[GreenKeys] 60ma loop supply

[Henry Minsky]

Is there someplace near the transformer that you can tap into a modern PC 
power supply to get
an isolated high voltage DC supply? I don't know how practicalthat is, but you
can get PC power supplies really cheaply these days.

         At 02:54 PM 9/9/2002 -0500, 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.
>
>
>
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