[TowerTalk] Driving Remote Relays--simply

[Jim Lux]

-----Original Message-----
From: "k6xx at juno.com" <k6xx at juno.com>
Sent: Dec 10, 2004 7:47 AM
To: towertalk at contesting.com
Cc: towertalk at contesting.com
Subject: [TowerTalk] Driving Remote Relays--simply


Remote Ant. Switch Operators:

Here is perhaps a simpler (and certainly more efficient) solution for sticky relays.

Given that relays need a higher 'activate' current than 'hold' current, what we'd really like to do is boost the supply voltage temporarily--long enough for the relay to move from OFF to ON-- then drop back to a lower voltage holding level. Yes, it is CURRENT we need, but we drive current with voltage, of course.

Several years ago, Tony, K1KP, presented a circuit designed to speed-up the large, clanky relays in Ameritron amplifiers. This circuit provides a short burst of double the supply voltage, then drops back to normal supply voltage. This speed-up functionality should be ideal for remote antenna switches (especially with long cables) as well. Virtually zero additional wear-and-tear on the relay, yet it gets hit hard, long enough to make solid, clean contact.

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Another approach is to use a 3 terminal regulator as a constant current source (set for the "hold current") and a fairly large capacitor across the output.  When the realy is open, the capacitor charges up to the max supply voltage through the regulator. You close the switch, and you get a high current pulse as the capacitor discharges, with a long pedestal on the back, at the current regulated by the regulator.  A resistor/capacitor combination can do the same, but if the temperature of the relay coil and wiring changes, its resistance changes, and the current will change.

A post yesterday or the day before discussed the 3 terminal regulator approach.  Something like an LM317 works well, and has minimal voltage drop across the resistor. 

Jim W6RMK