MOSFET means 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. 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.
TYPE OF MOSFET SYMBOL
MOSFET – Field Effect Transistors are difficult to test with a multimeter, but when a more power MOSFET blows, it is completely damaged. All the leads will show a short circuit. 90% of bad MOSFETs will have GS, GD and DS shorted.
HOW TO TEST MOSFET WITH YOUR OWN TEST CIRCUIT.
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.
BUILD YOUR OWN TEST CIRCUIT FOR MOSFET TESTING:
1. Touching with finger ( Test Point) 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.
Large devices such as the TO-220 types shown above do not like static electricity on the gate and you have to be careful not to spike the gate with any static. Generally this type of device is not super sensitive and you can use your finger or use large value resistor.
When replacing one of these devices, there are 2 things to match-up.
Voltage and Current.
In most cases, the “turn-ON” resistance (the resistance between Source and Drain) will be the same and the speed of operation will be ok. Check the voltage needed to turn the gate ON and make sure you can supply the required voltage.