Hey, we need electronics help.

Controllers Smoothie
,
1

Hey, we need electronics help.

On Smoothieboard v2 we want reverse polarity protection on the power inputs. On v1 we have it only on the main power input, but on v2 we’d like ideally to have it also on the mosfet input ( even though errors are extremely rare ).
The problem is it’s fairly expensive, we currently use a mosfet and a zener I believe ( see https://docs.google.com/spreadsheets/d/1vshB97WWt6Lceo59aggbdhRuXxTEhSL9lkotScR0oYE/edit#gid=0 ). but that’s like >$1. If we now have two of them that’s >$2 …
That’s a lot. Anyone has a trick or an idea for very cheap reverse polarity protection ? 12-24v, 0-20A.

Thanks a ton :stuck_out_tongue:

2

Can you point us to a schematic and what inputs you want protected?

3

There’s no current schematic. We’ve got a a 12-24v, 20A input and we want the cheapest way to protect it against reverse polarity, pretty much.

4

Reverse diode, blows a fuse. Then have an inline diode for the brain electronics just to be safe.

5

^ this is an option. Could blow the diode too though if the current is too high and the fuse doesn’t blow fast enough.

6

Not sur how hard it could be, but using sensor and relay, you could plug how you want and always have the right polarity?

7

Have you considered these techniques. I have not reviewed this yet…

https://www.maximintegrated.com/en/app-notes/index.mvp/id/636

8

A mosfet is awesome because it can handle the current without burning a lot of heat. Diodes usually have a decent voltage drop which results in quite a bit of heat generated. If you have it on the main input, why would you need it on the others? is there no regulator built in?

9

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.ti.com/lit/an/slva139/slva139.pdf&ved=0ahUKEwjAp5WHpqHWAhXBwFQKHX48AWAQFgglMAA&usg=AFQjCNEgbgKvs3cD_Ee5HJDeIE-qO5csLQ

10

I would suggest 40a low resistance FET. You would still need a fuse to protect the feet from too much current in case the heated bed is short

11

Unfortunately, mosfet is basically the best way at those currents. The zener is required since most mosfets have a Vgs(max) of 20v and thus 24v would damage it. If you limit voltage to 15v or so, you can ditch the zener.

12

Hey ! Thanks for all the answers !
So, we are aware of the different options ( diode+fuse, mosfet, etc ), that wasn’t really the question.
The question was more “do you know of specific parts ( as in : part numbers ) that are interestingly inexpensive and still do the job we need done here”. With all the talented PCB people around here I expect some of you have know parts we don’t.

Thanks again :stuck_out_tongue:

13

Seems like it would be best to diode protect the low-current circuit/voltage regulated circuits, and design the MOSFET output side to be tolerant of reverse connection by clamping the bias. Should be possible to avoid using any extra high-current devices, I hope.

14

A relay could work. If you’re going with 20A on both inputs it’ harder to find something <1$, but for 10A, 24VDC: https://www.digikey.pt/product-detail/en/omron-electronics-inc-emc-div/G5LE-1A-DC24/Z3114-ND/369005
(0.67€ @ 500 un.)