TESTING MOSFETs and FETs

MOSFETs and JFETs are all part of the FET family.
MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.
FETs operate exactly the same as a "normal" transistor except they have different
names for the input and output leads and the voltage between the gate and the source
has to between 2v to 5v for the device to turn on fully. A FET requires almost NO
CURRENT into the Gate for it to turn on and when it does, the voltage between drain
and source is very low (only a few mV). This allows them to pass very high currents
without getting hot. There is a point where they start to turn on and the input voltage
must rise higher than this so the FET turns on FULLY and does not get hot.
Field Effect Transistors are difficult to test with a multimeter, but "fortunately" when
a power MOSFET blows, it is completely damaged. All the leads will show a short
circuit. 99% of bad MOSFETs will have GS, GD and DS shorted.
The following symbols show some of the different types of MOSFETs:

Most MOSFET transistors cannot be tested with a multimeter. This due to the fact that

the Gate needs 2v - 5v to turn on the device and this voltage is not present on the

probes of either meter set to any of the ohms ranges.

You need to build the following Test Circuit:

Touching the Gate will increase the voltage on the Gate and the MOSFET will turn on

and illuminate the LED. Removing your finger will turn the LED off.