Finding a short circuit on a low voltage line

Thread Starter

basil_555

Joined Oct 20, 2021
15
Hi,

I have a board with a short circuit. The test point on a short circuited line reads 0v85, which I assume is 0.85V. When I put 0.85V to that line from my bench power supply, the current consumption is about 0.5A and the thermal imager shows that nothing is getting hot. Can I put more voltage to that line to see better which component is short circuited? And, in general, what are your advices for finding a short on a low voltage line? Thank you!
 

LesJones

Joined Jan 8, 2017
3,570
With 0.85 volts on the test point move along the track measuring the voltage until you get the lowest voltage reading. The short will be near that point. If you look at the schematic you may get a good idea of likely items connected directly between that track and the zero volt rail.

Les.
 

BobTPH

Joined Jun 5, 2013
4,030
.85V sounds like a forward biased diode(or transistor junction.)

What is the history? Is this a board that worked at one time, or one that you just built and it never worked?

Bob
 

Thread Starter

basil_555

Joined Oct 20, 2021
15
This board worked at one time. Then it had a short circuit, probably after a fall, but I’m not sure. Will I burn anything else if I put 1V or 2V on that 0.85V line? In my understanding, I can do it since it is already short circuited and all the voltage will go to the shorted component without affecting other elements. Am a right? Unfortunately, I don’t have schematics for that board.
 

Dodgydave

Joined Jun 22, 2012
10,085
You can use freezer spray and give it a coating of ice, then connect the power from your psu, take it upto 1.5v and note anything getting warm and melting, will give you an Idea where to look..
 

Thread Starter

basil_555

Joined Oct 20, 2021
15
You can use freezer spray and give it a coating of ice, then connect the power from your psu, take it upto 1.5v and note anything getting warm and melting, will give you an Idea where to look..
I followed your advice. I increased the voltage up to 1.5V. The current consumption became 2.5A, but nothing was getting hot at all. I'm really stumbled.
 

Thread Starter

basil_555

Joined Oct 20, 2021
15
I'm surprised that 2.5A on a pcb doesn't create heat!,
It turns out that there was something heating up and it was the CPU! 0v85 line was the CPU power line. It was low resistance and it was buzzing as if it was shorted. But there is a transistor and some other chips (PWM?) which buzz to ground nearby, which are actually shorted. Strangely, when I put voltage to 0v85 line, only the CPU was heating up, but not the shorted chips. Maybe because they are behind the coil? I have no way to remove the coil since it is a very tight SMD mount. So, probably, I’ll have to remove suspicious chips one by one to see which one is shorted.
 

LesJones

Joined Jan 8, 2017
3,570
Some more information probably woulf be helpful. What is the function of the board ? What is the CPU part number ? Waht are the part numbers of the chips that are getting hot ? The coil may be part of a switching regulator.

Les.
 

Thread Starter

basil_555

Joined Oct 20, 2021
15
Some more information probably woulf be helpful. What is the function of the board ? What is the CPU part number ? Waht are the part numbers of the chips that are getting hot ? The coil may be part of a switching regulator.
This is a video recorder board. I can't tell the CPU part number because it is covered with a massive heatsink. Here are the photos of the actual area.

Photo 1:
1. 2 chips marked with yellow dots. They are ok.
2. 2 chips marked with blue dots. They are the same part numbers as 2 above, but they sound shorted.
3. A46J cap sound shorted.
4. Coil marked with green is probably what connects this area to CPU.

Photo 2:
5. There is also a chip on the other side of the board with a marking "TE3". 2 contacts marked with yellow buzz to ground.
20211031_200514.jpg20211031_201141.jpg
 

Thread Starter

basil_555

Joined Oct 20, 2021
15
So you're trying to check components with a continuity tester with them still in-circuit? That isn't going to give you much useful information.
I thought that putting some voltage to any component on the shorted line will make the actual shorted component heat up. I don't understand why it doesn't work in my case.
 

dl324

Joined Mar 30, 2015
13,505
I thought that putting some voltage to any component on the shorted line will make the actual shorted component heat up. I don't understand why it doesn't work in my case.
Using a continuity tester isn't going to give you any useful information.

You need to use a sensitive volt meter and find the lowest voltage on the net in question. Another option is to use something like an HP547A current tracer.
1635706655759.png
"New" was in the mid-1970's.
 

dl324

Joined Mar 30, 2015
13,505
Is there anything like it poduced now?
Not that I know of. You can find them at a reasonable price on eBay from time to time. Now isn't that time; used test equipment prices are unreasonably high. There's one listing in the US for $600. I didn't pay anywhere near that for mine.
 
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