Can hexfets be checked with a dvm?

Think I've got a couple bad irfp9240's and 240's.

When I put the meter on diode test, I'll get readings between source and drain, and source and gate.

Depending on p-channel or n-ch. using appropriate lead on source.

Am I smoking something or is this a valid test?

I get no readings on the ones I suspect are bad.
Or very low readings.

Thanks

Fets are kinda different. Are they enhancement mode FETs? Are the Depletion mode FETs??
Depletion mode FETs act like tubes. For N-channel depletion mode FETs, the Gate needs to be a lower potential than the Source to be in it's linear range. So, they're kind of on and you have to bias them to turn them off. This behavior is a lot like a tube.

Enhancement mode FETs behave a little more like transistors. For N-channel enhancement mode FETs, the Gate needs to have a little higher potential than the Source to be in it's linear range. So, they're kind of off until you bias them on.

Basically a FET is a resistor Source to Drain and the gate allows or pinches off the Channel/Resistor. For testing FETs, you can use the diode test on the DMM. For an N-Channel FET (Depletion or Enhancement), connect the red lead to the Drain and the black lead to the Source. Tie the Gate to the Source with a jumper. The resistance through the channel (Source to Drain) should be low. Then tie the Gate to the Source. If the FET is Enhancement mode, the resistance of the channel (Source-Drain) should be very high, or open.

These tests are for MOSFET's which have an oxide passivated Gate. This MOS (Metal Oxide Semiconductor) gate is basically open. No diode type of behavior or anything. JFETs measure a little bit different.

There are a lot of variety of FETs. Let me check out the Intl Rectifier parts and be more specific. I'll add later.

These IR MOSFETs are Enhancement mode. The only thing that might prevent a DMM from testing these MOSFETs it that they have a VGS (Voltage Gate-Source) of 2~4 Volts. The meter may not have the voltage to turn them on enough to measure a low resistance down the channel. FETs can short, but I've seen more go open.

If you test them and you're meter seems to not be able to turn them on, you could have a power supply and a dropping resistor added to the picture to truly test them.

Jon_Logan said:

These IR MOSFETs are Enhancement mode. The only thing that might prevent a DMM from testing these MOSFETs it that they have a VGS (Voltage Gate-Source) of 2~4 Volts. The meter may not have the voltage to turn them on enough to measure a low resistance down the channel. FETs can short, but I've seen more go open.

If you test them and you're meter seems to not be able to turn them on, you could have a power supply and a dropping resistor added to the picture to truly test them.

Click to expand...

They are irfp 240 and 9240 that I got from China. I'm not sure what type they are.
Kind of first time dealing with them.
So far the only test I've made are using a Fluke 175.

On N channel Neg to source and pos to G and then D I get somwhere in the .450v range.

Opposite on P channel.

I get these readings on the ones I know are good.

On others that I suspect are bad I get small reading Source to Drain and OL Source to Gate.

Seems like they are open or shorted to me. But it's just an uneducated guess.

Thanks Jon

Hmmm, you might want to check the pinout. From Gate to either Source or Drain, you should measure wide open. I get the feeling you are measuring Source to Drain. .. or you're measuring your body resistance by holding onto the probes. There's a back diode across the channel, built in to this particular MOSFET.. Here's a link to a Fairchild version:
http://www.pcpaudio.com/pcpfiles/transistores/IRFP240.pdf The Fairchild, the Chinese, and the International Rectifier version should all follow the same pinout.

Also with anything "MOS", be sure you follow safe ESD practices. While these devices have very rugged specs while being soldered into a circuit, static electricity applied to the Gate can cause punch through and cause a leak or a short to the channel.

Using the datasheet, and using the meter's diode test, put the meter's red lead to the Cathode (K) of the internal zener diode and the meter's black lead to the Anode (A) of the internal zener diode. Then touch the gate lead, with a jumper, to the red lead.... measure that. Remove the jumper from the red lead. Now attach the Gate, with a jumper, to the black lead. Measure that. Post those results. If you're getting no difference, it is possible/likely that the meter's diode test does not force enough voltage to turn on the gate. The force voltage will be under the MOSFET's VGS threshold voltage. getting around that is more complicated and requires something to increase the Gate voltage, either through the use of an external battery or power supply.

Sorry it's not that easy to explain, but FET's especially MOSFET's have different flavors and transistor tests can't be directly applied in many/most cases.

I really appreciate you help and instructions.

When you say across the zener I assume from the diagram it's the slots in the case?

I think my meter won't turn on the zener cause I just get 0.0000v in all connections the way you describe.

Even on the good units it reads the same.

but looking at the data sheet, what confuses me is that I get voltage reading from S to G and D on good parts.

But on the bad ones I get readings from S to D but OL S to G.
When I look at the diagram of the mosfet I don't get how my meter is turning that connection on?

Just probably over my head but I appreciate the education.

Thanks

Negative or positive lead on the gate should read open on the DVM (set to measure diode) when the other lead is connected to the drain or source for either enhancement and depletion-mode.

Short the gate and source with a jumper. For N-Chan negatve lead on the drain and positive on the source should read a diode drop across the drain/ source body diode about 0.4 - 0.7V. Positve on the drain and negative on the source should read open.
P Chan is the opposite.

Gates should not be left floating when measuring drain to source. Can lead to erroneous drain/ source measurements and appear that the drain and source are shorted when they are not.

kconnor said:

Negative or positive lead on the gate should read open on the DVM (set to measure diode) when the other lead is connected to the drain or source for either enhancement and depletion-mode.

Short the gate and source with a jumper. For N-Chan negatve lead on the drain and positive on the source should read a diode drop across the drain/ source body diode about 0.4 - 0.7V. Positve on the drain and negative on the source should read open.
P Chan is the opposite.

Gates should not be left floating when measuring drain to source. Can lead to erroneous drain/ source measurements and appear that the drain and source are shorted when they are not.

Click to expand...

Thanks a whole lot for teaching me that.

The four that I thought were bad meet all the criteria of both tests so I'm assuming they are ok.

In circuit these measurements won't work I'm assuming?

I tested another that I was pretty sure was bad and wouldn't pass any and got strange readings so I guess that is toast.

I really appreciate the help..

John

No, they will not work in circuit because its highly likely that the meter will read paths through circuitry external to the MOSFET.
The test I described is a highly reliable indicator of the condition of a device. If it passes it is likely OK and does not need to be replaced.

Again thanks to you both for the help.

Definitely pointing me in the right direction.

Take care