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Re: [TowerTalk] Driving Remote Relays--simply

To: "" <>,
Subject: Re: [TowerTalk] Driving Remote Relays--simply
From: Jim Lux <>
Reply-to: Jim Lux <>
Date: Fri, 10 Dec 2004 16:25:46 -0800 (GMT-08:00)
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-----Original Message-----
From: "" <>
Sent: Dec 10, 2004 7:47 AM
Subject: [TowerTalk] Driving Remote Relays--simply

Remote Ant. Switch Operators:

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

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.


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 



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