burned small mosfet and flyback diode

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Michał Iwanicki

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Feb 2, 2022, 2:19:42 AMFeb 2
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Hi,

I'm in the process of wiring my Pick&Place machine based on Smoothieboard. Everything was working fine until I started playing more with the solenoid valves that control the vacuum for pickup nozzles. They are these: https://www.robotdigg.com/product/566/High-frequency-Solenoid-Valve-12-or-24VDC rated for 24V.

I hooked them up, I hooked up the pump, everything to small mosfets, to utilize the flyback diodes that are already there. The pump started working, the valve nicely turned on and off. But then I left it on for a while (say 30 seconds). Next thing I smell something burning.

Turned out that both mosfet and the flyback diode burned, and I'm trying to find the cause. There doesn't seem to be any shorts anywhere, the valve still works fine, but both the mosfet and the flyback diode are destroyed.

Looking at the datasheet of the diode used, it says that it's actually rated for 10V for reverse voltage only, while all the docs say that 24V is fine for the mosfet input votage (someone even mentioned it here: https://groups.google.com/g/smoothieware-support/c/OOvX1sTbwXs/m/6vfdoA1YBAAJ but there was no follow up). Could this be a reason for this failure? Say the diode burned first, due to 24V>10V for the reverse voltage, the mosfet got excessive current and burned next? Did I miss something in the docs saying that?

thanks!

Michal

Arthur Wolf

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Feb 3, 2022, 7:25:05 AMFeb 3
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We have many examples of 24v selonoids being used with these with no issue.
Maybe it's just a component failure? These are very rare, but they do happen. Or something wrong with soldering, possibly.

Where did you get the board from?

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Arthur Wolf

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Feb 3, 2022, 7:26:03 AMFeb 3
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I didn't read well enough, it died while doing nothing, not during a change of state, this means this is not related to actual flyback work. Makes it more likely this is a component/soldering issue.

On Wed, Feb 2, 2022 at 8:19 AM Michał Iwanicki <harr...@gmail.com> wrote:
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Michał Iwanicki

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Feb 3, 2022, 11:57:21 AMFeb 3
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Yeah, it died when the mosfet was on, so there shouldn't have been even any current going through it.

I got the board on amazon, here: https://www.amazon.com/gp/product/B0786SVQ9Z - which, as I understand is the genuine one, right?

My thinking is that the diode just got warmer (do they get warmer when reversed biased? I wouldn't think so, there's barely any current going through them that way, so there shouldn't be any real power dissipation, but I'm not an expert. Maybe just a bad solder joint nearby, like you suggest?), the breakdown voltage got lower as a result of that, at some point it started conducting that massive current and burned both the mosfet downstream as well as itself. FWIW, another of the small mosfets had another valve hooked up (also 24V) and it was just fine, even though I played with it in similar manner. And the last small mosfet had a pump running through the entire time (also 24V) and it's fine too. Not sure how much it matters, but I noticed that the diode that burned came from a different batch than the remaining two. Maybe just slightly different characteristics of the breakdown behavior or maybe completely unrelated.




Arthur Wolf

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Feb 3, 2022, 12:07:24 PMFeb 3
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I'd expect this is a problem with manufacturing or component supply. It's an odd one I've never seen before.
You should contact the vendor, and they'll offer a solution: either a replacement board, or some external extension that provides the same functionality.
If they don't, email me privately and I'll sort you out.



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Michał Iwanicki

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Feb 3, 2022, 12:24:30 PMFeb 3
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> You should contact the vendor, and they'll offer a solution: either a replacement board, or some external extension that provides the same functionality.
> If they don't, email me privately and I'll sort you out.

Will do! Thanks for your help.

Anything aside (I'm new to the whole electronics thing, just trying to learn something): shouldn't these diodes be rated for a higher reverse voltage? Or are these voltages actually much higher in practice than the ratings and it doesn't really matter in reality?

Arthur Wolf

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Feb 3, 2022, 12:26:57 PMFeb 3
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I wasn't part of the group that added the diodes, but I know your concerned was addressed, and they are fine, thousands of users have used them with no issue (it's not official, but some even use them at 32v / with higher current than the proper rating)



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