[GreenKeys] Loop Supplies (was: any missing USB-tty adapter orders?)

[Howard Weeks]

The only purpose of the series resistor (normally adjustable) is to set 
the loop current to 60 ma if that is the type of loop you are running. 
The exact value of that resistance is related to how much resistance is 
in the loop. That resistance is a function of how many selector magnets 
are plugged into the loop. Transmit contacts add little resistance if 
they are clean and in good condition. For a 120 volt DC loop voltage, a 
2500 ohm pot of the proper wattage will do the job nicely. Been running 
them that way for over 50 years.

There is nothing magic about a 120 volt DC supply. Just build one. Any 
AC transformer with at least one 120 volt secondary winding and 60 plus 
mils of current capability will do nicely. I have built several using a 
transformer that had split or dual 120 primary windings. Use one for the 
primary and the other for the secondary. Ignore the other windings.
A bridge rectifier and suitable filter cap will work. Put it all on a 
small chassis with a 100 ma meter and on/off switch along with the pot 
mentioned earlier.

Somewhere around here, I have several commercially built plug-in loop 
current regulators (solid state) that will automatically set a loop to 
60 ma and keep it there when it is closed. Think they were made by 
Stelma. Many were used in military tech control facilities back in the 
60 - 70 time frame.  Shouldn't be too hard to design and build.

I use a single 60 ma loop to tie everything together (TTY gear) in my 
shack. Even the computer driven stuff connects that way through loop 
keying adapters (home brew). The convenience is that I can plug any 60 
ma machine or computer into that loop by plugging it into any available 
loop jack and I am in business.

The loop is limited in terms of the data rates that it will support; 
however, it is a proven solution for the 60 - 100 wpm mechanical machines.

Howard K5JCP

On 5/18/2016 9:41 AM, Paul Birkel wrote:
> Cory:  What values/wattage did you select for your power resistor and
> adjustable rheostat?
>
>
>
> *From:*GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] *On Behalf
> Of *Cory Heisterkamp
> *Sent:* Tuesday, May 17, 2016 6:38 PM
> *To:* Greenkeys BOARD
> *Subject:* Re: [GreenKeys] any missing USB-tty adapter orders?
>
>
>
> Back in 2013 I decided I'd raid the parts stash for a power resistor and
> adjustable rheostat, then ordered a Hammond plastic project box and 1/4"
> insulated jacks off ebay <$10. Grabbed a mA meter there as well ($3.95)