Relays & protection diodes

[RicharD]

I had always assumed that one put a protection diode across a relay coil to protect from a possible spike.  I had also assumed that the relays I'm using have an internal protection diode since they are polarity specific.  Well you know what they say about ass*u*me.  First off, upon closer inspection of the relay cut sheet, there is not an internal protection diode.  2nd, not only does the diode protect the coil, but it also eliminates the audible "pop" I was getting during last night's experiment.  Bread-boarding is good.

[sluckey]

That diode's main purpose is to protect the driver device whenever the coil is deenergized. The relay coil is also an inductor. Whenever current is interrupted to de-energize the relay, a big CEMF voltage is produced by the coil to try to keep current flowing. This CEMF voltage spike quickly dies, but is often coupled into other sensitive circuits (your pop). Sometimes it is even sufficient to damage the relay driver, especially if the driver is a transistor or IC. The diode is connected such that it provides a short circuit to the reverse polarity spike and is most effective when it is physically located verrrry close to the relay coil.

[jjasilli]

Good stuff, but:

CEMF > google = Cutting Edge Music Festival

Diode across relay:  a) Are we talking DC?; b) schematic please?

[sluckey]

CEMF is counter electromotive force.

http://en.wikipedia.org/wiki/Counter-electromotive_force

Diodes are only used on DC relay coils.

[RicharD]

I'm using Omron G6A 12VDC relays.  G6A sounds like a Fender amplifier.

[LooseChange]

That little charge the relay makes is enough to hurt. Go ahead and touch it.

[PRR]

Same as the ignition coil in your car. Flow 12VDC at it, break the circuit, the coil kicks to 400V.

In theory: the current in a coil can't stop instantly, just like the voltage on a cap can't decay instantly. If you break the circuit, where does the current go? It "backs up" as a high voltage, trying to find a place it can go. In a spark coil, it breaks-over at the spark plug. In a relay, it may breakover at the switch, especially if the switch is a wimpy transistor. If there is no place to breakover, in theory the voltage rises to infinity. In practice an iron-core coil rarely kicks more than 10V the former DC voltage because the iron saps some energy (spark coils do a bit better).