How to Testing Field Effect Transistor (FET)  


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 thefet, 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.


testing fet

Author By : Jestine Yong




Repairing Articles
Free Repairing Tips
Download Service Manual
TFT LCD Display Equivalents
Repairing Bookstore
Other Resources
Contact Us
Free Subscribe Now
Site Map

LCD TV Repair

Plasma TV Repair

Projection Tv Repair

Learn Basic Electronic

Computer Repair Course

Learn Laptop Repair

CRT Tv Repair Course

LCD Monitor Repair ebook

How to testing Electronic Components like a Pro!

Power Supply Repair Guide- all types of SMPS


1 Feb 2007

Many thanks!  I have
downloaded the file and
will - hopefully - now be
able to fix the screen.
Like your website -
very helpful...
thanks again.
- Mike

31 Dec 2006

thank you for this
great site that i
found what i
search about.

15 Jan 2008

Thank you very
much for your
excellent site!
I know there would
be lots of people
around the world
who will appreciate
your very neat,
well-organised site.
-Matt Straford

DONATE Help the P2U

If you find my free stuff useful, why not leave me a little something with PayPal.

Donations are greatly appreciated, and help with the costs of providing FREE service manual & server fees.