I've been working on getting a 3D-printer interface board (a
RAMPS-FD v2.2, if it matters) up and running...finally got the bed-heater circuit working, which had been giving me trouble for some reason, and verified the rest of the board works. (I ended up desoldering and resoldering everything in that part of the board. Maybe I had a bad joint previously, though I also ended up replacing a possibly-bad LED.)
While debugging this part of the board, I needed to see if an IRLB8743 MOSFET was any good, or if I had somehow killed it. First thing I ran across was variations of this:
https://www.homemade-circuits.com/how-to-check-mosfet-using-digital/Basically, you use a multimeter in its diode-test mode: red to drain, black to source. Shorting gate to drain should switch it off, while touching the red probe to the gate and back to the drain should switch it on, and these conditions should be readable on the meter...but mine did nothing. Fail...or was it?
I then ran across this test circuit:
https://electronics.stackexchange.com/questions/319946/should-i-immediately-return-mosfets-shipped-in-non-esd-protective-packaging/319948#319948Just a couple of resistors, a switch, an LED, and a power supply...had it knocked together in no time, and this time the LED was blinking off when I pushed the button and turning back on when the button was released, just as described. It looked like maybe it was working after all, so I started resoldering things one component at a time, checking as I went. Once I remembered to reattach the board to the power supply (doh!), it started working like it should. Awesomesauce.
So...that first test of the FET with a meter. Should that have worked, or is there something weird about this type in particular that means that test won't work? The IRLB8743 is a rather beefy device in a TO-220 package that can switch up to 30 amps at up to 32 volts. I was using the diode-test function on an Extech MN26T; the manual says this about the diode-test function: "Test current of 0.3mA maximum, open circuit voltage 1.5V DC typical." Looking at the IRLB8743 datasheet, the gate-threshold voltage is quoted as 1.8V (typical). It's now looking like I'm answering my own question, but just to make sure I'm reading things right, is it the case that the multimeter test for MOSFET functionality isn't going to work for this particular type of MOSFET?