Testing a MOSFET short circuit

Thread Starter

nichols89_ben

Joined Dec 17, 2020
5
Hi, im trying to determine if I have a short in an SMD MOSFET (NTD4965NT4G) using a multimeter. My device is stuck in the on position and this seems to be the culprit. I was asked to check the voltage on the right pad on the FET, as it should be at zero volts in relation to the GND pin. Im new at this and not sure what that means so any help would be appreciated Thanks!
 

dl324

Joined Mar 30, 2015
12,917
Welcome to AAC!
I was asked to check the voltage on the right pad
This is meaningless without specifying the package and orientation. It would be better for you to use the terminal names. If you're referring to SOT-23, it could be gate or source.
on the FET
Since you're new, you should get in the habit of using more specific terminology. There are many kinds of FETs and MOSFET is just one of them.
 

Thread Starter

nichols89_ben

Joined Dec 17, 2020
5
Welcome to AAC!

This is meaningless without specifying the package and orientation. It would be better for you to use the terminal names. If you're referring to SOT-23, it could be gate or source.
Since you're new, you should get in the habit of using more specific terminology. There are many kinds of FETs and MOSFET is just one of them.
I attached some more info to the other reply
 

Thread Starter

nichols89_ben

Joined Dec 17, 2020
5
If the voltage between the drain and source is near 0V with the power on, then the MOSFET is switched ON.
Thanks for the response! Would this detect a short, causing the MOSFET to stay on all the time, even when it's switched off? Forgive my ignorance, do you hook the positive or negative to the drain/source?
 

dl324

Joined Mar 30, 2015
12,917
Would this detect a short, causing the MOSFET to stay on all the time, even when it's switched off?
With the circuit unpowered, you can do a continuity check across the source drain. One direction you should see a diode and the other should be open. If you get a short both directions then the device is shorted.
 

Thread Starter

nichols89_ben

Joined Dec 17, 2020
5
With the circuit unpowered, you can do a continuity check across the source drain. One direction you should see a diode and the other should be open. If you get a short both directions then the device is shorted.
Hi Denis, thanks for your help! Most of that flew over my head, so I need to read up some more. I assembled a wifi switch with mostly SMT pieces, hooked everything up, and worked fine except for it is stuck in the on position. The seller asked me to check the voltage on the right pad on the FET (verbatim). I have a multimeter but that's where I'm at
 

dl324

Joined Mar 30, 2015
12,917
I have a multimeter but that's where I'm at
If your meter has a diode check setting, you can use that. In one direction you should see a diode and in the other it should indicate over range. Other circuitry could interfere with the measurements, but you shouldn't read a short.
I assembled a wifi switch with mostly SMT pieces
That explains the quality of the soldering. I was wondering about that.
 
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