SMPS troubleshooting

There certainly is no point trying to measure a split open capacitor.

On Monday, July 8, 2019 at 8:52:36 AM UTC-5, HW wrote:
I have an automotive battery charger like this one:

http://www.carstel.com/index/product...fo/id/100.html

I have not been able to find schematics, and the manufacturer ignores
me.

There are two boards, one looks like the power supply section, while
the other one contains buttons and LED display. The SMPS board looks
like an ordinary flyback arrangement, with a transformer and opto
isolator for feedback.

The symptom is that the 5 A input fuse blows as soon as power is
connected. The fuse blows with a magnificent blue flash and produces
an audible pop, so the overcurrent is considerable.

The SMPS is based on an ST 3845B, driving a 9N90C MOSFET. Across the
MOSFET's D/S, there is a 470 pF / 1 kV capacitor, located very close
to the MOSFET. The capacitor is split open and has spewed its guts
onto the MOSFET. The capacitor measures open circuit with an ohmmeter.
I do not have a megger available, so I cannot check the capacitor at
high voltage.

I have unsoldered the MOSFET and the capacitor. Now the fuse does not
blow. Between the MOSFET's G and S pads on the PCB, I can see a nice
27 kHz square wave with declining amplitude for 7 ms, and then nothing
for 17 ms, before another burst of 27 kHz, so it looks like the
controller is trying to start from its bootstrap supply.

The MOSFET tests OK on one of those cheap ATMEGA-based component
testers.

What is the most likely fault scenario?

What should I check next?

Could the blown capacitor be the only problem, or is it only a result
of the actual fault?


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What is the most likely fault scenario?

What should I check next?



wire yourself up a dual outlet box with a line cord an on off switch and a duplex outlet.

Cut the tabs on the outlet and wire the two sections in SERIES.

Plug your UUT into one outlet and a 60 Watt incandescent lamp into the other outlet.

Now you can troubleshoot. If there is a short, the light will limit the fault current.

Also beware using a scope that the ground in your circuit may be live.
I would buy or make an isolation transformer as well for your safety.

Replace the FET and cap and turn it on with the 60Watt bulb in series.

m




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On Mon, 8 Jul 2019 11:40:51 -0700 (PDT), Terry Schwartz
wrote:

There certainly is no point trying to measure a split open capacitor.


Sure. But since I had removed two components and the short went away,
I felt it was worth checking them both for a short. It turns out none
of them are shorted, which is where it gets interesting.

Some capacitors do fail short. In fact, some are designed specifically
to fail short.
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On Mon, 8 Jul 2019 11:30:50 -0700 (PDT), John-Del
wrote:

I would have bet almost anything the mosfet is shorted. Put it on an ohmmeter and check for continuity across all the pins.


The transistor responds just as it should to the ohmmeter. I have not
tested it at high voltage or current.

In circuit, I believe the MOSFET (and the capacitor) sees about 640 V
as it turns off. 240 V rectified is about 320 V, and I believe the
transistor seed twice that at turnoff?

Those tiny blue disc 1kv caps are known to both split and blow the device they're across. I stock the 331, 471, 681, 102, and 222 in 2kV


Yep, that's the capacitor type here. I have ordered a kit with 20
different values, 10 of each.
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On Mon, 8 Jul 2019 07:27:25 -0700 (PDT), "
wrote:

The MOSFet will set you back a princely $4, including shipping.
The cap will set you back, perhaps, a whole dollar ($1).
The controller - perhaps one more whole dollar ($1).


One of the goals with this repair attempt is to learn something about
SMPS. So, before I start throwing parts at it, I want to scratch my
head a little and try to go the methodical route.

Replace them all. But only *after* you have checked any and all diodes for open or short conditions.


OK, I will check the diodes.
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I have not been able to find schematics, and the manufacturer ignores me.

They ignore everybody. What you have to learn is reverse engineering, if you want to make fifty bucks an hour that is what you have to do.

You are lucky to be dealing with a problem like that. Follow those wires, and dealing with power those are big traces on the board you can see. I might be able yo show you if you brought a six pack, but I can't really even think of how I could tell you.

I financed four childhoods on this and I still can't explain it.

I mean mine...

You have to figure out **** on your own. That board, usually there is a ground plane, power plane. You CANNOT find out how it it but you know how IT HAS TO BE.

Get it in your head "How they make this mother****ingtron do what it does ?".

Take that attitude. I made more money than many engineers. But you do have to know theory.

One of these days I think I am going to publish a course in the basics that doesn't require degrees in Latin and calculus. It is so much simpler than they make it seem.

On a certain unit if you are really dumbfuddled, think "I have to do this, how do I do it ?". Approach it from the beginning.

But you have to learn it, I could not do it in 400 pages. I mean learn to disconnect this and that and take readings and all that, how to figure out how to fake the thing out to run with a few things missing. People have begged me to teach it.

Like a Dojo, you do not want to stand to with me when it comes to this. But I am not malicious, it is just a matter of not typing, or answering the phone, or email, or even the door. (I got Hydra-shoks) I get too ****ed off I just remove myself from that person's life. After a month they get the idea - YOU ARE ****ING GONE. And more than half of them owe me money.

I got a bit ****ed off at the boss and didn't answer the phone, he showed up at my house. I NEVER GAVE HIM MY ADDRESS !

Anyway online there are many articles on the different types of switching regulators, boost or buck, doesn't matter. Learn how the HAVE to work and then go down in there.


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There is always a balance when contemplating a repair between, for lack of better terms:

Accuracy - how suitable is the repair to the need?
Precision - how few parts are necessary to complete the repair?

To which I would add "cascade artifacts": some part or component up-line fails or glitches such that one-or-more parts down-line fail visibly and spectacularly. This will fool the typical tech into addressing the obvious failures, and not looking for the first-cause.

Hence: Check any and all diodes. If any zeners, special attention there! And while in there anyway, caps and so forth, switches for dirt.... you get the picture. General clean-up and policing.

Now, consider the analogy of a spun crankshaft bearing on a V6 engine. Only one bearing actually spun - but does a good tech replace only that bearing? Not hardly. You had a spectacular failure (spun bearing) of a small part - but are entirely unaware of the relative condition of the parts immediately adjacent to it. And no visible means to become aware.

Which is why, for an additional $4, it would pay to replace all the "bearings" as you have already torn down the engine in any case.

I am re-capping a vintage AR received I lent to a friend 20 years ago, and now it needs service. It really needs only two (2) small caps on the tuner board. But, as I am in there already, it will get new matched output transistors, re-biased, and about another dozen or so electrolytic and small-value caps replaced. The cost is small, the time is not-much, and it will have another 20 years of useful life, at least.

Peter Wieck
Melrose Park, PA
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On Monday, July 8, 2019 at 6:28:43 PM UTC-4, HW wrote:
On Mon, 8 Jul 2019 11:30:50 -0700 (PDT), John-Del
wrote:

I would have bet almost anything the mosfet is shorted. Put it on an ohmmeter and check for continuity across all the pins.


The transistor responds just as it should to the ohmmeter. I have not
tested it at high voltage or current.


My first rule of repair is that you'll never see everything in your career. In 50 years, I have seen just one high voltage high current semiconductor check normally on an ohmmeter but "short" under circuit conditions, then check normally again on the meter. They generally do you a favor and short themselves silly. The one I did find was an early 1980s RCA TV where the horizontal/hv output transistor would go into full conduction with *any* dc on the base and blow the fuse. It checked fine out of circuit but a new one stopped the immediate and spectacular blowing of the fuse. That's the only one I ever saw.

So, while you wait for your parts to come in, follow Peter's advice and check every diode you find in the circuit as that's the next part failure percentage-wise that follows your kind of failure. Often, a zener will check OK front to back but will conduct significantly off it's printed voltage (generally lower). Also carefully check all low value resistors.
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In article ,
says...

Now, consider the analogy of a spun crankshaft bearing on a V6 engine. Only one bearing actually spun - but does a good tech replace only that bearing? Not hardly. You had a spectacular failure (spun bearing) of a small part - but are entirely unaware of the relative condition of the parts immediately adjacent to it. And no

visible means to become aware.

Which is why, for an additional $4, it would pay to replace all the "bearings" as you have already torn down the engine in any case.



At work we had a motor speed controler to go bad. This was on about a
200 HP motor. Called in the factory repair man and he located 2 bad
diodes in the 3 phase circuit. I asked him to replace the 3 rd one. He
said it checked good and they were about $ 200 each. While it may or
may not have been bad, he at my request replaced it. I told him it was
costing us over $ 1000 an hour to hae the equipemtn down,so in the time
it took him to get there and many ohter things, it was maybe a 50 to 100
thousand of lost time if not more. I felt that $ 200 was good insurance.
While the diode was probably good, I felt that if 2 were bad, it may
have weakened the 3 rd one.

Same as when I had a timing belt changed on schedule on my Toyota. The
mechanic recommended replacing the water pump as it was in the same area
of the tare down. Said no more labor,just the cost of the pump and he
doubted it would last another 70,000.
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On Tue, 9 Jul 2019 06:56:53 -0700 (PDT), John-Del
wrote:

On Monday, July 8, 2019 at 6:28:43 PM UTC-4, HW wrote:
On Mon, 8 Jul 2019 11:30:50 -0700 (PDT), John-Del
wrote:

I would have bet almost anything the mosfet is shorted. Put it on an ohmmeter and check for continuity across all the pins.


The transistor responds just as it should to the ohmmeter. I have not
tested it at high voltage or current.


My first rule of repair is that you'll never see everything in your career. In 50 years, I have seen just one high voltage high current semiconductor check normally on an ohmmeter but "short" under circuit conditions, then check normally again on the meter. They generally do you a favor and short themselves silly. The one I did find was an early 1980s RCA TV where the horizontal/hv output transistor would go into full conduction with *any* dc on the base and blow the fuse. It checked fine out of circuit but a new one stopped the immediate and spectacular blowing of the fuse. That's the only one I ever saw.

So, while you wait for your parts to come in, follow Peter's advice and check every diode you find in the circuit as that's the next part failure percentage-wise that follows your kind of failure. Often, a zener will check OK front to back but will conduct significantly off it's printed voltage (generally lower). Also carefully check all low value resistors.

I had a late 70s RCA console that would not start intermittantly out
in the boonies but worked perfectly in town. The voltage at this farm
varied between 110 to 115 volts. Replacing the horizontal output
transistor repaired the set.
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On Tuesday, 9 July 2019 09:10:55 UTC+1, Jeff Urban wrote:

On a certain unit if you are really dumbfuddled, think "I have to do this, how do I do it ?". Approach it from the beginning.


the only problem with that approach is that with a battery charger you could use maybe 10% the parts they did. Too many things have junk they don't need nowadays.

I once replaced a whole large psu pcb in a Sony tv with a rectifier & lightbulb. Yep, it worked. Wonder if that's where the fashion for surround lighting came from with TVs.


NT


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On Wednesday, July 10, 2019 at 9:12:53 AM UTC-4, wrote:

the only problem with that approach is that with a battery charger you could use maybe 10% the parts they did. Too many things have junk they don't need nowadays.


Mad Man Muntz had this minimalism down to a science. That being written, please consider:

Any device sold to the great unwashed is a balancing act between first-cost, utility, warranty and longevity. Add into that a significant measure of product safety, and you would have a very basic formula for the design of a consumer item.

What do battery chargers (as discussed) do? They interact with a very large, highly energetic devices called 'lead-acid' batteries in a way that *can* lead to spectacular results. A charger that creates such results on a regular basis will not do its manufacturer any favors.

It is takes no power of imagination whatsoever to figure out that a manufacturer will add nothing more than what is absolutely necessary to any device per the basic formula already described.

Peter Wieck
Melrose Park, PA
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On Thursday, 11 July 2019 19:19:26 UTC+1, wrote:
On Wednesday, July 10, 2019 at 9:12:53 AM UTC-4, tabby wrote:

the only problem with that approach is that with a battery charger you could use maybe 10% the parts they did. Too many things have junk they don't need nowadays.


Mad Man Muntz had this minimalism down to a science. That being written, please consider:

Any device sold to the great unwashed is a balancing act between first-cost, utility, warranty and longevity. Add into that a significant measure of product safety, and you would have a very basic formula for the design of a consumer item.

What do battery chargers (as discussed) do? They interact with a very large, highly energetic devices called 'lead-acid' batteries in a way that *can* lead to spectacular results. A charger that creates such results on a regular basis will not do its manufacturer any favors.

It is takes no power of imagination whatsoever to figure out that a manufacturer will add nothing more than what is absolutely necessary to any device per the basic formula already described.

Peter Wieck
Melrose Park, PA



Consumer devices are a slowly moving target on the various performance points you mentioned. In the 60s you got no charging control & were lucky if you got a fuse. Today most chargers are 'smart', which is to say too dumb to charge a flat battery. Most battery chargers have a lot more features than they need so they can boast & sell more. To the average consumer more crap is better, though it often isn't.

Avoiding fireworks is a low barrier for chargers, except for lithium cells.


NT
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On Mon, 08 Jul 2019 15:52:32 +0200, I wrote:

Could the blown capacitor be the only problem, or is it only a result
of the actual fault?


The capacitors arrived. Out of curiosity, I reinstalled the old
transistor and tested the charger (without the capacitor). The charger
started up normally, no fuse blowing. Then I added the new capacitor
and ran the charger at full load for about 15 minutes with no
problems.

In the meantime, I also bought a cheap megger and tested the blown
capacitor at up to 1 kV. It does show some conductance, but it is in
the hundreds of kohm range, so it should not be able to blow the 5 A
fuse.

So, the charger works again. Although the blown capacitor seems to
have been the root cause of the problem, the analysis does not quite
add up. Maybe there is an intermittent problem somewhere?

Thank you everyone for your comments.
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On Wednesday, 17 July 2019 23:29:44 UTC+1, HW wrote:
On Mon, 08 Jul 2019 15:52:32 +0200, I wrote:

Could the blown capacitor be the only problem, or is it only a result
of the actual fault?


The capacitors arrived. Out of curiosity, I reinstalled the old
transistor and tested the charger (without the capacitor). The charger
started up normally, no fuse blowing. Then I added the new capacitor
and ran the charger at full load for about 15 minutes with no
problems.

In the meantime, I also bought a cheap megger and tested the blown
capacitor at up to 1 kV. It does show some conductance, but it is in
the hundreds of kohm range, so it should not be able to blow the 5 A
fuse.

So, the charger works again. Although the blown capacitor seems to
have been the root cause of the problem, the analysis does not quite
add up. Maybe there is an intermittent problem somewhere?

Thank you everyone for your comments.


it may be that as the cap blew the fuse it also blew its own conductive path out


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On 7/17/2019 6:29 PM, HW wrote:
On Mon, 08 Jul 2019 15:52:32 +0200, I wrote:

Could the blown capacitor be the only problem, or is it only a result
of the actual fault?


The capacitors arrived. Out of curiosity, I reinstalled the old
transistor and tested the charger (without the capacitor). The charger
started up normally, no fuse blowing. Then I added the new capacitor
and ran the charger at full load for about 15 minutes with no
problems.

In the meantime, I also bought a cheap megger and tested the blown
capacitor at up to 1 kV. It does show some conductance, but it is in
the hundreds of kohm range, so it should not be able to blow the 5 A
fuse.

So, the charger works again. Although the blown capacitor seems to
have been the root cause of the problem, the analysis does not quite
add up. Maybe there is an intermittent problem somewhere?

Thank you everyone for your comments.


Without a schematic you can't say for certain, but this is
a possibility: with the bad cap in there it may be that the
Mosfet was turned on 100% of the time, thus a dead short
through the inductor to ground, instead of sending pulses
to the inductor to charge it. The fuse saved the Mosfet
and/or the inductor.

Ed



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