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 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.
For PMOS, follow the setup below.
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 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.
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 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
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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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
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
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.
Nice .
Thanks for explanation of mosfet.
Dear sir
same quary as how to first if it Noms or Pmos and value of Diod request to reply
[email protected] again many Thanks
Which tool do you use to draw the picture and the table？
Itis very beautiful.
just microsoft power point and excel…
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.
Can we use this procedure when MOSFET is on board??
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.
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?
Hello dear, thanks for this amazing knowledgeable article.
Thank you.
When you test a mosfet, you get only one value unlike transistors where you get two values does that mean mosfet is still good.
Ivan James
[email protected]
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)
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.
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.
You also need to check for the touch increasing resistance, because the diode can be not shorted but if the touch resistance increases when you touch it with the tip of nail or finger and stays whn you stop touching the thing is ok, but if you touch and the thing is increasing and increasing and never stop even when you stop the is faulty too!, check videos of mosfets with mini lamps or leds… You can do a simple test with the negative (I believe) in the center pin and you (multimeter in diode) touch the left pin (n mosfets) and then whit same positve multimeter touch the right and stay there, and with one nail or finger touch the left pin (fast) you will notice how in the multimeter the reading jumps a little bit, and stays, and touch again and jump a little bit and stays, and if you fully touch the resistance go to max etc. if the thing does not stay at same number (droping voltages) well that mosfet will not work.