Protection for fan output shorts
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I understand that a minor revision is planned for the Duet2 series. If it hasn't been asked for yet, I would like to request some sort of protection on the fan outputs to prevent idiots like me from blowing the output mosfet's.
A related question - is the Duet3 more idiot proof? If so that would be a big bonus for me. -
@jens55 you can put a flyback diode in yoru device wiring for just that purpose.
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@sinned6915, it's about shorting the output by accident and not about inductive voltage spikes.
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@jens55 Maybe one fuse per fan output?
That would take board space, but would protect the FETs.
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@alankilian, IIRC, the issue is that the outputs are already fused but the mosfet's die quicker than the fuse can react (in the case of a short). It makes neat little craters in the mosfet
I can't point to this being specifically mentioned but I am pretty certain that @dc42 did mention this at one time. -
@jens55 The single fuse needs to be rated for the total current of ALL fans and so a single FET can blow if one fan draws excessive current.
These kind of things to protect from accidental miswiring can cost quite a bit of money and 99 and 44/100th % of the people will never need it, so building in all that cost when very few people will miswire a fan is a cost to everyone.
It's completely possible to build a bulletproof board, but it's going to cost something like US$500 for a board.
I believe most people want a less expensive board, and will spend the time to use care not to damage it.
Just my opinion.
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@alankilian, while I agree with your opinion in general, I think that the fan output pins are somewhat different. Many people have managed to blow those mosfets. It isn't just miswiring them that is a problem but accidental shortening.
Anyway, I wanted to add my vote for an upgrade if that is considered financially viable and if there is enough board space. It was never my intention to debate the pros and cons of updating the circuit.
It is now in the hands of more talented people than me and I will leave it at that. -
@jens55 I've wired up a few fans.
How do you manage to short them while wiring them up?
Do you wire while the power is on or something?
Because that's 100% not the right way to handle a board like this.- Wire up your fans.
- Check your wiring.
- Turn on the power.
I don't see a way to short the fan outputs unless you are being careless and wiring a live board with the power on.
What did you do to damage this board?
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@alankilian, I have already admitted to being an idiot .... I really don't think that elaborating on that would be a good thing to do.
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@jens55 said in Protection for fan output shorts:
I really don't think that elaborating on that would be a good thing to do.
I need to keep that in mind when talking to my wife.
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LOL ....
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@jens55 said in Protection for fan output shorts:
I understand that a minor revision is planned for the Duet2 series. If it hasn't been asked for yet, I would like to request some sort of protection on the fan outputs to prevent idiots like me from blowing the output mosfet's.
We looked into providing fan output short-circuit protection some time ago. Unfortunately, thermal fuses are too slow to provide protection. Electronic fuses for 12V circuits are readily available, but not for 24V circuits.
A related question - is the Duet3 more idiot proof? If so that would be a big bonus for me.
Yes it's more idiot-proof, for example against shorting the external +3.3V or +5V rails to a higher voltage. The fan mosfets have a higher surge current rating; however they will still fail if you short a 24V fan output. They may survive shorting a 12V fan output, but I haven't tested it.
