How much protection in board-board power is appropriate?

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daqq
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Posted by daqq: Mon. Oct 31, 2011 - 12:43 PM
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Hello,

How much protection (fuses, overvoltage, reverse polarity protection and such) would be appropriate in the following setup on the Board component:

Power supply -> Cable -> Board
             -> Cable -> Board

The Cable is of the type that it's very hard to put in the other way around (kinda like a Molex computer cable).
The board is a module that should be (re)placed by the user.

So, the question: How much protection is appropriate? Fuses? Overvoltage? Reverse polarity?

Thanks,

David

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Learning and Information, General Electronics
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kk6gm
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Posted by kk6gm: Mon. Oct 31, 2011 - 02:24 PM
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The engineering calculation is (cost of failure * probability of failure) < or > (cost of protection * all units). I would consider a polarized cable connector pretty much equivalent to a PCB trace. If anything, connectors usually fail open.

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DO1THL
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Posted by DO1THL: Mon. Oct 31, 2011 - 02:57 PM
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While kk6gm's formula's are absolutely correct, i think you simply cannot expect to get a universal answer to your universal question. I mean, you didn't mention if we're talking about your main development board vs. a final product, something that gets used by yourself exclusively vs. being used at the other side of the planet by someone else, if there's a hyper-sensitive part in the $$$-range or if there's mainly a cheap AVR lurking around...... So, make your own decision based on your own expectations/fears!

Einstein was right: "Two things are unlimited: the universe and the human stupidity. But i'm not quite sure about the former..."

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bobgardner
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Posted by bobgardner: Mon. Oct 31, 2011 - 03:37 PM
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A diode in series is a good reverse polarity protector. We used to ship stuff that had the 5V on the rear connector and the customer was supposed to supply regulated 5V power. There's a big problem if the regulated ps is feet away. Big turn on spikes, fumble fingered techs hooking the 5V up to the 12V line, then they send it back and say 'Your crappy gizmo doesnt work'. Nothing better than a robust wide input range power supply to protech your board. In my opinion.

Imagecraft compiler user

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DocJC
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Posted by DocJC: Mon. Oct 31, 2011 - 05:02 PM
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The first board needs LOTs of protection. Hard to imagine what the user will use for a power supply, or how it will be connected.

The second board may or may not need as much protection. A series diode for reverse polarity means the board is running at a lower voltage. If the first board passes an unregulated power to the second board, then you have headroom for a formal protection circuit, and its own on-board regulator.

You also need to consider the impact of the failure. Mission critical application vs no-big-deal. (Ventilator for patient, fly-by-wire flight controller, traffic light controller, etc. vs coffee maker).

JC

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jayjay1974
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Posted by jayjay1974: Mon. Oct 31, 2011 - 09:31 PM
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Polarized connector, polyfuse, series diode, tranzorb followed by a wide(r) range PS.

Should make it quite bulletproof for not too much cost.

If power efficiency is really important and power loss of the series diode is too great, you can use a MOSFET.

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daqq
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Posted by daqq: Mon. Oct 31, 2011 - 09:59 PM
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Thanks for the input! There will be a polarized connector on both ends of the cable. I think I'll add a polyswitch and a protection diode.

Quote:
Hard to imagine what the user will use for a power supply, or how it will be connected.
The power board, the cable, the board - all is supplied. The only thing the user does is plugs them together.

The whole setup is nothing mission critical nor anysuch.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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valusoft
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Posted by valusoft: Mon. Oct 31, 2011 - 10:21 PM
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daqq wrote:
The power board, the cable, the board - all is supplied. The only thing the user does is plugs them together.
In that case ... no two connector pairs of the same type and all polarised. Never underestimate the ability of a USER to prove your foolproof design isn't.

Ross McKenzie ValuSoft Melbourne Australia

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petervk
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Posted by petervk: Tue. Nov 1, 2011 - 09:58 PM
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daqq wrote:
Thanks for the input! There will be a polarized connector on both ends of the cable. I think I'll add a polyswitch and a protection diode.

Do yourself a favor and add a tranzorb as well.

It doesn't sound like this is the case for your application, but normally one also needs to think about the "quality" of the power supply. Is this a nice clean well behaved regulated supply, or is it a power feed from a horrible nasty supply like a train or even a car.

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