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How to Check a MOSFET Using a Digital Multimeter

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In this post I have explained how to test mosfets using multimeter through a set of steps, which will show help you to accurately learn the good or faulty condition of a mosfet

Table of Contents

Mosfets are Efficient but Complex Devices

MOSFETs are outstanding devices when it comes to amplifying or switching of various kinds of loads. Though transistors are also largely applied for the above purposes, both the counterparts are hugely different with their characteristics.

The amazing efficiency of mosfets are to a great extent neutralized by one drawback associated with these devices.It is the involved complexity which makes these components difficult to understand and configure.

Even the simplest of operations like testing a good mosfet from a bad one is never an easy task especially for the beginners in the field.

Though mosfets usually require sophisticated equipment for checking their conditions, a simple way using a multimeter is also considered effective most of the time for checking them.

We take the example of two types of N-channel mosfets, the K1058 and the IRFP240 and see how these mosfets can be tested using an ordinary digital multimeter through slightly different procedures.

checking mosfet with DMM

How to Check N-Channel Mosfets

1)  Set the DMM to the diode range.

2) Keep the mosfet on a dry wooden table on its metal tab, with the printed side facing you and leads pointed towards you.

3) With a screwdriver or meter probe, short the gate and drain pins of the mosfet. This will initially keep the internal capacitance of the device completely discharged.

4) Now Touch the meter black probe to source and the red probe to drain of the device.

5)You should see an "open" circuit indication on the meter.

6) Now keeping the black probe touched to the source, lift the red probe from drain and touch it to the gate of the mosfet momentarily, and bring it back to the drain of the mosfet.

7) This time the meter will show a short circuit (sorry, not short-circuit rather "continuity).

The results from the point 5 and 7 confirms that the mosfet is OK.

Repeat this procedure many times for  proper confirmation.

For repeating the above procedure each time, you will need to reset the MOSFET by shorting the gate and the drain leads using meter probe as explained earlier.

How to Check P-Channel Mosfets

For P-channel the testing steps will be as per 1,2,3,4 and 5, but the polarities of the meter will change. Here's how to do it.

1)  Set the DMM to the diode range.

2) Fix the mosfet on a dry wooden table on its metal tab, with the printed side facing you and leads pointed towards you.

3) With a any conductor or meter probe, short the gate and drain pins of the P-mosfet. This will initially enable the internal capacitance of the device to discharge, which is essential for the testing process.

4) Now Touch the meter RED probe to source and the BLACK probe to drain of the device.

5) You will find an "open" circuit reading on the meter.

6) Next, without moving the RED probe from the source, remove the black probe from drain and touch it to the gate of the mosfet for a second, and bring it back to the drain of the mosfet.

7) This time the meter will show a continuity or a low value on the meter.

That's it, this will confirm your mosfet is alright, and without any problems. Any other form of reading will indicate a faulty mosfet.

If you any further doubts regarding the procedures please feel free to express your thoughts in the comment section.

How to Test an IRF540 Mosfet

The procedures are exactly similar to the above explained N-channel mosfet testing procedures. The following video clip shows and proves how it may be implemented using an ordinary multi-meter.

Practical Video Tutorial

Simple Mosfet Tester Jig Circuit

If you are not convenient with the above mentioned testing procedure using a multimeter, then you can quickly construct the following jig for checking any N channel mosfet efficiently.

N channel mosfet testing jig compressed

Once you make this jig, you can plug-in the relevant pins of the mosfet into the given G, D, S sockets. After this you just have to press the push button for confirming the mosfet condition.

If the LED glows only on pressing of the button, then your mosfet is fine, any other results will indicate a bad or defective mosfet.

The cathode of the LED will go to the drain side or drain socket.

For p-channel mosfet you could simply modify the design as per the following image

mosfet tester jig 1

Reader Interactions

Comments

  1. Swagatam,
    I built your jig today and tested 30 MOSFETS; they are TO3 packaged and are for stereo amplifier outputs. 5 “P” channel and 2 “N” channel devices showed slight glowing of the LED before pressing the button. Pressing the button brought the LED to full brightness. I assume the subject MOSFETS are leaking and should be replaced. What say you? I am experiencing negative rail voltage on the right channel and have tested all the other devices on the board either by replacement or removal and replacement. Thank you, Jack

    Reply

    • Thank you Jack,
      It could be that the MOSFETs are leaking or maybe the LED is just too sensitive and might be detecting even a leakage in millivolts.
      You can try pressing the button without any MOSFET connected, if the LED does not glow then we can assume that the MOSFETs might be leaking slightly. You can also try the same with a BJT and verify the results.

      Reply

  2. Hello, I often see RF Power transistors such as the MRF454 referred to as a MOSFET, but the data sheet calls them a RF Line NPN Silicon Power Transistor.
    My question is can the MRF454 be tested in the same fashion?
    If yes, then I would guess that the test circuit you show should work as well.

    Reply

    • MRF454 is not a MOSFET, it is a BJT, so it cannot be tested using the methods explained above.
      It can be tested just like any other BJT is tested.

      Reply

  3. Hello dear.two irfp450 are same factory.and ones bipping in multimeter and other is 200 ohm range.are they fine?

    Reply

  5. Hello Swagatam,
    I am troubleshooting a DC-AC power inverter and suspect the MOSFET’s
    They are SSF7509 N-type
    Have taken out and tested both.
    1. Shorted G-S
    2. Meter in Diode mode, Red probe-D, Black-S = Nonconductive
    3. Red-G, Black-S 4.Red-D, Black-S = Low Reading, but diminishes to nonconductive
    Is this Normal?
    From what I’ve found and understand is that once charged, D-S will be conductive till G is reset. Is this correct?
    If this correct does this mean these MOSFET’s are bad?
    Thank you much,
    Ken

    Reply

    • Hello Ken,
      As explained in the article, initially the drain and the gate must be shorted together.
      If you are not getting results exactly as suggested in the above article, then the MOSFET could be faulty.
      Instead of checking the MOSFET with a meter I would recommend using the MOSFET tester circuit as given at the bottom of the article. This Jig circuit will be able to confirm with 100% accuracy whether your MOSFETs are good or bad.

      Reply

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