Passive amplifier keying device.
The schematic above is an Amplifier keying line "back-pulse" damper. The resistor limits current both "in-rush" and steady-state type. The diodes are for "back-pulse" canceling, the reason for two diodes, is that it would be very unlikely for both to fail, so there would always be protection. The Ceramic disk capacitors are for "RF by-pass" should any RF appear on the keying line, these caps should by-pass it to ground, before it arcs the relay contacts, or cause other RF feed-back problems. Also, the .1uF cap absorbs a small portion of a voltage surge before it reaches the resistor.
The Small metal box shields the circuit and gives a mounting point for the RCA jacks.
The C-2 end of the unit should be placed in the amp key line, C-1 should go to the rig key line.
Feel free to copy this drawing and build your own.
The circuit has been built by me and tested up to 24vdc @ 150mA. with an ICOM rig.
It should work well up to 1Amp. of current.
However, if you are using an ICOM rig with it, do NOT exceed 16vdc @ 0.5Amps. I have used the damper with 24vdc @ 150mA. but I would not go any higher than that. If you have an amp with a 12vdc keying line, and below 500mA current, this key-line damper should work well and give the rig a minimum level of keying relay protection. For amps that exceed the voltage/current levels here, use the interface relay on my other keying page. These will give the maximum level of protection.
Copyright © 2005 M.A. Erickson, KK5DR. All rights reserved.