[AMRadio] Re: AMRadio Digest, Vol 44, Issue 41

D. Chester k4kyv at charter.net
Wed Sep 19 01:54:36 EDT 2007

>From: "Rick Brashear" <rickbras at airmail.net>
>Well, the problem with an RC circuit is it applies to the start up as well
>as the release and I need instantaneous start up or maybe I'm thing wrong.

A lot will depend on the resistance of the relay coil.

You might try putting a  capacitor in series with a  resistor, and then put 
the whole thing in parallel across the relay coil.

When you switch on the control voltage,  current from the control voltage 
source will divide into two branches. Some of  the current will immediately 
flow through the relay coil to turn on the relay.  The rest of the current 
will flow through the resistor to charge the capacitor.  But the resistor 
will limit the current surge to the capacitor, thus preventing the capacitor 
from pulling down the control voltage enough to delay the relay at start up. 
When you switch off the control voltage, the capacitor will discharge 
through the series resistor and relay coil.  With the appropriate value of 
resistance, the discharge current should be enough to hold the relay for a 
split second after the control voltage is removed.

First, try the circuit without the series resistor, and see if you can find 
a capacitor in the junkbox that produces somewhat more than sufficient delay 
in the release time.  Now add just enough series resistance to avoid pulling 
down the control voltage when it is first applied.  The relay will close 
instantly, but there will be a  delay upon opening as the capacitor 

I sent you an e-mail with attachment for the article I wrote in ER a couple 
of years ago regarding a sequencing circuit I designed.  It is more complex 
than what I described above. I still use it in my station to contral the T/R 
function.  I didn't post the attachment here, because most e-mail reflectors 
won't accept attachments.

Don, k4kyv 

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