Basic MOSFET testing, questions

spinnaker

Joined Oct 29, 2009
7,835
Most people aren't going to click on links to unknown sites. So you need to explain in more detail.


Yes the tab is the metal part with the hole.

Read the sticky at the top of this forum. This subject is not test and measurement.
 

Thread Starter

UnnamedUser159

Joined May 3, 2016
501
I read that i should have low reading on DMM/diode testing mode/ when i have COM probe on the S and just touch the G and then move the probe to the Drain.
I have new one mosfet here.
P10NK70ZFP
Use DMM that has 3 Volts on diode testing mode. When touch the Drain i see
1391 and then 1. Why? Shouldn`t i see a "low reading" ?

thanks for unconfusing me;
 

jpanhalt

Joined Jan 18, 2008
9,115
The gate threshold voltage can be as low as 3.0V and as high as 4.5V for that device according to the DS.

Your "probe" needs to charge up the gate capacitance. Be sure you do not touch the gate after charging it. If that still doesn't work, try a 4V to 5V source to charge the gate, then retest. If you are uncomfortable using a supply voltage directly on the gate, add a series resistance of 100 to 1000 ohm.

Edit: That gate is rated +/- 30V. You could use a 9V battery to charge the gate too. Be sure the battery negative and COM are connected.
 
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Thread Starter

UnnamedUser159

Joined May 3, 2016
501
can i just touch the mosfet G with positive probe of one 9v battery and at the same time the Minus of battery to the Source and after seconds make that D-S check with DMM or the seconds are a lot and the gate will be auto discharged ?
 

jpanhalt

Joined Jan 18, 2008
9,115
Yes, you can do that. While it is true that the capacitor takes time to charge (it is about 2 nF in that device), the time for doing that will be very short in human terms. That is, you don't need to "wait" very long.

The important thing is that there is a "common ground." That is, the meter's negative (COM) and the battery negative need to be connected when you touch the gate. After that and assuming you do not touch the gate again, it does not matter.
 

Thread Starter

UnnamedUser159

Joined May 3, 2016
501
have reread the topic again.

in this case with my mosfet i can use 9v battery alone to charge gate`s capacitanse with only + and minus to G and S.
After that quick catch the DVM in doide test mode and check DS and expect that "low reading" ?

about test #2 if i dont have such probe that will be constantly connected to Source can i put my ESD gloves and just make a short G-S with tweezers and then without acting quick measure d-s again with DVM?

thanks
 

jpanhalt

Joined Jan 18, 2008
9,115
about test #2 if i dont have such probe that will be constantly connected to Source can i put my ESD gloves and just make a short G-S with tweezers and then without acting quick measure d-s again with DVM?
Yes. In fact, even without ESD gloves, touching it with your finger will discharge the gate. Or a resistor to source will do the same thing.
 

Thread Starter

UnnamedUser159

Joined May 3, 2016
501
and maybe for the final/for now/ - for discharging gate`s capacitance just short with metal tweezers without ESD gloves?

thanks
 
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jpanhalt

Joined Jan 18, 2008
9,115
Anything that will connect the gate to the source/ground will work. The connection can be relatively high resistance or a short. In actual circuits, it is sometimes a diode across the gate resistor.* I just use my finger. The voltage is low (say 9V max). You can discharge with a resistor, tweezers, even a larger capacitor.

*A gate resistor may be used to control "ringing" (oscillation) or to slow turn on.
 

bertus

Joined Apr 5, 2008
20,461
Hello,

Do you have a link to the original tester?
The image does not tell me much.
With transistor, they could also mean just a BJT.

Bertus
 
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