Electricity Primer

Introduction

This primer will help you power your Phidgets while being safe to the electronics. It mostly applies to Phidgets that use external power (such as being plugged into the wall power or a battery). If you're looking to learn concept for how to properly power a self-sufficient, wireless, battery-powered robot, you've come to the right place. Or, if you're looking to use more than one motor controller and not destroy your controller or your PC in the process, you've also come to the right place.

Find Your Needs

Let's say you want to run the SBC off of a battery. Or a motor controller with a power supply you bought from the hobby store. Will it work? What will you need? It is worth it to spend a moment with pencil and paper to work through this section and identify your power needs.

Voltage And Amperage

Power supplies - whether wall power or batteries - are rated based on voltage and amperage.

These two concepts can be described with an analogy: a circuit is kind of like a water faucet. Voltage is the pressure on the faucet, and the water supply is amperage, also known as current. Too much pressure behind your faucet, and the water mains or faucet will break. Likewise, if you have too much voltage from a power supply, your circuit will break.

But the faucet doesn't care whether there is a big reservoir or small reservoir feeding the system, as long as the pressure is managed. Likewise, you can choose a power supply with more amperage than you need (a big reservoir to draw from) as long as the voltage matches.

Picking a Power Supply

The power requirements for Phidgets are given in volts and watts. If you have an infinite

${\text{Amperage}}={\frac {\text{Watts}}{\text{Operating Voltage}}}$

Selecting Cables

In general, use the shortest cables possible.

Hooking Up The Pieces

Here, things can be tricky. You might think: just plug everything in and go! But often it is not that simple. Systems that require special attention in hooking things up are:

• Projects with long cables
• Projects with special USB cables
• Projects with two different power supplies, including:
  • Battery and wall power
  • Power from two different batteries
  • PC power over USB and battery power
  • PC power over USB and wall power

Hubs

Avoid hubs where possible. Unpowered hubs are good for reading data from memory keys, but not for powering many external devices.

Basics

• Your circuit is a collection of garden hoses
  • Voltage is pressure
  • Amperage is the amount of water
• Interference can be created and absorbed by your circuit, both are undesirable
  • This interference is EM energy that travels through the air
  • It is especially produced by sudden changes
    • Even common things do this such as plugging in a long extension cord with nothing on the other end
      • The cord must equalize its electron balance with the wall power
      • The electron flow that makes this happen creates EM waves that affect (and potentially disrupt) electronics in the area

Picking a power supply

• Over-voltage rating matters, this will probably kill your circuit
  • Similar to putting so much pressure within a garden hose it blows up
• Over-amperage does not matter, the circuit can already control this
  • Similar to using a smaller nozzle on a garden hose - less flow
• Under voltage or under amperage and your circuit will:
  • Just not turn on
  • Turn on and then realize demands are too high, then turn off
  • Turn on and off, trying to fill the demands and then protecting itself for a short time before trying again
• Power supplies (even AC) have a set voltage, but that voltage is relative.
  • When a connection is first made, the board and supply settle their relative voltages.
  • This can generate a spark and feedback loop within the board
    • The board will get hot and should be unplugged within the first few seconds to prevent permanent damage
    • How to prevent?

Shielding

• Hard to do right
• Emissions hit shield and travel back to ground with resonance

Cables

• USB cables should be thick, and to spec
• USB depends on the fluctuations going out on +5V and back on ground to be well matched in time and distance
  • Their nearness causes their emissions to cancel each other out
  • Some cables have ferrite beads, which are low-pass filters (low frequencies pass)
    • This helps prevent a situation called USB common mode, where
• Some voltage is lost along the USB cable
  • Thin cables are more susceptible to this loss because they have higher resistance
  • The loss happens both ways, so the Phidget is running on a slightly reduced voltage gap from 5V
  • The thinner the cable, the more likely the Phidget will drop below its 4.5-4.6 V reset point

Size of circuit

• Circuits are always loops, and loops will resonate like antennas at a frequency determined by their size
• The smaller the loop, the higher the frequency
• Higher frequencies have a smaller potential to interfere with circuit frequencies
  • Keep hookup wires short

Multiple power sources

• USB is one source, wall and battery power is another
• With only one device, not really a problem
• With more than one device, you create a closed loop between the two devices and the power source
  • Electrons can return via the grounds connecting both devices and the PC motherboard rather than just straight to wall or battery ground
  • Solutions:
    • Make the connections between all devices and battery or wall really desirable to electrons
      • Low resistance
      • Big fat wire
      • As short a wire as possible
    • Use a USB isolator
    • Use Ethernet for data rather than USB (or wireless), only for future Phidgets
• SBC complicates things...(three phidgets)