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Ruining mosfets with a transistor tester and tweezers
VSV_electron:
Hello!
I've never had any experience with MOSFETs before so I bought myself two kinds of mosfets (or "MOSFETS" if you like) on AliExpress.
I know, I know - those are not the real thing and they are not up to specs (if working at all) but the majority of those who are aware of what they buy on Ali say: "These are valid devices with the 1/2nd or 1/3rd of the Drain current rating". Others, expecting too much cry in despair in their 1-star reviews: "Fakes-fakes-fakes!".
Sorry for the lengthy intro, I'll try to be as concise as possible (not my thing really...).
- I bought a bunch of 2n7000 signal E-MOSFETs (50 pcs).
- I bought 10 IRLZ44N "power" E-MOSFETs (parenthesis intentional)
I as a always do: put on my antistatic wrist strap (checked and working at 1 MOhm), got my 2 antistatic bags of 2n7000 and IRLZ44N out (transistors never come in ESD-safe bags from Ali but I always repack them hoping for the best case). Got my simple and common GM328A transistor tester out and ready.
Note: Before starting the test procedure I read a few instructions on the Web on careful handling of the MOSFETs. Well, one thing is to read something and another is to get it all right and not to slip somewhere.
- I started testing the mosfets from the less expensive 2n7000. The tester identified them correctly as an E-MOSFET and a half a dozen randomly picked out of the bag were all fine.
What I could do WRONG, please comment: after the tester finished the identification of each 2n7000, I removed them from the ZIF socket and with the tweezers shorted all the three leads together (just to make sure the Gate is discharged). I actually did that for a few more times in a row to one 2n7000 and after 3 or 4 times (tester + tweezers) it was still identified as the valid E-MOSFET.
- After that I moved on to IRLZ44N and then there was frustration waiting for me:
Once I inserted the IRLZ44N into the tester ZIF socket it was identified as an E-MOSFET. I waited until the tester LCD dimmed (at that point there is no testing voltage being applied to the DUT; I checked that with a MM). I pulled out the IRLZ44N, did the same thing as I did to the 2n7000 - shorted all the 3 leads with the tweezers to discharge the gate. (Note: My ESD wrist strap is on me all the time...).
On the second insertion of the IRLZ44N into the tester it was already a dud. I didn't believe it so I did all the same thing with the second IRLZ44N... dud again!
I went on to the 3rd IRLZ44N and... it was already a dud out of the box.
So I got out the 4th one and tested it in the tester and it was fine, valid E-MOSFET. This time, however I decided to stay on the safe side and didn't short the leads of the 4th E-MOSFET. I waited for the tester to finish its procedure and got the E-MOSFET out
On the second insertion into the tester ZIF socket... it was already a dud. I didn't do anything special to it that time.
After wasting 3 of the counterfeits (not counting the 1 out of the box dud) with the simple tester and tweezers manipulation I stopped. I still had 6 from the batch untouched so I feared to ruin what I still got left. I got my UNI-T UT61E+ MM set on 'Resistance' and checked the D-S resistance of the destroyed IRLZ44Ns first. The Red lead on the Drain and black on the Source gave me around +/- 1 kOhm for each of the duds; the same with reverse.
I did that the same to the remaining potentially still good 6 IRLZ44N. All of them showed around 3.5 to 3.9 MOhm Drain to Source (Red-Black) and the '0' reading for reverse. So they all went back into the bag this time leaving me in the a little upset condition. I decided to stop there completely for today.
So - is that explainable that I did the same thing to the 2n7000 and IRLZ44N and the former was fine after several insertions into the tester ZIF with the subsequent shorting of the leads and they survived all the mild "abuse" and the latter were duds for no apparent reason ? What did I do wrong?
thm_w:
Shorting all the leads together with tweezers is not necessary. But I don't see that damaging any common FETs like we have here. And if anything IRLZ44 should be more robust than the smaller part, which is a bit suspicious. Reading 1k Drain to Source with the gate discharged is definitely bad.
See if you can get a hold of some genuine power FETs, maybe from salvaged electronics boards.
Maybe also measure the voltage your GM328 tester is applying to the part with a multimeter, and see when the GM328 is done if it leaves any voltage on the part or not. I assume it should discharged all outputs to near 0V.
Your wrist strap is connected back to mains earth? What power source is supplying the GM328?
BTW if you want cheap genuine parts, can just buy from LCSC.
VSV_electron:
--- Quote from: thm_w on April 15, 2024, 09:47:29 pm ---...
Maybe also measure the voltage your GM328 tester is applying to the part with a multimeter, and see when the GM328 is done if it leaves any voltage on the part or not. I assume it should discharged all outputs to near 0V.
Your wrist strap is connected back to mains earth? What power source is supplying the GM328?
...
--- End quote ---
To be honest I hesitated to buy any "power" stuff in the TO220 package off Ali for a long time. I knew Chinese signal BJTs of large variety and 2n7000 (but not JFETs) were fine according to the reviews but reviews of their power BJTs and MOSFETs were always doubtful.
I would have to connect a scope to the GM328 to see what it supplies to the DUT but with the MM it shows cycling through some voltage ranges from 0V up to maybe 5V which is expected according to the tester's algorithms.
My wrist strap is connected to the mains earth.
GM328 is powered by a wall adapter rated at 12V. GM328 confirms that the "battery 12V is OK" and something like "5.05V is OK" - I think that's about the regulated voltage to the Atmega328P.
I went back to Ali and found a popular listing with the cloned IRFZ44N (should be close to my logic level IRLZ44N). Except for 1 undelivered review the listing is almost 100% 5-star reviews with people reporting good tests on all received units.
https://aliexpress.com/item/1005003080494688.html
That was in line with my understanding of what to expect: don't drive them too hard and they should be OK for hobby projects. My situation frustrates me as I blame myself for destroying potentially fine units. Well, that's actually stupid and I probably just need to refine my approach but you see how desperately ESD-safe I am?
I never handle any stuff I receive off Ali directly without a wrist strap; Never handle any chips by their leads, etc.
DimitriP:
--- Quote ---What did I do wrong?
--- End quote ---
--- Quote ---so I bought myself two kinds of mosfets (or "MOSFETS" if you like) on AliExpress.
--- End quote ---
--- Quote --- know, I know - those are not the real thing and they are not up to specs (if working at all)
--- End quote ---
I rearranged it a bit so it's easier to follow :)
VSV_electron:
--- Quote from: DimitriP on April 16, 2024, 01:19:01 am ---...
I rearranged it a bit so it's easier to follow :)
--- End quote ---
Oh, please, I understand your humor but it's inappropriate here. Be helpful, not sarcastic.
I found the listing of the shop I bought my IRLZ44Ns - people were happy with what they bought for their needs. Their MOSFETs are not exactly up to specs but useable and they successfully tested them with their testers.
I'm trying to figure out where I went wrong with my handling and/or testing of the units.
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