Electronics > Beginners

Different voltage drop on two identical mosfets

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mzzj:

--- Quote from: u666sa 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.

--- End quote ---
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.

u666sa:

--- Quote from: mzzj on October 01, 2023, 06:05:03 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.

--- End quote ---

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

Hiemal:
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.

fourfathom:

--- Quote from: Hiemal on October 01, 2023, 10:41:18 pm ---Try shorting the fet out completely (all pins shorted to each other) then just read from source to drain.
--- End quote ---
How can this not just read zero Ohms?  Gate, source, drain all shorted together.  Please explain.

EPAIII:
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?




--- Quote from: fourfathom on October 01, 2023, 11:28:54 pm ---
--- Quote from: Hiemal on October 01, 2023, 10:41:18 pm ---Try shorting the fet out completely (all pins shorted to each other) then just read from source to drain.
--- End quote ---
How can this not just read zero Ohms?  Gate, source, drain all shorted together.  Please explain.

--- End quote ---

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