[Elecraft] Inductive relay "kick"

[Carl, WCØV]

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