How to protect Raspberry Pi when powering via 5v pin?

Question

I have reglaurly heard that while it is possible to power the Raspberry Pi via the 5v rail, you do so at your own risk. This is because the rail lacks any of the protections that the USB port has included.

I have a reasonably cheap 5v, 2a power source that I’m using for another component (Adafruit thermal printer - requires 1.5a) that I am planning to run the Pi off in parallel. The tutorials I’m basing my project off just connects the Pi via the rail and that works. But I’m wondering, how I might go about providing similar protection to what the USB port provides.

I have a 5v step-up/step-down regulator, would connecting the Pi via that work? Do I need to throw a fuse in there? Is there anything I’ve missed?

How do I provide the same (or similar) protection that power over USB provides, when powering the Pi via one of the 5v pins?

Answer 1

The Pi Zeros have no additional protection when powered via the microUSB socket over a 5V pin.

The other Pis have three types of protection when powered via the microUSB socket.

1. a polyfuse which limits current flow and offers some protection to the Pi.
2. transient voltage spike protection which stops transient overvoltages from damaging the Pi.
3. the socket design ensures that if you use a standard lead you don't mix up +ve and -ve.

You could add a fuse to your 5V pin supply.

If you have a decent external power supply you shouldn't need transient overvoltage protection.

As far as mixing up +ve and -ve you just need to be careful when connecting to the 5V and ground pins.

Answer 2

The protection circuitry on the Pi is there to "protect" the Pi from the power supply.

The vast majority of cheap 5V plug packs (including those sold for use with the Pi) have poor regulation (for which the circuitry provides no additional safety) and almost no protection from mains induced transients. Indeed it is difficult to provide isolation in a double insulated supply with no earth - which is why so many people report tingling or shocks when using these kind of devices, and certainly not for the prices people are prepared to pay.

The best "protection" is to use a quality supply - which your question seems to rule out. Interspersing a buck-boost regulator will provide no benefit.

Bearing in mind the $35 cost of the Pi do you need additional protection? I have designed many PSU circuits over the years, but most cost an order of magnitude more than this.

Personally I normally run several Pi from a 5V switch mode supply (of the kind sold to power LED lighting strips). My tests on this indicate very good regulation, low ripple and noise, with the added benefit that it can be earthed. Even then I supply the Pi via the µUSB socket (connected with decent sized wiring).

Answer 3

The Pololu unit you have sounds like a good and reliable way to protect your Pi hardware when you power it from the 5v rail. As far as fuses are concerned: don't waste your time as it will all be over but the crying before (most) fuses can react. If you didn't have your regulated power supply already, you could have considered a simple hardware hack involving a series resistor and a Zener diode (good explanation here). And if you want something a little more elaborate, this Maxim solid state device might provide a foundation for a solution :)

At the end of the day however, I'm sure you recognize that by stepping off the beaten path, and doing something "non standard" you're taking on some greater risk. Weigh it up, then go ahead!