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Re: [TowerTalk] Driving Remote Relays--simply

To: "Jim Lux" <>, <>,<>
Subject: Re: [TowerTalk] Driving Remote Relays--simply
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Fri, 10 Dec 2004 20:00:12 -0500
List-post: <>
> 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

Actually you should never have that problem with Ameritron
switches (or any other relay systems I can think of, except
perhaps the RCS-4 or similar).

The relays are worse at high temperatures, and even then the
pull in is significantly less than 9-10 volts DC. The
control box puts out 15V or higher, and relay current is
generally about 100mA.

You have at least 5V of headroom even under extreme
temperature, and to drop 5V at 100mA takes 50 ohms of loop
resistance. That's a LARGE length of very thin wire. If you
doubt it look at a wire table.

My suggestion is if you have a pull-in problem, you should
find out what is defective.

By the way, I have used a series resistor and shunt storage
cap in systems that require a large high-current pulse.
Switching large vacuum latching relays of a few ohms coil
resistance in 200 ft runs of cable often  requires 100 volts
or more of voltage and a VERY large cap. But the normal
relays in your typical antenna switches don't.

I have a pull in circuit for amplifiers on my website that
sequences the relays properly. The newer Ameritron amps now
use a similar system with two relays.

Whatever you do, if you play with amplifier relays be SURE
the sequencing isn't harmed. You can very easily blow an
expensive bandswitch if you hose up the sequencing.

How can you hose up sequencing? Very easy in some systems!

Say you use the external voltage accelerator. If you hit the
relay, dump the charge in the storage cap, and then drop and
re-hit before that cap recharges the charging load can upset
the timing in a dual relay system.

In a single relay system you can generally do whatever you
like and not modify sequencing. NOT is a dual relay system.
Keep that in mind.

When I want to speed a single relay up, I simply add a
transistor constant current source and use a supply two or
three times the relay voltage. I set the source current at
rated relay current. It is not necessary or useful to add a
cap, because much of the time lag is caused by the slowly
rising current in the coil's inductance as the magnetic flux
builds. The constant current source allows nearly full
supply voltage to appear across the relay coil with NO
chance of excessive current whacking the relay or relay
keying transistor in your exciter. You can add a single NPN
transistor to limit the open circuit voltage at the rig.

All of the suggestions work and are very good in general
cases, we just have to be careful applying them to
everything. Occasionally there can be problems. If you used
the external accelerator on newer Ameritron amps for
example, you will throw relay timing off and you could even
burn out the keying system in the amp. It does NOT like to
have negative voltage applied to the keying jack. I suspect
there are other amps like that also! Be careful.

73 Tom


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