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

[mstangelo at comcast.net]

Back on the days of 50 baud telex we did keying on the 120 volt 60 millamp loops with mechanical relays or transistor keying circuits. We had an RC snubber network across each keying device. They were so pervasive that RC snubber networks were packaged in a form similar to a radial mylar capacitor.

I used to have a nomograph to calculate the values. I think I still have some of those snubber devices at home.

Mike N2MS

----- Original Message -----
From: Dennis DuVall <duvallddennis at gmail.com>
To: Hubert Miller <kargo_cult at msn.com>
Cc: tetrode at googlegroups.com, Milsurplus <milsurplus at mailman.qth.net>
Sent: Mon, 22 Feb 2016 23:22:58 -0000 (UTC)
Subject: Re: [Milsurplus] A relay transient suppression circuit.....?

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