I received the 4 wire milliohm meter today. There was no doubt in my mind that it was going to outperform the 2 wire meter. I zeroed the meter and then measured a random precision shunt and it was pretty close to dead on. That was exactly what I expected.
People think I was stupid for buying the 2 wire meter. They completely missed the reason why I bought it. So anyway...
I didn't buy the 2 wire meter expecting it to have the precision of a 4 wire meter. I was never under that expectation. I just wanted to see if it was "good enough". Why buy a $100 meter when a $40 one can do what you need? Not everyone needs to measure down to 1 or 2 milli ohms. Lots of folks don't even need to measure down to 1 ohm. For people needing better than 1 ohm and down to .03 ohms, well the $40 MESR-100 with the factory test leads will do just fine for them. Once I have the Kelvin clips that will make it a bit better than it is now. I think .01 ohms will be doable reliably. The crappy alligator clips it came with are not exactly spectacular. So depending on what you are looking for, It may very well be "good enough". It just depends on what you need. It definitely measures resistance in or out of a capacitor so that's proof that while it may have been built for use on capacitors, it also serves other low ohm uses equally well. With the factory alligator clips, I wouldn't rely on it for measurements lower than about .03 ohms. We will see about using Kelvin clips. I'm sure they will help in getting better results.
So then...like I've said all along...the results will be the results and here you go! I used the same shunt on both meters. Of course the 4 wire meter was close to dead on, that was never in doubt or anything I ever questioned.
I will label 10 AOT290's 1 to 10 for this test. I'll arbitrarily pick 10 mosfets from my "stock".
1. The first test will be to bias the AOT290's to 10 volts on their gates and measure Rds with both the 2 wire and 4 wire meters and the newest component tester which measures Rds.
2. For the second test, I'll measure a resistor to put in series with source to drain and measure voltage drop across the mosfet and the resistor. The gate will still be biased at 10 volts. I'll use the same 10 volts for measuring Rds. The current through the resistor will be the same current flowing through the mosfet. Once I know the voltage drop across the resistor, V=IR will tell me the current through the resistor. That same current and the voltage drop across the mosfet will tell me its Rds. This will make a nice double check of what the 3 meters directly measure for Rds.
3. The final test will use the same test set-up as number 2, but the gate voltage will be increased slowly. According to the AOT290 spec sheet, the mosfet gate threshold voltage is supposedly a minimum of 2.9 volts and a maximum of 4.1 volts and typically at 3.5 volts. This is the value that I have seen to be widely different from the spec sheet on my component testers. I have not been able to test Rds previously for obvious reasons, but I will use the 4 wire meter to measure Rds at the minimum, typical and maximum threshold voltages. I will also put a mosfet in a couple of component testers to see what they measure for the threshold voltage. This test will be to prove that what my component testers measure is in fact accurate or not for this spec.
That ought to settle any questions about AOT290 mosfet consistency or lack there of and whether an inexpensive component tester is a contemptible POS like some folks think or a worth while and inexpensive test tool. The results will be the results...as always!
Since I have 25 brand new, never been pulled from the antistatic packaging IXYS IXFP180N10T2 mosfets, I'll do the same tests on them. We will see what happens! I'm also thinking of doing the exact same thing on some TI mosfets, but I would need to buy them first. I have some IRF4110's that I bought on ebay a couple of years ago and a bunch that came from digikey I think. I've kept them separate the entire time I've had them. It might be interesting to see if my ebay mosfets are as good as the digikey ones. With all of these tests, I'm looking for consistent results between the various test methods and the test tools and that the mosfets really work according to specs.