Different voltage drop on two identical mosfets - Page 1
Forum section: electronics / beginners

author: u666sa
author_rank: Frequent Contributor
timestamp: October 01, 2023, 03:43:47 pm
content: 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 0.4858Is that okay or no okay?

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: October 01, 2023, 04:14:59 pm
content: Did you short the source and drain before applying the DMM?

author: u666sa
author_rank: Frequent Contributor
timestamp: October 01, 2023, 04:52:08 pm
content: Of course! Did it for both, measured several times, each time shorting.

author: CaptDon
author_rank: Super Contributor
timestamp: October 01, 2023, 05:03:19 pm
content: 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.

author: u666sa
author_rank: Frequent Contributor
timestamp: October 01, 2023, 05:16:44 pm
content: 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.

author: mzzj
author_rank: Super Contributor
timestamp: October 01, 2023, 06:05:03 pm
content: Quote from: u666sa on October 01, 2023, 05:16:44 pmSo 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.

author: u666sa
author_rank: Frequent Contributor
timestamp: October 01, 2023, 06:07:13 pm
content: Quote from: mzzj on October 01, 2023, 06:05:03 pmIn other words, they are both good?

author: Hiemal
author_rank: Regular Contributor
timestamp: October 01, 2023, 10:41:18 pm
content: 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.

author: fourfathom
author_rank: Super Contributor
timestamp: October 01, 2023, 11:28:54 pm
content: Quote from: Hiemal on October 01, 2023, 10:41:18 pmHow can this not just read zero Ohms? Gate, source, drain all shorted together. Please explain.

author: EPAIII
author_rank: Super Contributor
timestamp: October 03, 2023, 02:23:08 am
content: 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