Re: [IRCA] Kevin's remote terminated loop article

["Kevin S" <satya@xxxxxxxxxxxx>]

Thanks, Steve, for reminding me of that!  With an R1 of at least 100 ohms,
it shouldn't be a problem, but you're right: if one tries to maximize the
current and gets rid of R1 in order to bring the Vactrol's resistance down
to the extent possible, the pot may fry up (again).

Kevin



> I would suggest you do not want to go down to zero ohms or approach the
> max
> current through the Vactrol LED, unless you switch the pot out of the
> circuit first. My experiences building this for Kevin had full current at
> first, going through the pot, but I noticed the resistance scale wouldn't
> reset to the same points after running the pot down for max LED
> current/minimum Vactrol resistance. The small amount of resistance in the
> pot was being permanently changed from too much current. Maybe a better
> pot
> could be found than the one I happened to use, I don't know. Pots aren't
> designed to handle much current; rheostats are. Maybe changing to a
> rheostat
> is all that's needed if you want to run more current to the LED.
> 73,
> Steve
>
>> Date: Sun, 3 Jan 2010 15:15:37 -0700
>> From: "Chris Knight" <chris@xxxxxxxxxxxxxxx>
>> To: "'Mailing list for the International Radio Club of America'"
>> <irca@xxxxxxxxxxxxxxxx>
>> Subject: Re: [IRCA] New article on terminated loop remote resistance
>> control
>> Message-ID: <E70DBC2D579149848ED0CB30B4E2221F@ChrisPC>
>> Content-Type: text/plain; charset="us-ascii"
>>
>> Hi Kevin,
>>
>> Very nice article! The only question I have right now is how are you
>> interfacing the control unit to the vactrol? What I would do is
>> interface
>> a
>> coax cable to the control box via a SO-239 connector and outside to the
>> antenna. The coax impedance wouldn't matter as the control voltage is
>> DC.
>> The Vactrol will operate down to -40 degrees C according to the
>> datasheet.
>> So cold Colorado weather won't be a problem. ;-)
>>
>> I made notes on your article. The LM7805 can have input voltages ranging
>> from 7V to 35V DC. This will work great if powering the control unit
>> from
>> a
>> car battery or a ham radio power supply. Power at V1 shouldn't drop
>> below
>> around 1.7V for full range on the vactrol LED. I calculated voltage in
>> your
>> circuit to be 2.75V at V1 which is perfect. The LED can drop up to 1.7V
>> and
>> the resistor, Rs, would safely drop the remainder (V1-Vled).
>>
>> So, the only thing I can think of as an added option would be the
>> ability
>> to
>> bring Rout down to zero ohms, maybe through a switch that would also
>> trade
>> out the impedance matching transformer with one better suited for a
>> loop.
>> This should give you a bidirectional pattern, if you need it, and more
>> gain
>> over a terminated loop.
>>
>> I don't know if you are aware of the existance of digitally-controlled
>> variable resistors. These could spawn a new generation of remote phasing
>> control among hobbyists using computers as the control unit. Think of
>> the
>> possibilities!
>>
>> Now on to reading your next article. Keep up the great work!
>>
>> 73,
>>
>> Chris
>>
>> Chris Knight (N0IJK)
>> Fort Lupton, Colorado
>> http://sites.google.com/site/ftluptonulrlogs/
>
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