Author Topic: Different voltage drop on two identical mosfets  (Read 854 times)

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Offline u666saTopic starter

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Different voltage drop on two identical mosfets
« on: October 01, 2023, 03:43:47 pm »
Mosfets are FTW20N50A, I have them off the board. Both check out okay using testing technique in diode mode. Except, one shows voltage drop 0.2501 and the other one 0.4858

Is that okay or no okay?
 

Offline WaveyDipole

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Re: Different voltage drop on two identical mosfets
« Reply #1 on: October 01, 2023, 04:14:59 pm »
Did you short the source and drain before applying the DMM?
 

Offline u666saTopic starter

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Re: Different voltage drop on two identical mosfets
« Reply #2 on: October 01, 2023, 04:52:08 pm »
Of course! Did it for both, measured several times, each time shorting.  :-//
 

Online CaptDon

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Re: Different voltage drop on two identical mosfets
« Reply #3 on: October 01, 2023, 05:03:19 pm »
How do you test a MOSFET with drain shorted to source??? I short the gate to source and look for an open between D and S or the body diode if measuring in the opposite direction. I have also done gate shorted to drain to see that the MOSFET is conducting. If I did D shorted to S I would hope to see an open circuit in both directions measuring to the gate and to not blow up the gate by stressing the barrier from excessive gate voltage.
Collector and repairer of vintage and not so vintage electronic gadgets and test equipment. What's the difference between a pizza and a musician? A pizza can feed a family of four!! Classically trained guitarist. Sound engineer.
 

Offline u666saTopic starter

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Re: Different voltage drop on two identical mosfets
« Reply #4 on: October 01, 2023, 05:16:44 pm »
I short gate to drain with tweezers.
Black probe source, red drain. Reads open.
Black probe source, red gate and then back to drain. It reads voltage drop.

Same precedure on both mosfets. Except each one reads different voltage drop.
 

Offline mzzj

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Re: Different voltage drop on two identical mosfets
« Reply #5 on: October 01, 2023, 06:05:03 pm »
I short gate to drain with tweezers.
Black probe source, red drain. Reads open.
Black probe source, red gate and then back to drain. It reads voltage drop.

Same precedure on both mosfets. Except each one reads different voltage drop.
So you are actually turning on the mosfet with the test voltage from the DVM.
Measured voltage drop after that is highly dependent  on gate voltage threshold and how high voltage your particular DVM happens to apply.
Gate voltage threshold varies a lot between invidual mosfets resulting in one of them to turn on more completely than on than the other.
 
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Offline u666saTopic starter

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Re: Different voltage drop on two identical mosfets
« Reply #6 on: October 01, 2023, 06:07:13 pm »
Gate voltage threshold varies a lot between invidual mosfets resulting in one of them to turn on more completely than on than the other.

In other words, they are both good?  :-/O
 

Offline Hiemal

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Re: Different voltage drop on two identical mosfets
« Reply #7 on: October 01, 2023, 10:41:18 pm »
Yeah agreed, you're turning your MOSFET on by applying the red lead to the gate. Try shorting the fet out completely (all pins shorted to each other) then just read from source to drain. Gate doesn't need to be involved.
 

Offline fourfathom

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Re: Different voltage drop on two identical mosfets
« Reply #8 on: October 01, 2023, 11:28:54 pm »
Try shorting the fet out completely (all pins shorted to each other) then just read from source to drain.
How can this not just read zero Ohms?  Gate, source, drain all shorted together.  Please explain.
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Offline EPAIII

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Re: Different voltage drop on two identical mosfets
« Reply #9 on: October 03, 2023, 02:23:08 am »
Step by step:

1. Short all pins.
2. Remove that short.
3. Immediately, with on other steps, take your reading from source to drain.

This would be much like shorting a capacitor before reading it with an Ohm meter. You remove the short, then take the reading.

Or did I forget to check that this was not posted on April 1?



Try shorting the fet out completely (all pins shorted to each other) then just read from source to drain.
How can this not just read zero Ohms?  Gate, source, drain all shorted together.  Please explain.
Paul A.  -   SE Texas
And if you look REAL close at an analog signal,
You will find that it has discrete steps.
 


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