[Elecraft] Inductive Kick

[David Cutter]

The usual diode across the coil is simple and effective for the vast 
majority of situations but the rate of change of current can induce 
undesirable effects in nearby sensitive circuits which the diode 
exacerbates.

For most relays we see in ham rigs a 1N400x series diode is often used, when 
a 1N4148 is not big enough to dissipate the energy pulse of bigger relays.

I have wondered if the slow 1N4000 types can turn on fast enough to catch 
those first few ns, but I have not looked into this.

I have used a capacitor between base and collector of the driver to slow 
down the rate of change and thus eliminate both the overshoot problem and 
coupling energy into nearby sensitive circuits.  Some speed-up circuits 
which apply a series resistor also help with this potential problem.

David
G3UNA


----- Original Message ----- 
From: "David Robertson" <darjar at comcast.net>
To: <w3fpr at embarqmail.com>
Cc: "Elecraft" <elecraft at mailman.qth.net>
Sent: Sunday, April 05, 2009 1:11 PM
Subject: [Elecraft] Inductive Kick


> 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
>
>
>
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