If you accidentally wire reverse voltage to a circuit, what determines the likelihood of damaging components? Is the mere presence of the voltage destructive, or is it reverse current flow? If the power source is a current source limited to 40-80mA, does that improve the odds of survival? Finally, if a circuit appears to be working properly after the reverse voltage condition is fixed, can it be trusted, or is there still likely to be hidden damage that increases the risk of future failures?

The thing that specifically got me wondering about this is a board I designed (my first production board) that I failed to include reverse voltage protection on. We inadvertently had one wired up backwards for a few minutes to a 5V supply with a current limit (40 or 80mA, I forget which.) One we fixed the wiring, it seemed fine, but now I don't know if I should trust it.

The only components I'd worry about are the SS495 Hall effect sensors or the MCP6542 comparator. Here's the schematic:

 

If no current flow, there is no destruction. But...to destroy or degrade a revers biased junction is might not take much current at all -microamps of reverse breakdown current on a MOSFET gate can degrade its leakage current performance.

For integrated circuit, reversing the power supply connection can destroy the part if there is enough current available.

Your Hall effect devices and comparator are probably fine. It is not a good idea to assume that a component that was exposed to a reverse power supply connection is good unless you can verify it by testing and measurement. Always eye such parts with suspicion.

 

If no current flow, there is no destruction. But...to destroy or degrade a revers biased junction is might not take much current at all -microamps of reverse breakdown current on a MOSFET gate can degrade its leakage current performance.

For integrated circuit, reversing the power supply connection can destroy the part if there is enough current available.

Your Hall effect devices and comparator are probably fine. It is not a good idea to assume that a component that was exposed to a reverse power supply connection is good unless you can verify it by testing and measurement. Always eye such parts with suspicion.

Thanks! So, I had hoped that failures would be all-or-nothing, but it sounds like that's not the case.

So here's a follow-up question. Should I add reverse voltage protection to the next production run of this board? Would most here consider that a standard expectation, or more of a bonus feature? I felt kind of dumb when I realized how easy it would've been to add one zener diode from power to ground and provide reverse voltage and over voltage protection.

The context for this board is that it gets installed into an espresso machine here at our factory, and it's connected with a four-wire polarized connector that we wire ourselves. So, once we wire it up, service techs should just disconnect the plug, not rearrange the wires, so it should be somewhat idiot-resistant, aside from that first step here in the factory.

Any thoughts on how standard such protections should be?

 

Reverse current protection is not required if as you state its wired via a polarized plug. Just need to take care when prototyping.
if its critical a diode is fine, depends on current used by load. Cost may be an issue.
I'm always pleasantly surprised if reverse protection has been added to something I'm playing with on the bench and I make a wrong connection. Happens to all players at some time.