[GreenKeys] 60ma loop supply

[Gregory W. Moore]

I think I have the answer in my own teletype. Since I have an M28ASR on the
way, I will have the built in loop supply in the machine. All that is going to
be seen by this is a Mite, and the output from the FSK converter, and the
input to the TU for level conversion to key a low level fsk input on my xcvr.
Sri Gentlemen, I had confused myself for a minute, and that is pretty
embarrassing for a former teletype repair type.
73 es tnx de Greg WA3IVX

jhaynes@alumni.uark.edu wrote:

> 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.