attempting to diagnose faulty components on a motherboard

[photonmagic]

hello everyone

firstly id like to thank those of you taking the time to read my post, hopefully with your help ill be able to learn  =]

i have an old asus p5g41t-m lx plus motherboard that turns on but no video. after surfing around youtube and google (for literally many days) i see many with similar issues testing mosfets for shots as well as other troubleshooting to try and pin point the area around the motherboard that was damaged. i took off everything non essential and powered it on, felt around with my fingers for hot components and i noticed the audio ic was VERY hot as well as 2 mosfets (nikos P2003BDG)located near the 24pin connector. i have taken out the audio ic but the issue still remains, so now im trying to test mosfets however, im a bit confused because from what i understand so far a to-252 mosfet should only have continuity at the source and after taking out one of the ones that was getting hot it doesnt give any continuity in any pin, ive tried opening and closing the gate by shorting the drain with gate but i get no beeps at all, i feel it must be bad pretty strongly because the same exact mosfet that is next to it does give me continuity at only the source pin. havent found any mosfets on this motherboard that do have continuity on both gate and source. dont really know if im doing something wrong so i figured it was time to ask for help, my apologies if im noobin =D

[Manul]

First of all I'm not sure what you mean by "continuity at the source". Continuity may only exist between two points. So for example between source and drain or between ground and some component in a circuit, etc.

Anyway, on a good N-ch mosfet you will see no continuity between any two points if gate is not on. You will also be able to measure diode voltage drop if you connect your multimeter negative to drain and positive to source. If you bring gate positive and above gate threshold voltage - you will measure continuity between drain and source (but there is no need to try that usually).

Bad power mosfet will usually be shorted. So it will have some constant resistance between drain and source (anywhere from miliohms to tens of ohms). Sometimes also gate to source.

This is of course out of circuit. When you measure in circuit in some cases you may fool yourself, so keep in mind, that other components around may affect your measurements.

But if in circuit it shows low resistance between drain and source or between gate and source - thats a high chance you have a failed part.

[bob91343]

You are misusing the test equipment.  Continuity testing is intended for cables, connectors, and the like.

If you are looking for shorts, use instead the Ohms function.

[helius]

You are misusing the test equipment.  Continuity testing is intended for cables, connectors, and the like.
If you are looking for shorts, use instead the Ohms function.

The continuity test on a DMM is simply a resistance (ohms) measurement with an added beep. The beep activates when the measured resistance is below a specified cutoff, which may be 20 ohms, more or less. This leaves margin for cable and probes and any wires in the circuit. The resistance of a semiconductor can vary quite a lot with an ohms test, depending on the test voltage and the device parameters.

For testing semiconductors, the Diode Test function is usually more appropriate, as it tells you the voltage drop across the semiconductor junction. 0 volts drop == the junction was melted.

[photonmagic]

First of all I'm not sure what you mean by "continuity at the source". Continuity may only exist between two points. So for example between source and drain or between ground and some component in a circuit, etc.

Anyway, on a good N-ch mosfet you will see no continuity between any two points if gate is not on. You will also be able to measure diode voltage drop if you connect your multimeter negative to drain and positive to source. If you bring gate positive and above gate threshold voltage - you will measure continuity between drain and source (but there is no need to try that usually).

Bad power mosfet will usually be shorted. So it will have some constant resistance between drain and source (anywhere from miliohms to tens of ohms). Sometimes also gate to source.

This is of course out of circuit. When you measure in circuit in some cases you may fool yourself, so keep in mind, that other components around may affect your measurements.

But if in circuit it shows low resistance between drain and source or between gate and source - thats a high chance you have a failed part.

i sincerely apologize, as you can tell im new to this, i meant that when removing the mosfet from the board i didnt get any continuity from drain to gate, drain to source, or from source to gate, i will now try to take measurements according to your suggestions, im realizing i do not know ALOT of things but i do want to learn! =]

You are misusing the test equipment.  Continuity testing is intended for cables, connectors, and the like.
If you are looking for shorts, use instead the Ohms function.

The continuity test on a DMM is simply a resistance (ohms) measurement with an added beep. The beep activates when the measured resistance is below a specified cutoff, which may be 20 ohms, more or less. This leaves margin for cable and probes and any wires in the circuit. The resistance of a semiconductor can vary quite a lot with an ohms test, depending on the test voltage and the device parameters.

For testing semiconductors, the Diode Test function is usually more appropriate, as it tells you the voltage drop across the semiconductor junction. 0 volts drop == the junction was melted.

thank you very much i will search how to properly use the diode function for testing, i know how to search for datasheets but i do not not how to interpret them yet =]