Rev 3: Hot MOSFET & low Voltage

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11 months 1 week ago #10396 by rmcrae
I am reading 0v between Pin 15 and ground on the controller socket and 0v betweeen the HVD and GND.

So I guess I need to see how lucky I am, huh?

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11 months 1 week ago #10397 by Ty_Eeberfest
Yeah I'm afraid so. The MOSFET is conducting even with no gate drive, which is not how it's supposed to work.

One last test before trying your luck with the de-soldering: with the ATMega still removed form its socket, stick a temporary jumper from HVD to ground and see if by any chance your VIN and VCC come up to normal levels and the MOSFET cools off. Grounding the gate will get rid of any residual gate charge that nagically came out of the ether.

I have no idea how much de-soldering you've done in your life but I warn you that these plated-through holes are tricky. Go as easy as you can with the heat and if you have a solder sucker use it.

Look into it later when the dust is clearing off the crater.

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11 months 1 week ago #10398 by rmcrae
Still no luck. Guess I'm going to be buying a desoldering pump and some desoldering wick later today. I've had to desolder a thing or two off this board already--not fun, but I know I can do it. Hopefully doing it will bear some fruits.

Thanks for all the help so far. Will keep you updated.

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11 months 1 week ago #10399 by Ty_Eeberfest
Well there is one other option... cut the MOSFET's pins off flush with the board. Deal with getting the stubs de-soldered and removed later. Or for that matter you could cut the pins a little bit above the board, never remove them, and tack a new MOSFET onto them later (or the old MOSFET in the event it's not actually bad).

Look into it later when the dust is clearing off the crater.

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11 months 4 days ago #10400 by rmcrae
Just pulled the MOSFET. Here are the results

From GND to VIN it is reading 12.5v.

From GND to VVC it is reading 4.9v.

From GND to 170v it is reading 11.8v

Guessing this means to MOSFET was bad, right?

Also to answer your question about the stuff on the back of the board (sorry just saw that) they were just cat hairs.

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11 months 4 days ago - 11 months 4 days ago #10401 by Torsten Lang

rmcrae wrote: Guessing this means to MOSFET was bad, right?

I would say so. As most MOSFETs are sensitive to static electricity - which of the following countermeasures do you use at your working place? Antistatic floor, antistatic shoes (or no shoes at all), antistatic mat on the table, anti-static wrist strap? And what about your soldering station?

I don't know which type of MOSFET was used here, but if it is an IRF840 or similar it has no gate protection diode and so can easily be destroyed by wrong handling. Once you have charged the gate more than 20V above or below the source potential by an electrostatic event it will break through irreversible...

Regards,
Torsten
Last edit: 11 months 4 days ago by Torsten Lang.

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