Hi, I have an MSI Z77A-G43 motherboard with a short on the 12V rail, there isn't any bulged capacitor, how can I find the short?

 

How do you know there is a short on the 12V rail?

Do you have a schematic or can you draw one?

 

There is continuity across 12V and GND, so any power supply will trip.

 

The 12V is for motors like hdd, cds, etc,, the only way to find a short is to isolate the 12v tracks along it's path, which means cutting the copper pcb at various points, .. I would start by checking any silicon devices like diodes, mosfets etc...

 

I think I would try passing a current of between 100 mA and one amp between the +12 volt and ground inputs of the mother board. Then using a DC milivoltmeter measure the voltage between the terminals of components that you think are across the 12 volt rail and ground. the component with the lowest voltage reading is probably the one that is shorted. A mother board will have many layers so following the + 12 volts track will probably be difficult.

Les.

 

Second LesJones' method. Did that all the time when I was still working. Depending on the board, I would use as much as 5 amps. Then just follow the path(s) with a millivolt meter until you find the lowest voltage, and start looking for your short in that area.

 

First off verify it's actually the motherboard, by disconnecting everything from it.

If it's definately the mobo then you could pass a current thru it from a CC supply and look for the hot spot.

 

I tested for continuity on the 8-pin connector next to the cpu socket with the motherboard out of the case and nothing connected to it, without cpu and ram.
Wouldn't it be much easier just freezing the board, apply current and check where it melts first?

 

I tested for continuity on the 8-pin connector next to the cpu socket with the motherboard out of the case and nothing connected to it, without cpu and ram.
Wouldn't it be much easier just freezing the board, apply current and check where it melts first?

Heat require current and *voltage*. If it is a short circuit, there will be no voltage across it, hence no heat.

 

Heat require current and *voltage*. If it is a short circuit, there will be no voltage across it, hence no heat.

That's true for a constant current power supply, but what if I apply 12V to it with nothing but a current limiting resistor, where will all the power go to?

 

That's true for a constant current power supply, but what if I apply 12V to it with nothing but a current limiting resistor, where will all the power go to?

Just as an example... If you take 12 volts and apply it through a resistor into a 'shorted' board, most of the voltage will drop across the resistor. Really no different than using a CC supply. So you may only have a volt or so on the board itself. Assume the short is on the far side of the board from where you are connecting power: There will be no heat generated at the short, because no voltage will be across the short -- or look at it as P = I²R, if r is zero, no power dissipated.

The voltage will be dropped across the runs in the board between the source and the 'short'. The heat may be well distributed across the board. So you can't use location of heat to determine location of short.

But this is getting a bit off subject. Put some current into the board and follow the voltage drops with your voltmeter. You'll find the short.

 

Heat require current and *voltage*. If it is a short circuit, there will be no voltage across it, hence no heat.

Just as an example... If you take 12 volts and apply it through a resistor into a 'shorted' board, most of the voltage will drop across the resistor. Really no different than using a CC supply. So you may only have a volt or so on the board itself. Assume the short is on the far side of the board from where you are connecting power: There will be no heat generated at the short, because no voltage will be across the short -- or look at it as P = I²R, if r is zero, no power dissipated.

The voltage will be dropped across the runs in the board between the source and the 'short'. The heat may be well distributed across the board. So you can't use location of heat to determine location of short.

But this is getting a bit off subject. Put some current into the board and follow the voltage drops with your voltmeter. You'll find the short.

Nah, I disagree.
Maybe you're right if we're talking theory and you're proving your point in Spice sim.
But this is real life and there's so such thing as a true SHORT on a board with 0.00000000000 ohms that would drop no voltage across it.
Any shorted component is likely to have higher resistance than PCB tracks, and heat up before they do.

 

I think I would try passing a current of between 100 mA and one amp between the +12 volt and ground inputs of the mother board. Then using a DC milivoltmeter measure the voltage between the terminals of components that you think are across the 12 volt rail and ground. the component with the lowest voltage reading is probably the one that is shorted. A mother board will have many layers so following the + 12 volts track will probably be difficult.

Les.

I third LesJones use a meter

 

You can use your ohmmeter to find where the resistance between 12V and gnd is the lowest. That is where you will most likely find your short. It's probably an electo cap somewhere. If the cap is internally shorted, there won't be any bulging.

 

You can use your ohmmeter to find where the resistance between 12V and gnd is the lowest. That is where you will most likely find your short. It's probably an electo cap somewhere. If the cap is internally shorted, there won't be any bulging.

You'd need a 4 wire milliohm meter. A normal ohmmeter is not sensitive enough.

 

Any shorted component is likely to have higher resistance than PCB tracks, and heat up before they do.

Perhaps. Perhaps not.

Put in the current, use the voltmeter.

 

Test any and all diodes, transistors and varistors using a multimeter.... You will have to remove any that appear faulty (ie they sit across the 12v rail) in order to rule them out and It might take a while.