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).

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.