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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, with recommended PSU (Power Supply Unit) current capacities ranging from 700mA for the Model A to 3 amps for the Pi 4, but these numbers don't include sensors or motors.

Since you're building a robot, not an alarm clock, at very least a 3 amp supply is probably recommended. With sensors and motors you may need something even bigger. You may need to add up the estimated power consumption of all your components and add a bit extra for safety. Or use a very large power supply and actually measure the power consumption using a multimeter while your robot is running.

Jaycar sell a Mean Well 70W 5V 14A Power Supply that actually delivers 70 watts: 5 volts at 14 amps for NZ$54.90. It's not as pretty as the Apple unit but as a bench power supply it looks pretty good. Here's the specs:

• Full range 85~264VAC input 
• Withstand 300VAC surge input for 5 seconds 
• High efficiency, long life and high reliability 
• Operates from -30°C to 70°C under air convection without a fan 
• Extremely low no load power consumption 
• Complete protective functions 
• Power on LED indicator 
• 5G anti-vibration capability 
• Complies with international safety standards.  
• 100% full load burn-in test 
• 3 year warranty 
• Input Frequency: 47-63Hz 
• Cold Start Inrush Current: 50A

Rather impressive.

The above is a portion of the table from the Raspberry Pi FAQ.

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 5 watt USB adapter

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

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 milliamps). It says this nowhere on the box; I had to unpack the unit and read the 0.8mm light grey text, then return it to the store.

Apple 12 watt USB adapter

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.

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 if I need 3 amps (I use it for the 8" monitor for my Pi). At least it on their packaging and their website this was clear. I've another USB charger that has two sockets and says it delivers 3.1A, but doesn't say if that's for the whole unit or per socket. As it's slightly smaller than the 3SixT adapter I'm guessing the whole unit.

It's a tricky business, and rather difficult to figure out. If your power supply is running hot or your Raspberry Pi resets sometimes due to a brownout (the voltage may dip below 5 volts if the adapter can't supply the necessary amperage), your power supply is probably not big enough.

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