Why are Some of My Mosfets Getting Super Hot?

[Mad Scientist]

Oddly enough my battery holders came individually packaged in static resistant bags(keep in mind static electricity wouldn't do anything to a battery holder) but the mosfets came in plain old plastic strips 0_o

After building a test circuit operating at 4v with a .6ohm resistance I tested 10 of the remaining mosfets in question. After testing them only 2 of the 10 tested were working properly or at what I would consider reasonable as far as heat generated vs time engaged, though two others worked without getting ridiculously hot extremely fast, but they still got significantly hotter than the other 2 in a much shorter amount of time. Next time I have a bit of time I will try and up the voltage and see if it is simply that these knock offs or whatever they are need a higher working voltage to turn on properly. I suppose we'll see, but either way I'd keep away from the mosfets with no little black spot on the back.

Measure the voltage drop across the source to drain when firing to compare them. You can then calculate the on resistance of the MOSFET and how much power it is actually dissipating using Ohm's law and the power equation. E = I R. P = E I.

Compare the measured on resistance to the specs, and compare dissipated power to what it's rated for. You should be able to weed out the bad ones in a jiffy.

 

[yo han]

Not knowing the regular price for 3034 FETs I bought 5 for $5 from a Chinese eBay seller a while ago.
I'm almost certain these are fakes but they're working like they should and don't even get luke warm with the same box mod setup you're using.
So probably some of the fakes perform well enough?

 

[K3nn0Q]

Regular price on the standard 3034 (the TO-220 package) is roughly 3.50USD, depending on qty and distributor. There are others that can be used that vary in price by a few dollars

 

[MonkeyTokes]

Not knowing the regular price for 3034 FETs I bought 5 for $5 from a Chinese eBay seller a while ago.
I'm almost certain these are fakes but they're working like they should and don't even get luke warm with the same box mod setup you're using.
So probably some of the fakes perform well enough?

I know one seller from china on eBay has genuine mosfets for about a dollar a piece - $8.75 for 10 plus $2 for shipping. I only paid about $2.20 including shipping from both vendors I purchased from. Some places are $5+ for one... Which is ridiculous, though that's what people are charging because they've become popular and people will pay. Which is why I'm browsing datasheets to find a different mosfet to use, perhaps an SMD.

If the logo on the mosfets you bought is the transistor symbol, not a circle with the transistor symbol missing from it, then my guess would be they are fake. They could be fake and just good fakes, I'm not sure. Also the date code on some of the mosfets I bought was in the future from the time I bought them, so that combined with everything else kind of led me to believe they're fake. You can check the date code key in the datasheet.

 

[MonkeyTokes]

Measure the voltage drop across the source to drain when firing to compare them. You can then calculate the on resistance of the MOSFET and how much power it is actually dissipating using Ohm's law and the power equation. E = I R. P = E I.

Compare the measured on resistance to the specs, and compare dissipated power to what it's rated for. You should be able to weed out the bad ones in a jiffy.

Thanks for the tip! I wasnt exactly sure how to test a mosfet aside from testing to see if it is completely dead or still functioning. I gave all but 5 to a friend that wanted to mess around with them, but I will try it out on the ones I've got.

Say the numbers come back too low or too high, is there a way to correct it? Higher or lower resistance across gate and source, adjusted voltage, etc?

 

[Mad Scientist]

Thanks for the tip! I wasnt exactly sure how to test a mosfet aside from testing to see if it is completely dead or still functioning. I gave all but 5 to a friend that wanted to mess around with them, but I will try it out on the ones I've got.

Say the numbers come back too low or too high, is there a way to correct it? Higher or lower resistance across gate and source, adjusted voltage, etc?

If your gate drive voltage is within spec then the MOSFET must have a RDSon at least as low as the specification. If it doesn't, it's toast

 

[yo han]

If the logo on the mosfets you bought is the transistor symbol, not a circle with the transistor symbol missing from it, then my guess would be they are fake. They could be fake and just good fakes, I'm not sure. Also the date code on some of the mosfets I bought was in the future from the time I bought them, so that combined with everything else kind of led me to believe they're fake. You can check the date code key in the datasheet.

These are the ones I got: http://r.ebay.com/wxPDNA
Comparing it with your description it should be a fake. But like I said, they perform well for a $1 (including shipping) FET

 

[MonkeyTokes]

Haven't gotten around to testing their voltage drop yet because I need a new battery for my multimeter, but just for kicks I wired two batteries in parallel (rather than series like i always use) and the 3 singles I had didn't get nearly as hot and two that I soldered in parallel don't get got at all though the one does when connected to 4.2v... Not really sure what this means. But at least that means they may have some use somewhere down the line if their only problem is needing more than 4.2v to turn on completely.

 

[yo han]

I'm running mine with 12V
I came across this on Reddit:

Mosfet gate drive is absolutely critical, you want ~10 to 12V ideally on the gate for it to meet it's Rds(on) spec under high load. poor gate drives < 4.5V increases self heating and risk of damaging the die.