[Elecraft] Inductive Kick

[David Robertson]

Carl,
I completely agree with you. I have seen 12 volt DC relays produce as much as 1000 volts peak or more when the current to the relay coil was terminated. This term was taught to me as Inductive Kick.

 When any coil of wire has DC current pass through it, a magnetic field is produced and magnetic flux builds up around the coil. This would be like stretching a spring. If you disconnect the current source, the magnetic field can quickly collapse causing the lines of magnetic flux to quickly cut accrossed the windings of the coil resulting in a high voltage spike. This would be equivalent to letting go of your stretched spring which can snap back at high speed resulting in possible damage.

Placing a diode in parallel with the coil with the cathode toward the positive side of the current source has no effect when current is normally applied. When you interrupt the current. the magnetic field collapses causing the coil to become a current source. The diode carries the current and reduces the voltage because of it's low impedance when switched on.  Remember the stretched spring. Having the diode in the circuit is like slowly releasing your spring instead of it snapping back.

Hope this helps.

73
Dave KD1NA


Don, seriously disagree about the magnitude of the "kick". For years I taught a course on industrial installations and demo'ed the "kick" issue by soldering two bare wire to the coil of a small Potter and Brumfield "ice cube" 12 vdc relay. I would pick the biggest guy in the class to hold the wires while I touched them to a nine-volt transistor radio battery. And then watch as he would throw the relay across the room. A reverse diode would vastly reduce the effect.

Well, I got yelled at and was told "Hey, Stupid (and stupid wasn't the word they used)! Put that on a storage scope to see what you're messing with." The scope showed a spike of 450 vdc! The diode reduced it to less than 75 vdc. 

The magnitude is due mainly to the large inductance of the coil, but until some measurements are taken, some caution is advised. The inductance of any coil inside a K2 or K3 is bound to be much smaller. But the phenomenon is the same.

73, Carl WC0V