electromagnet help

[jeff262]

ok am doing a project and i needed an electromagnet so i took one out of a chevy Air Conditioning compressor. the magnet energizes and pulls the metal clutch in to engage the compressor. now in doing this i noticed that there was a diode that was going across the + and - wires going to the magnet. i lost it somewhere in the junk yard. why would the magnet have a diode going between the wires? also what way would the diode go? the car is a 12v system and the magnet says 7 volts stamped on it if that makes any difference
thanks jeff

[Ron van Sommeren]

http://en.wikipedia.org/wiki/Flyback_diode
Ring on diode towards (+) terminal, diode blocking.

Vriendelijke groeten ;) Ron

[Hugh P]

Back EMF protection. When the power is removed from the coil the magnetic field decays and introduces a voltage spike of the oppsite polarity which can damage solid state electronics. Placing the diode across the coil so that it does not conduct when power is applied lets the diode suspress the reverse voltage spike keeping the smoke in the expesenive electronics