[Milsurplus] A relay transient suppression circuit.....?

[Dennis DuVall]

The RC circuit here actually allows for a faster keying speed than would be possible than with the diode used alone.  With the diode ONLY across the coil the relay will remain energized until the resistive losses in the coil windings allow the transient coil current to drop below the level necessary to hold  the relay closed.  The addition of the simple RC circuit here allows a portion of the transient coil energy to be transferred for dissipation to the RC circuit in the form a voltage “spike."  Net result is a shorter delay in relay coil release, how much depending on the choice of circuit constants.  The value of the capacitor probably has the greatest effect here.  The resistor would mainly be chosen to insure the capacitor was discharged and ready to go again prior to the following keying cycle.

Fundamental thing here, when interrupting the current to a relay coil (or other inductance) the stored energy has to go somewhere.  In the case of turning ON and OFF with mechanical contacts arcing across these contacts suffices in many cases.  In the case of large inductances as used in power supplies spark gaps specifically for this purpose are provided.  I have also observed cases where arcing through wiring insulation was apparently considered to be acceptable….   :^(

Dennis D.  W7QHO
Glendale, CA

*************
> On Feb 21, 2016, at 6:18 PM, Hubert Miller <kargo_cult at msn.com> wrote:
> 
> Going thru some manuals today, deciding which to keep and so on, i find one for a keying ‘control panel’ by RCA. It says this: 
> “Each relay has an RC circuit in series with a diode across the coil to provide transient suppression. This RC circuit lowers the maximum keying
> rate to about 80 WPM based on a standard word equal to 25 centerholes of Morse tape.”  
>  
> The relays are provided with a -12 dc. The relay coil has connected to this hot -12 end, a diode 1N459 anode ( reverse biased ); the diode
> other end ( cathode ) goes to a paralleled 10kR and 2.2 uF connected reverse to the operating voltage across the relay. Is that RC to 
> prevent damage to the diode by the reverse EMF of the coil, on break (opening ) the relay circuit ( by ungrounding the cold +12 end of the
> coil, where a hand key or auto keyer is connected ? And probably the delay effected by the capacitor discharging slows the coil release? 
> Why the capacitor at all? The diode and resistor should do the job, i would think, and the inrush current into the charging capacitor ( by
> the reverse coil EMF ) i would think, puts a big initial current spike thru the diode? 
> What think?
> thanks-
> Hue Miller 
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