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bobc
I have updated the schematic and layout with some thermistor changes and other things Cefiar mentioned. I have routed a separate ground from the thermistor header to two of the ground pins on the Due header.
I wondered if we should be using AREF? It might be a cleaner reference for the thermistors, and I think it would be quite easy to route.
As long as we can pull the appropriate amount of current from it, and it'll be stable, I guess it makes sense. I'll have a proper look and leave a better opinion soon.
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bobc
I've also been looking at the D102 crowbar diode. It's supposed to protect against reverse polarity by blowing the fuse. But the 1N4004 only has a continuous current rating of 1A, and a peak current of 30A for 8.3 ms. That does not seem nearly long enough to blow a fuse, which might take 0.5s.
So for that to even work, we would need to change it to something nearer 10A rating. Call that Option A. A possible part is : [uk.rs-online.com]
However I'm still not sure that provides much protection, since -12V is still applied for maybe 100-500 ms. It would seem to make more sense to use a blocking diode in series, which would need to be rated 10A+, there are some available at 15A in TO220 packages. (Option B ) We might be able to get by without a heatsink on it, there is also the voltage drop to consider.
I am fairly sure the 1N4004 is a waste of space though. I think we should at least change it to Option A. Thoughts anyone?
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uncle_bob
I'm not a big fan of series diodes. You paid good money for that power and now you are just throwing it away :).
This is the reverse-biased diode across the supply (ie: get the supply into the board backwards, it forward conducts so the fuse will blow).
Usually what happens is that the diode here fails as part of it's job of protection. I've seen a number of posts recently (eg: about Rumba) where people have connected the PSU backwards and the crowbar diode dies, going short circuit. This protects the rest of the electronics at the cost of a diode and some soldering. Unfortunately I don't know if the failure mode for large currents is ALWAYS to go short, or to go open. Of course, this also depends on things like the tracks on the board (ie: Will the failure be the diode or the circuit around the diode).
I don't think there is any perfect answer here, but something slightly better than a 1N4004 would always be a good idea.
Re: The forward diode on VIN - Unfortunately a lot of the Arduino boards I've seen end up with ~5V on VIN if you connect them to a USB port (ie: no other voltage on VIN). If there is no forward diode between the supply and VIN, then you'll get +5V back onto your supply, which means things like a fan on an aux connector, etc could try and run. Draw too much current backwards through the on-board reg on the Arduino and you'll end up with a fried reg on your Arduino when you connect via USB with no PSU connected/on but with other devices still connected (or if your PC isn't great a dead USB port - or both). This also happens with 3 pin pin regs, so the 12V reg doesn't save you either if you're using that (for > 15V supply).