MOSFET mystery

[JimBanville]

Re: MOSFET mystery

There is a diode inside between drain and source.

The fluke 115 diode test range (according to the spec.) is 2.00v, now check the data sheet for this mosfet On Characteristics, Vgs Gate threshold voltage

Thanks. Will do.
Sorry if I'm sounding like a broken record, besides the possible inability to get the diode to charge (if that's what it's called - I'm a novice), what are your thoughts on the probes having to be reversed? Or am I doing it wrong?
What "should" you see if you place the positive probe on the source and negative probe on the drain on an N channel mosfet?
And why can't I seem to "short" the gate and drain?

[R_J]

Re: MOSFET mystery

Thanks. Will do.
Besides the possible inability to get the diode to charge (if that's what it's called - I'm a novice), what are your thoughts on the probes having to be reversed? Or am I doing it wrong?
What "should" you see if you place the positive probe on the source and negative probe on the drain on an N channel mosfet?

Are you blind? can you not see that there is an internal diode junction across the mosfets drain and source, it is called a body diode. That is why you will ALWAYS test a diode junction one way across the drain and source of this mosfet.
Shorting the gate and drain would discharge the gate and the mosfet would check open. You need to charge the gate to turn on the mosfet. and in this case the gate voltage is higher than your meter can supply.

[JimBanville]

Re: MOSFET mystery

Are you blind? can you not see that there is an internal diode junction across the mosfets drain and source, it is called a body diode. That is why you will ALWAYS test a diode junction one way across the drain and source of this mosfet.
Shorting the gate and drain would discharge the gate and the mosfet would check open. You need to charge the gate to turn on the mosfet. and in this case the gate voltage is higher than your meter can supply.

Lol. Calm down. I said I'm a novice. I probably never uttered the word "mosfet" in my entire life prior to a couple days ago.
I'm aware there is a diode there. That's not a question. All the tutorials I've read on testing N channel mosfets have specially said to use the NEGATIVE probe on the source and the POSITIVE probe on the drain to test the mosfet. They never mention using the POSITIVE probe on the source and the NEGATIVE probe on the drain for any reason. That's the question I've been asking since I started this thread.
So here's my question yet again...Why do I only see any voltage reading when my probes are used in opposite polarity than all the tutorials I've seen? This one in particular... https://youtu.be/gloikp9t2dA?si=uM9MvsEnYdtSSrGg

[eccerr0r]

Re: MOSFET mystery

Thank you RJ I was about to spout the same, it was getting quite repetitive indeed

Anyway to shut off mosfets you need to make sure Vgs < Vth ... so you should be shorting gate and SOURCE not Drain.

There are 6 ways to connect two probes to a MOSFET. This is for a N-channel Enhancement. Reverse + and - for P-channel

+ to G, - to S => should be OL. This will charge the gate if the DMM's test voltage is high enough, and thus turn the mosfet on
- to S, + to G => should be OL. This should discharge the gate and turn the mosfet off regardless of DMM test voltage
+ to D, - to S => test channel, should be OL if off, very low ~0.0 if on
+ to S, - to D => Test body diode, should be 0.5 volts drop for most mosfets
+ to D, - to G => should read OL. I don't know the value of this measurement because it's indeterminate what it will do.
- to G, + to D => should read OL. I don't know the value of this measurement because it's indeterminate what it will do.

If you have a probe on G and other probe on D or S and it reads anything but OL, the MOSFET should have a one way trip to the round file. Also note that there are leaky mosfets where the gate will discharge on you and will self shut off. Whether this will cause the circuit to malfunction depends on the circuit but if you're fast enough changing connections you still should be able to see D-S with a low resistance.

[JimBanville]

Re: MOSFET mystery

Thank you RJ I was about to spout the same, it was getting quite repetitive indeed

Anyway to shut off mosfets you need to make sure Vgs < Vth ... so you should be shorting gate and SOURCE not Drain.

There are 6 ways to connect two probes to a MOSFET. This is for a N-channel Enhancement. Reverse + and - for P-channel

+ to G, - to S => should be OL. This will charge the gate if the voltage is high enough, and thus turn the mosfet on
- to S, + to G => This should discharge the gate and turn the mosfet off regardless of DMM, will read OL
+ to D, - to S => test channel, should be OL if off, very low ~0.0 if on
+ to S, - to D => Test body diode, should be 0.5 volts drop for most mosfets
+ to D, - to G => will read OL. I don't know the value of this measurement because it's indeterminate what it will do other than check the gate is not shorted to drain.
- to G, + to D => will read OL. I don't know the value of this measurement because it's indeterminate what it will do other than check the gate is not shorted to drain.

If you have a probe on G and other probe on D or S and it reads anything but OL, the MOSFET should have a one way trip to the round file.

Thank you!!!!

"+ to S, - to D => Test body diode, should be 0.5 volts drop for most mosfets"

^That^ is what I've wanted to know all along. Do you know why some people are saying to do - to S and + to D to get the .5 voltage reading, like this tutorial video I was referred to ... https://youtu.be/gloikp9t2dA?si=IgNn2CgL-ofGiDxC
is it just wrong? Or did I misinterpret something? Thanks for being patient and helping my to learn 👍🏼

[eccerr0r]

Re: MOSFET mystery

fsck youtube, I won't even bother watching that crap there, it's mostly garbage full of conspiracy theories and snake oil. Granted W2AEW tends to show accurate information so probably misinterpretation.

[JimBanville]

Re: MOSFET mystery

fsck youtube, I won't even bother watching that crap there, it's mostly garbage full of conspiracy theories and snake oil.

Uh. Ok. How about Reddit? The moderator of one of the electronics repairs subreddits referred my to the garbage YT video with bad info. Lol.
As you can imagine, diametrically opposed answers to these questions doesn't help someone learn.
I tried to recreate what W2AEW did in the video by placing - on S and + on D and, unlike W2AEW who got a .5v reading, I got OL. Only by reversing the probes did I get a .5v reading.

[eccerr0r]

Re: MOSFET mystery

reddit has a lot of garbage too. Again if you didn't see my amendment, W2AEW tends to be accurate but quite technical so likely you can easily misinterpret stuff.

[JimBanville]

Re: MOSFET mystery

reddit has a lot of garbage too. Again if you didn't see my amendment, W2AEW tends to be accurate but quite technical so likely you can easily misinterpret stuff.

Ah. The video is pretty short. Can you tell me what I got wrong, please? I literally followed along and did exactly what he did. I'd like to know. Thanks! ✌🏼

[eccerr0r]

Re: MOSFET mystery

I'm not going to watch the video. Point out the exact h:m:s of the video that you got confused.

Also note that he is using a BS170 or 2N7000 or something like that, these have different characteristics than a FDP26N40.

[JimBanville]

Re: MOSFET mystery

I'm not going to watch the video. Point out the exact h:m:s of the video that you got confused.

Also note that he is using a BS170 or 2N7000 or something like that, these have different characteristics than a FDP26N40.

Maybe my misunderstanding is that my mosfet requires more voltage to get the results he got. I watched several videos and read some articles and none mentioned that was a factor. I'd never tried testing a mosfet before. People were using DMM's similar to what I've got so I thought I'd be good. Thanks.

[redwire]

Re: MOSFET mystery

FDP26N40 VGS(th)on is spec'd at 3.0-5.0V at turning on slightly 0.25mA so some bogeys could stay stone off "bumped on" by a typical multimeter.
You have to know your multimeter, what compliance voltage it puts out on Ohms or Diode Test. Most will light up a blue LED so ~3V is common, but I have an old bench multimeter that puts out 15V. Measure it with another multimeter.
edit: the '328 component testers apply 5V which explains those different results.

Also, even if you discharge the gate and then connect the multimeter + to D, - to S, some charge gets transferred to the gate so if that pin is floating it will actually be bumped up above 0V due to the mosfet's internal capacitance.

[JimBanville]

Re: MOSFET mystery

I'm not going to watch the video. Point out the exact h:m:s of the video that you got confused.

Also note that he is using a BS170 or 2N7000 or something like that, these have different characteristics than a FDP26N40.

I have kept moving these probes around and discovered this...
I place - on drain and touch + to gate.
Then release both and place + on source and - on drain.
I then see a .3v reading.
While leaving + on source, touch gate with - then put - back on drain and see a .5v reading.
This is all contingent on me using the + probe on source.
If I do what W2AEW did in his video, - on source, I get nothing.

[R_J]

Re: MOSFET mystery

I was not getting mad or anything of the sort, I was just trying to answer your question as to why you kept measuring .49 when checking drain to source, it was because of the internal body diode.
As long as the mosfet is not shorted It is likely good and whatever equipment these are from, the mosfets are likely good.
and as explained by others, the video likely tested some other mosfet or igbt and they are not all the same.

[JimBanville]

Re: MOSFET mystery

I was not getting mad or anything of the sort, I was just trying to answer your question as to why you kept measuring .49 when checking drain to source, it was because of the internal body diode.
As long as the mosfet is not shorted It is likely good and whatever equipment these are from, the mosfets are likely good.
and as explained by others, the video likely tested some other mosfet or igbt and they are not all the same.

Gotcha. I appreciate it. Based on what research I had done on N channel mosfets in the last couple days, having never dealt with them before, including "recommended" videos on "how to test N channel mosfets", I was, and still am, puzzled how something can be tested in so many ways. Lol. Oh well, based on how I was finally able to trial-and-error figure out on my own how these particular mosfets are tested, I suppose they're fine and I'll move on to all the other components to figure out what's making this smps power cycle itself and not fully turning on. Thanks ✌🏼

[R_J]

Re: MOSFET mystery

Post make/model and pictures of the power supply, many times when a power supply will start and then restart it can be the startup (vcc) cap for the controller ic is weak, or maybe there is a short in the secondary causing it to shut down (over current detection)

[JimBanville]

Re: MOSFET mystery

Post make/model and pictures of the power supply, many times when a power supply will start and then restart it can be the startup (vcc) cap for the controller ic is weak, or maybe there is a short in the secondary causing it to shut down (over current detection)

Will do! 👌🏼