Over on YouTube our hacker [VIP Love Secretary] shows us how to make a simple MOSFET tester.
This is a really neat, useful, elegant, and simple hack, but the video is kind of terrible. We found that the voice-over constantly saying “right?” and “look!” seriously drove us to distraction. But this is a circuit which you should know about so maybe do what we did and watch the video with subtitles on and audio off.
To use this circuit you install the MOSFET you want to test and then press with your finger the spare leg of each of two diodes; in the final build there are some metal touch pads attached to the diodes to facilitate this. One diode will turn the MOSFET off, the other diode will turn the MOSFET on, and the LED will show you which is which.
Apparently this works through stray capacitance, an explanation which makes sense to us. We were so curious that we ran over to the bench to build our own version (pictured with the schematic above) just to see if it worked as advertised, and: it did!
We tested it with a faulty MOSFET and when the MOSFET under test is faulty then the LED won’t turn on and off like it should when the MOSFET works. Also, if you build one of these, you want to feed in a two or three volt supply (it will depend on the specs of the LED you use); it’s not mentioned in the video but two volts is what we used that worked best for us.
Thanks to [Danjovic] for writing in to let us know about this one. If you’re interested in MOSFETs maybe it’s time to learn the truth about them.
Nice video. A better explanation is that the stray capacitance of the high-impedance gate should retain whatever charge is imparted upon it to keep the LED turned on or off. The human acts as an antenna for lots of stray EMF and the diodes are acting as peak detectors for the antenna voltage. Touching the cathode drags the gate down to the peak negative voltage (minus the diode drop), while touching the anode raises the gate up to the peak positive voltage (again, minus the diode drop).
Be cautious using this as ESD from touching the diodes could destroy your MOSFET. If you add a ~10V zener diode from gate to source, it’ll protect it without affecting the tester functionality.
Agree, but a typical zener will do little to protect against ESD; and many MOSFET gates can blown to smitthereens by HBM ESD.