Increasing Load on PSU for short testing

[macattack600]

I am using atx psu 12v rail to find short on 12v rail motherboard. There is absolute short on this rail but 45 smd components in front of it. I started removing about 10 of the legs and gave up

I believe the board has a internal short through the VIA and not a component but I want to confirm.

I have now 1 12v car light bulb attatched to ground as the load the meter reads 2.0amps .


question ----If I add another bulb and so on would that would raise the amperage by 2 ?

Nothing is getting hot no components get warm, barley any voltage on the rail , the bulb in fact turns on so its completeting the circuit and not switching off the PSU

I figure I need a heavy load to really feel this internal short to confirm.
The more amps I tack on the more should be pulled through this heavy short right and I would be able to feel what is really getting hot, Providing i'm not going over say 12 or 16amps?

[petehall347]

Re: Increasing Load on PSU for short testing

quick tip ... transistors and diodes short in the main ... capacitors also short but not so often ... ic chips short also ... resistors burn up .
get the schematic and look for culprits then test on the board . if showing short remove and test again .
the way you are going you will make smoke and things will be worse than they are now ..
you could always get really involved and start checking voltage drops across resistors and checking voltages at semiconductors to find the short .

[cpt.charlie]

Re: Increasing Load on PSU for short testing

If you connect another bulb in parallel yes, current should increase but you may ruin your board.

[macattack600]

Re: Increasing Load on PSU for short testing

so for every bulb it should raise up 2 amps ? I'm thinking about attaching a long LED rope light I need a very demanding load.

[macattack600]

Re: Increasing Load on PSU for short testing

quick tip ... transistors and diodes short in the main ... capacitors also short but not so often ... ic chips short also ... resistors burn up .
get the schematic and look for culprits then test on the board . if showing short remove and test again .
the way you are going you will make smoke and things will be worse than they are now ..
you could always get really involved and start checking voltage drops across resistors and checking voltages at semiconductors to find the short .

I would like to learn what is thermal resistance short finding something to do with heating the board with heat gun and measuring resistance drop you know about this?

[cpt.charlie]

Re: Increasing Load on PSU for short testing

LEDs won't make a high load BTW, use a car lamp instead.

If you had a miliohm meter it could be handy to approach to the fault.

[macattack600]

Re: Increasing Load on PSU for short testing

If you connect another bulb in parallel yes, current should increase but you may ruin your board.

I also have a 50w 12v light bulb do you know if the amps i'm looking for will jump up by how much?

[cpt.charlie]

Re: Increasing Load on PSU for short testing

50/12=4.16A in theory, consider some extra resitance at the short and contacts so current will be a bit lower

[macattack600]

Re: Increasing Load on PSU for short testing

Great I have 2 I will report back later. I really don't think I'm going to burn this rail up i'm getting no action @ 2 amps I'll go 1 at time , it's a large smd internal VIA pad connecting 45 components to both sides. I have hunch its shorted inside but I need to know for sure.

[petehall347]

Re: Increasing Load on PSU for short testing

check voltages at semiconductors before adding more current and burning the board and more stuff .

[macattack600]

Re: Increasing Load on PSU for short testing

I have no voltage on the line I remove the resistor for voltage i'm missing per schematic. In diode mode--- red probe ground black on pad I get Dead short to ground and so on removing legs one by one stopped after 15

Cross reference working board gives 0.0500

[cpt.charlie]

Re: Increasing Load on PSU for short testing

Do you have a short in the general 12V rail (24 pin connector) or the short is related to 12V VRM rail?

[macattack600]

Re: Increasing Load on PSU for short testing

In VRM rail , when I remove the resistor everything behind it gives 12v the board is making what's necessary but stopping short .

[macattack600]

Re: Increasing Load on PSU for short testing

The psu says the 12v is rated @9.2amps so I guess I cannot go over that right?

[budm]

Re: Increasing Load on PSU for short testing

BTW, the cold resistance of the bulb will be about 10~15 less than the hot resistance so watch out high inrush current.