# Basic MOSFET testing, questions

#### UnnamedUser159

Joined May 3, 2016
501

the case or the tab if you wish
the case is the plastic, right?

the tab is metal part with the circle in it, right ?

With the meter positive still connected to the drain, touch a finger between source and gate
the negative probe is not now on any pin?

thanks...

#### 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.

#### mvas

Joined Jun 19, 2017
537
the case is the plastic, right?
the tab is metal part with the circle in it, right ?
With the meter positive still connected to the drain, touch a finger between source and gate
the negative probe is not now on any pin?
thanks...
The article clearly states ...
" ... So: connect the meter negative to the MOSFET's source ..."

#### shortbus

Joined Sep 30, 2009
7,607
Most people aren't going to click on links to unknown sites
But, 4qdtec, is a well know site to people interested in DC motor control.

#### 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? Shouldnt 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.

Last edited:

#### 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.

#### UnnamedUser159

Joined May 3, 2016
501

in this case with my mosfet i can use 9v battery alone to charge gates 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.

#### shortbus

Joined Sep 30, 2009
7,607
Edit: That gate is rated +/- 30V.
That is why I said that the 3 to 4.5V wasn't enough to turn the mosfet fully on, in an earlier post. Never saw numbers like that mosfet uses.

#### 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

Last edited:

#### 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.

#### jpanhalt

Joined Jan 18, 2008
9,115
I only see a 2x16 LCD character display attached to something. What is the something?

#### UnnamedUser159

Joined May 3, 2016
501
Transistor-L/C ESR Tester
QS-15-102

#### UnnamedUser159

Joined May 3, 2016
501

#### 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