Electricity Primer

From Phidgets Support
Revision as of 19:04, 12 January 2012 by Cora (talk | contribs)

Introduction

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

Basics

Picking A Power Supply

Selecting Cables

Hooking Up The Pieces

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)