[GreenKeys] 60ma loop supply

[jhaynes@alumni.uark.edu]

I think what you are asking is about a power supply that will run several
60ma loops.  That's kinda hard these days, because the parts to build it
are not easy to find.

You'd like to use transformer isolation rather than running it right off
the AC power line, so you can ground one side of the loop supply.  You 
need about a 50 watt transformer.  You can get 120 volt isolation 
transformers - there's a 50VA one in the Mouser catalog.  Or you could
get a couple of the 25 volt transformers at Radio Shack and run them
back-to-back.  Cost about the same either way.  The single transformer
is obviously more compact and more efficient.  

Then you need a bridge rectifier, and that is the easy part.

Then you need an electrolytic capacitor that is good for 200 volts or so
(you are going to get about 150 volts)  and you'd like to have 3600 uf
or more.  The Mouser catalog has a 2000 uf at 200 v that's almost 20
bucks and you need two of them.  So we're up to about $70 by the time you
throw in a fuse and other miscellaneous parts.  Maybe you can find a 
surplus power supply, or find the parts on surplus, for less money.

The 150 volts is higher than the standard 120 volts that you want, but 
it's about the only thing easy to get, if you don't find a ready-made 
power supply.  Just means you need 2.5 K loop resistors instead of the
normal 2K.

Now here's something that is archaic but you might want to do it anyway.
In series with each of your loops, or especially the ones you use for
testing, put a light bulb.  60 watts is a nice size.  The bulb won't light 
much, if any, at 60ma.  But if you accidentally short the output the
bulb resistance goes way up when the bulb lights, so you don't blow a 
fuse.

Seems like you ought to be able to find a surplus Model 28 power supply
for a lot less money, and it would have a transformer that gives you 120
volts.  There were two sizes.  The one with the little transformer is good
for 120ma if I remember correctly, and the one with the tall transformer
is good for 500ma.