How to Know if MOSFET is Defective

Below are the guides on how to know if MOSFET is defective. These are the most common techniques that can be used to check if MOSFET is defective.

Step #1 on How to Know if MOSFET is Defective

:Diode Check

The first thing we will try on how to know if MOSFET is defective is to check the diode drop. Get a DMM and set it to diode mode. For NMOS, follow below setup.

How to Know if MOSFET is Defective

For PMOS, follow the setup below.

How to Know if MOSFET is Defective through diode check

A good MOSFET should have a reading of 0.4V to 0.9V (depends on the MOSFET type). If the reading is zero, the MOSFET is defective and when the reading is “open” or no reading, the MOSFET is also defective.

When you reverse the DMM probe connections, the reading should be “open” or no reading for a good MOSFET. If the reading is zero, the MOSFET is defective.

Step #2 on How to Know if MOSFET is Defective

:Resistance Check

The next method to use on how to know if MOSFET is defective is to conduct a resistance check. A good MOSFET should have high resistance between drain-source regardless of the DMM probe polarity.

The gate to source has also high resistance in either way for a good MOSFET. However, you should take note that when you put the positive of the DMM to the gate and the negative to the source of an NMOS, the MOSFET will turn on. You might mistakenly judge that the MOSFET is defective when you measure drain-to source resistance since the DMM will read 0 ohm. So, to avoid this scenario, ensure to discharge the charge on the gate.

The same to PMOS, when you put the positive terminal to the source and the negative terminal of the DMM to the gate, the MOSFET will turn on.

How to Know if MOSFET is Defective through resistance check

If you measure the resistance of the MOSFET terminal to terminal, you should consider the associated resistors for these affect the reading. For example in above circuit, when you measure the resistance across gate to source, what you read is not high resistance but the value of R1 which is 10k. Removing the 10k resistor will make the reading high.

Step #3 on How to Know if MOSFET is Defective

:Continuity Test

The third method on how to now if MOSFET is Defective is to do continuity test. Set the DMM to continuity mode. In modern DMM continuity mode usually have audible sound when the measured points are connected. Put the positive to drain and the negative of the DMM to source or vice versa, when the reading is zero or the DMM sound persists, the device is damage; shorted drain-source.

Use the same approach to the other leads of the MOSFET and use the same judgement. However, when you put the positive of the DMM to the gate and the negative to the source for NMOS or other way around for PMOS; the device will turn on and when you measure continuity between drain to source; the reading is zero. You might misjudge that the MOSFET is defective. So, ensure always to discharge the gate to source.

To verify, turn off the device and measure again the continuity. To turn off an NMOS, put the DMM positive terminal to source while the negative terminal to the gate. Do otherwise to turn off a PMOS. A good MOSFET should have no continuity between terminals. If it has, it is defective indeed.

Summary on How to Know if MOSFET is Defective

N-channel MOSFET Troubleshooting
P-channel MOSFET troubleshooting
MOSFET resistance and continuity readings
Figure 7

Above techniques are common on how to know if MOSFET is defective. I know there are few other techniques out there. So I suggest combine the above techniques to the other techniques you learned so that you can perform accurate troubleshooting. If you are interested to know how to troubleshoot diodes, read How to Know a Defective Diode. On the other hand, if you are interested to learn how to troubleshoot BJT, read How to Know if a Transistor Defective. If you want to go further about MOSFETs, read Power MOSFET Design Equations.

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17 comments

  1. Dear sir
    Whats the value of diode
    That has bn used in this
    Article ?

    N how to know
    Wether the mosfet is
    Nmos or pmos ?

    Thannnx

  2. Dear sir
    Whats the value of diode
    That has bn used in this
    Article ?

    N how to know
    Wether the mosfet is
    Nmos or pmos ?

    Thannnx in advance

    1. In the table above, follow how the DMM probe connection goes. If the reading match the NMOS, it is NMOS. On the other hand if the reading match the PMOS, it is PMOS.

  3. With a 3 phase 12v setup and neg prob on gate, pos prob on resistor I get a reading of
    .625. When I put pos lead to gate, and neg lead to resistor I get open line. This is the case with all three mosfets. Does this mean the mosfets are bad.

    1. I will not advise. You may get wrong reading. However, if there is no other MOSFET connected in parallel, you may still check the diode drop. For the gate resistance, you may also get the reading if there is no other circuit directly connected to the gate.

  4. The table’s estimates of HIGH and LOW are so vague t , a range of the HIGH or the LOW could help.. example, on a tested resistance of A (salvaged component) MOSFET TF 10N60, where does the (Drain-Source) 3.6 k ohm reading fall into? a high or a low?

  5. Dear Sir,

    Recently I have a motherboard failed, and I checked that all N-MOS (FDMS3660S) resistance between D/S is zero, is this quite common when a electronic device use for a long time? (After motherboard failed, the power supply still worked)

    1. How long?
      Depending on the design, but usual product MTBF is around 100,000 hours. So, if your product is running 24 hours per day and everyday, this will result to around 11 years before any of the parts start to get damage.

  6. Dear Sir,

    Thank you very much for your valuable article. I just want to know that is this the procedure doing with analog multi meter.

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