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

[Bob Camp]

Hi

An MJE340 worked fine back in the 1960’s as a current regulator.  They are still out there for about 38 cents each if you buy ten. About all
the rest of the circuit consists of is a zener (say 6.8V) an emitter resistor (say 100 ohms 1W) and a bias resistor to keep the zener fired (say
about 100K or so. You should put a couple of neon bulbs to clamp the voltage and protect the setup when it switches to open loop. 

With a 60 V transformer in the loop supply it will do a pretty good job on a half dozen machines or so. You will need to heatsink the transistor
to handle the 5 or 6W it will dissipate. 

Is it  cheaper / better / easier / more reliable than a resistor …. not at all clear. A rotary pot at 12.5 W will run you $40 or more. A variable
wire wound is about $14 or so. The nice thing about the constant current loop is that when you (over) load it up, at least the machines don’t 
all go open loop ...

 Bob


> On May 18, 2016, at 12:34 PM, Howard Weeks <weeksh at att.net> wrote:
> 
> 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)
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