Electricity Primer

Introduction

This primer will help you power your Phidgets while being safe to the electronics.

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)