Help me understand the purpose of this Diode

[FIRE UP]

Originally Posted by nothermark

The physics behind it are that a voltage is generated any time a conductor is moving in and electric field. (Faraday effect) Normally we think about spinning a magnet and holding the wire stationary but it also works when the field changes. Ignition coils work that way. A relay has the same problem so the diode is used to limit negative voltage swings by shorting them to ground. That causes the magnetic field to collapse faster as well as preventing spikes on the power line. One can also use a zener diode to capture postive spikes over the zener voltage. That comes into play when controlling large coils with a solid state switching device.

Wow,
I think I'm gonna have to schedule a therapy session reading this. Waaaaaaaaaaaaaaay over my head. But it's good to know folks do know about this stuff and how it all works.
Scott

[jelag]

When a coil of wire is charged with power there is a magnetic field around the coil... that is the basic principle of how and ignition coil works... a magnetic field around a 12 vdc coil collapses around a coil of wire that has thousands of winding's of a finer wire and thus the 50K volts that are needed to fire a spark plug are produced.. nothing more than a magnetic field collapsing..



This same principle is happening in a solenoid when it's powered down...he resulting collapse of the magnetic field can create a back flow of electricity... the result might be a power spike backwards in the circuit which might damage a circuit board.. Thus the diode to prevent that damage...

[Coachmen Joe]

It prevents a back EMF pulse when the coil collapses. Is it actually needed for a solenoid?
Probably not but it certainly can't hurt until it shorts out and you will know it when that happens.

When I designed circuits every ice cube relay had a diode across it. Because like I mentioned when the coil collapses there can be quite a bit of voltage for a few microseconds causing the relay not to operate correctly or stick.

[YC1]

https://www.google.com/search?q=coun...hrome&ie=UTF-8

Counter EMF. Just another term for all the great posts that have nailed it.