[Elecraft] Fw: Inductive relay "kick"

[Don Wilhelm]

Carl,

If you look at the datasheet for the relay driver used in the KPA100, 
you will find that it does indeed have integral clamping diodes.
Based on that fact, the inductive kick in the KPA100 should be almost 
zero, but in reality, there will still be a residual hit.  I believe 
that saying it should never be greater than twice the relay coil voltage 
is quite a conservative one - we were discussing the KPA100 application 
in particular, not back EMF in the  general.sense.

You are quite correct - without clamping, and relay coils with 
relatively low resistance, the back EMF can grow very large.  The energy 
in an inductor (the relay coil is an inductor) is proportional to the 
inductance times the square of the current, and if the relay coil 
resistance is low, the square of the ON current through it can be very 
large.

73,
Don W3FPR

Carl wrote:
>>>> 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
>>>>         
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