Testing Mosfets using a DMM

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rkosiorek
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Testing Mosfets using a DMM

Post by rkosiorek » Jul 05, 2010 1:30 am

the only way to confirm that a MOSFET is blown it to test it out of circuit. unfortunatetly being in circuit influences the readings very much.

the tests are very simple. and can be performed with any meter that has a "diode" test setting. the "diode" setting injects of voltage of between 2.5 and 3VDC into the circuit and is current limited to something between 5mA and 20mA. the RED lead vill be the (+) and the black will be (0V). the value displayed will be the voltage across the leads in mV. when testing diodes one direction will show and "open circuit" and the other would show the forward voltage drop across the diode.

FETs or MOSFETs have two basic states. they are either off or they are on. and they come in two different polarities, P-Channel or N-Channel. since the MOSFETs in controllers are almost exclusively N-Channel the balance of this discussion will apply to them.

to turn a MOSFET "on" a charge is applied to the GATE pin. the GATE is reacts much like a very good capacitor. once charged it will hold that charge for quite a while keeping the MOSFET "on". the MOSFET will remain "on" until the charge is bled off by some external means. and this is one of the problems of testing the MOSFET in circuit. if the driver circuit is constantly keeping the gate either charged or discharged the MOSFET will remain fixed either "on" or "off" rendering continuity measurements meanigless.

so after removing the MOSFET from the circuit you can start testing it. first set the meter to the "diode" range. then lay the MOSFET down on the table. once the test is started make sure that the MOSFET leads do not touch anything other than the meter leads. do not touch either the meter probes or the fet leads with your fingers. touching the leads may affect the gate charge of the device and affect the readings.

TEST #1 Turn MOSFET "off" and Test Gate-Souce Function
Touch the Gate with the BLACK (-) lead and the Source with the RED (+) lead. the reading should be an open circuit. any other reading shows that there is a short between the Gate and the Source. if it is shorted no further testing is needed - replace the device. this test will also turn off the device

TEST #2 Test Drain-Souce Junction.
being turned 'off' the Drain-Source should be an open circuit. test by probing with the BLACK (-) lead on the Source and the RED (+) lead on the Drain. the expected reading is an open circuit. most MOSFETS have a reverse protection diode built in. to test this diode, reverse the polarity of the leads placing the RED (+) on the Source and the BLACK (-) on the Drain. the meter should read the forward voltage of the diode. a reading of 250 to 500mV is normal. if the reading is "open Circuit" check the device specifications. if it does not include the protection diode this would be normal. if it is supposed to have one than the diode has blown "open". if the readings show the device is shorted or the diode is blown open - replace the device.

TEST #3 Switch MOSFET "on"
probe the Gate with the RED (+) and the Source with the BLACK (-) leads. the expected reading is "open circuit". this double checks the Gate for short circuits. if shorted - discard the device.

TEST #4 Check Drain-Source junction for proper conduction.
make shure that you have not touched the Gate lead since you turned the device "on". probe with the RED (+) on the Drain and the BLACK (-) on the Source. the expected reading is a short circuit. then probe with the polarity reversed with the RED (+) on the Source and the BLACK (-) on the Drain. this should also show a short circuit. if either of these shows an open circuit than the Drain-Source is blown open and the device needs to be replaced.

on most devices, once the Gate is charged, it will remain charges for several minutes. long enough to do the tests. you just have to be careful not to disturb the gate lead and accidentally discharge it.

if the device passes all 4 tests it is likely a good device.

rick
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