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Hey, John!
That's EE 101, 102, and 103! Have you paid your tuition yet?
Relays serve a simple function with many variations on a theme. In essence, a relay is a switch that is controlled by an electrical signal instead of your finger.
An electrical current flows through a winding and creates a magnetic force which pulls down an armature. This mechanical action changes the physical state of electrical contacts, effectively opening and/or closing other (often multiple other) electrical circuits. So a simple electrical input, of low current or power, can control the electrical condition of several other -- usually much larger -- electrical circuits.
When packaged with some simple electronics, relay assemblies (the Bosch relays in our cars, for example) can do even more. Consider these:
- Turn on a heavy current load with a small "on" signal (horn relay, electric fan relay, door locks relays).
- Turn on a circuit only when two "on" signals are received (fuel pump relay).
- Electrically isolate one circuit from another for noise reasons (radio suppression relay, which powers injectors but keeps electrical interference away from wiring harness).
- Perform an electrical "toggle" function but always resets to a known state on power-up (OD relay).
- Perform an electrical "toggle" function but retains previous state regardless of power history (headlight hi-low relay).
- Provide a "timed-on" function after only a momentary "on" signal (interior lamp relay, rear window defrost relay).
- Provide a timed, or "pulsed" on condition with a steady "on" signal (turn signal flasher, intermittent wiper relay).
- Provide other obnoxious and irritating functions (seat belt buzzer and key-in alarm).
- Sense electrical current conditions and alarm when a condition is met, or not met (bulb-out warning relay).
Relays are a basic building block of electrical circuits, from your car to enormous power stations -- they have relays (circuit breakers, really) bigger than a house! Asking for a book on relays is, well, like asking for a book on motors. Gas, gasoline, electric, steam? Open or closed system? Reciprocating or turbine? Internal or external combustion? The subject is vast.
You might try your local bookstore and ask for a book published by TAB. Or even try Radio Shack -- they used to have useful booklets on these subjects.
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