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 checked using an ordinary digital multimeter through slightly different procedures.
STEPS for type K1058 :
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 piece of insulated wire stripped at the ends, short the extreme left lead and the extreme right leads of the mosfet. This will initially keep the internal capacitance of the device completely discharged.
4) Now Touch the meter black probe to center lead and the red probe to right leads of the device.
5)You should see an "open" circuit indication on the meter.
6) Now keeping the black probe touched to the center lead, lift the red probe from the right lead and touch it to the left lead of the circuit momentarily and bring it back to the right lead of the mosfet.
7) This time the meter will show a short circuit.
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 you will again need to short the gate and the source leads using a piece of wire as explained earlier.
STEPS for type IRFP240:
Follow the procedure exactly as given in the steps 1,2,3,4 and 5.
Next remove the red and the black probes from its positions and touch the red to the left pin and the black to the center pin of the mosfet.
After a few seconds, put the red probe and the black probe back to the center and the right pin of the mosfet respectively.
This time the meter should show a short circuit, proving that the mosfet is in a good condition.