Photon A0 pin tolerance to negative current?

I am looking at possible fault conditions with my Photon which is powered from a ‘12V’ battery, via the Relay Shield.

If the battery is connected backwards, it is clear that the Relay board won’t mind, but I also have a divider which monitors the battery voltage via Photon pin A0/PC5. With a reversed battery, the Photon will be powered down and A0 could see up to -3.3V through 796 Ohms. That seems to imply that the maximum (negative) current under those conditions will be -3.75 mA. I am hoping that, being under 5mA, there won’t be any damage, but would like to hear from someone who knows for sure.

The pins can sink current (negative amperage) but what you are proposing is negative voltage and that’s a no-go.
See here

Yes, I had read that, though it appeared that those specs all assume that the Photon was powered up.

I was hoping that with no power you didn’t have to worry about the usual things like latchup. My suspicion was that with a negative voltage the input would behave like a forward-biased diode, either one added for protection or a substrate diode which could tolerate a small current without damage.

I may want to try some careful tests to see how it behaves with no power.

When I do this, it hurts!!! Perhaps you should be adding a protection diode to prevent reverse battery connections. It’s never a good idea to hope that an input will behave a certain way under conditions it was never planned to operate under.

How can you be certain about “no power”?
Even if the regulator won’t provide positive voltage and the rest of the system won’t start running, you still would (most likely) have some electric potential on the GND rail and any potential bellow that GND level can (most likely will) damage the semiconductor junctions that are not meant to deal with such a condition.

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I think I have an answer. I carefully applied a negative voltage to the Relay Shield battery input terminals and monitored the voltage and the current out of Photon pin A0. As I’d hoped it acted very much like a diode with its cathode on A0, so my main concern should be that I don’t want its current to get too high. The spec sheet suggests that it might be OK with 5mA.

The first plot exhibits a typical diode V-I curve, while the second one shows that the current increases rather linearly with negative battery voltage at the divider input. At -15V the current looks like it would be ~ -3mA, though my power supply only could go to 12.5V so that remains to be proven.

After doing the test I cranked up the Photon again and could see that its battery voltage measurements were still within a few mV of my meter reading. Looks like, when powered through the Relay Shield, it could survive a serious mistake like battery reversal. (Though not recommended in any case)

Thanks much for the help.

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