We have a fleet of pi 3's plugged (via microUSB) into 5v/3A USB adapters, hooked directly into vehicles.

I measured one such usb adapter at 7.5v, and the pi 3 that was hooked into it was dead; it flashes on then immediately dies as soon as I plug it in to any power source.

I need to talk to the people responsible for the wiring, how likely is it that the extra voltage overrode the pi's voltage protection?

The adapter in question was a Kimdrox 3A/15W Dual Power Adapter

  • 3
    That much voltage will definitely release the magic blue smoke and kill a Pi. – Steve Robillard Oct 17 '16 at 18:47

it flashes on then immediately dies as soon as I plug it in to any power source

Although you are probably out of luck since it is intended to protect against over current, not over voltage (over current would occur, e.g., if you tried to draw too much via the breakout pins), there is always the chance that the main polyfuse triggered, since the problem with excess voltage is that it produces too much current.

So, you could set that Pi aside and see if it recovers (from other reports we've had here, a few days should suffice) but again, probably not.

Could voltages over 5v damage a pi?

The recommended maximum is 5.25V, and yes anything in excess of could be catastrophic for the device (although as per joan's comment below, you may actually get away with somewhat more). The problem is that current is a product of voltage and resistance (Ohm's law). The resistance is engineered in the device's design to deal with a specific, narrow range of voltage (in the Pi's case, 4.75 - 5.25V).

If you supply less than that, there is too little current and the device will not work.

If you supply more than that, the amount of current will increase correspondingly. Excess current will cause components to heat up, and at a certain point, that heat will physically damage them. Polyfuses excepted, such damage cannot be undone.

I believe the polyfuse is less likely to protect against overvoltage because more delicate components in line after the fuse will heat up to a critical point first. However, the big picture is not necessarily that simple (see Ghanima and Milliway's comments below regarding the presence of a TVS diode in the 5V power regulation circuit; above a certain threshold, this will mean voltage will flow to ground with little resistance, causing a surge in current which hopefully trips the fuse), and it does not hurt to cross your fingers.

I measured one such usb adapter at 7.5v

That is probably defective, since almost anything plugged into a USB port is going to be expecting 5V. Considering they are often used to recharge batteries which can explode, it is a potentially dangerous defect, so you might want to give some thought to where they came from.

| improve this answer | |
  • raspberrypi.org/documentation/hardware/raspberrypi/schematics/… the polyfuse with the SMBJ5.0A zener diode should actually protect against overvoltage. Voltages over breakdown of the SMBJ should lead to a large current thus triggering the polyfuse. I think there is at least one other question here addressing this issue. – Ghanima Oct 17 '16 at 19:22
  • 1
    Voltages less than 6V are pretty unlikely to be catastrophic to the Pi. I wouldn't push the limit but I have quite often measured 5.8V on the 5V line of my Pis. – joan Oct 17 '16 at 19:23
  • 1
    The diode is a Voltage Suppressor, not a Zener, although they have similarities. The Voltage Suppressor has a very fast response, and is capable of absorbing quite a bit of power. They are intended to absorb transients. It is possible that the Voltage Suppressor drew sufficient current to blow the polyfuse, so the Pi may recover. Then again they do not have the same sharp voltage response, so still allow high voltages (typically 6.4V) to pass. – Milliways Oct 17 '16 at 21:53
  • @Milliways Right you are -> more things I learnt about today. Although I dunno if the difference between the "reverse standoff voltage" (5) and the "breakdown voltage" (6.4 - 7.2) will make the difference between something that protects the pi for future use vs. something that just protects against it bursting into flames. Hopefully sweethome gets back to us one way or the other, this is an interesting test case. – goldilocks Oct 17 '16 at 22:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.