Electricity Primer: Difference between revisions

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#REDIRECT [[Improving Phidgets Hardware Reliability]]
==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)

Latest revision as of 18:53, 22 October 2019