Checking component's that have
two leads such as the resistor, capacitor, diode and etc are much easier
than checking transistor and fet which have three leads. Quite often
technicians get confused with the three leg devices. In today's article i
will share with you on how to accurately check field effect transistor (fet)
using analog multimeter. First, identify the gate, drain and source pin from
semiconductor data book. Once you have locate each pin of the
fet, then use your analog meter set to times 10K ohm range. If you are
measuring an n-channel fet then place the black probe to the drain pin. Then
touch the gate pin with the red probe to discharge any internal capacitance
in the fet. Now move the red probe to source pin while the black probe still
touching the drain pin. Use your finger and touch the gate and drain pin
together and you will see the analog meter needle will kick forward to
middle range of the meter's scale. Removing the red probe from the source
pin and touching it back again the source pin the needle will still remain
at the center of the meter's scale. To discharge it you have to remove the
red probe and touch one
time on the gate pin. This will discharge the internal capacitance again.
Now using the red probe to touch on the source pin again, the needle would
not move at all because you already discharge it by touching the gate pin.
I know is a little bit of confusion but after some practice you will be able
to test all types of fet.
When measuring the fet and you observed all the readings moved toward the 0
ohms range then the fet is considered shorted and need to be replace.
Checking the P-channel is similar to checking an N-channel fet, just switch
the probe polarity when measuring the P-channel. If you having an analog
multimeter with a times 100k Ohm range, then you might not be able to
accurately check the fet due to the missing of 9 Volt battery in the meter.
The missing of 9 volt battery will have not enough power to trigger the fet.
Make sure your meter have the time 10k ohm range. Typical N-channel fet part
numbers are K792, K1118, IRF630, IRF 840. P-channel fet part number are
J306, J512, IRF9610 and etc.