Wiki: USB Power Supplies

When you're working on your robot you'll likely want to plug it into a wall socket rather than run it from batteries. For that you'll need a reliable USB Power Supply with enough current capacity to run your processor (e.g., Raspberry Pi), your sensors, motors and any other accessories (such as displays).

We'll focus on the Raspberry Pi since it generally requires more power than other microcomputers/microcontrollers.

How Much Power?#

There are a number of companies providing USB Power Supplies specifically for the Raspberry Pi. Each of the different Pi models has differing power requirements, but at least a 3 amp supply is probably recommended.

Dishonest Advertising?#

The problem is, the companies selling these units are not always very honest about the power ratings of their products. Apple seems to be the worst: they sell two adapters (listed as the Apple 87W USB-C Power Adapter and the 96W USB-C Power Adapter) that are likely the most expensive on the market and are not showing their output ratings in watts but seemingly their power requirements, i.e, not what they deliver but what they use. Of course it says nothing about this on the box. It's 87 watts, right?

Apple is really relying on the ignorance of its customers here.

Apple 5 watt USB adapter

I made the mistake of buying the 87 watt unit for NZ$120 (wow!), opening the box and finding in fine grey print on the adapter a confusing bunch of different amperage ratings at voltages, but at 5 volts the unit delivers 5 volts at 3 amps (i.e., 3000 mA or milli-amps). To give you an idea of how far this is off from that advertised 87 watt promise, delivering 87 watts at 5 volts it should be delivering 17.4 amps. Apple's unit is clearly using 87 watts to deliver 15 watts (3 amps at 5 volts) of power. You can check this yourself by using an online Watts to Amps Calculator or Amps to Watts Calculator.

Apple 12 watt USB adapter

What's strange is that Apple has delivered honest reporting on its other units. Their tiny 5 watt unit delivers 5 volts at 1 amp. Their 12 watt unit delivers 5.2 volts at 2.4 amps. They provide technical specification for these smaller models, but the "Tech Specs" for the 96 watt model just says "USB C". What gives?

This is where having a bit of your own knowledge is good. These are the two pertinent formulas:

  amps = watts ÷ volts
  watts = amps × volts

If the Raspberry Pi people say your Pi needs 3 amps, that's not how much power the adapter is using (including power wasted through conversion, heat, etc.), it's how much it's required to deliver.

Some of the units I've found say they supply 3 amps but over two separate USB connections, or say they deliver 3.1 amps but have two USB sockets, so is that 3.1 amps for the whole unit or only 1.55 amps per socket? It's hard to tell when vendors aren't honest. I've got a 3SixT USB charger that has this on the bottom:

    Input: AC 100-240V~, 50/60Hz 0.8A Max
    USB 1 Output: 5V = 2.4A Max
    USB 2 Output: 5V = 2.4A Max

Of course, on the box it said it was a "4.8A" unit. But it only delivers 2.4 amps per USB socket, which won't do. At least it on their packaging and their website this was clear.