[GreenKeys] Basic Loop supply

[Ralph Mowery]

That is similar to what I did years ago for my first loop supply.  I just
used what I had on hand.  A transformer of a high enough voltage, made a
bridge rectifier out of 4 diodes and about 100 uf at 450 volt capacitor ( 20
uf and less voltage could have been used, but that is what I had).  Then
used several resistors of around 300 to 800 ohms at 10 to 20 watts and a
wire wound adjustable resistor to get up to around 2500 ohms.  Probably
easier and cheaper to find an adjustable resistor than a potentiometer type
resistor.  Just start with the adjustable tap at the high end and work down
to the current needed.  Just be sure to bleed off the capacitor before
sticking your hands into it. It might be safer to add a bleeder resistor of
say 100,000 ohms across the capacitor.

I had a metal box about 4x5x6 inches.  I cut out part of it and put in a
piece of plastic. Then mounted on the plastic 5 closed circuit jacks and one
open circuit one.  Closed circuits jacks are needed so the loop will stay
closed when a device is unplugged.  I used the open circuit one just in case
I wanted to break the loop for some reason.  Probably really used it because
I had it on had in the junk box.

No use over thinking the loop supply.


 

-----Original Message-----
From: GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of
epvgk at limpoc.com
Sent: Wednesday, May 18, 2016 1:52 PM
To: Lester Veenstra
Cc: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Basic Loop supply

On Wed, May 18, 2016 at 01:39:55PM -0400, Lester Veenstra wrote:
> 
> The pricey item is the variable wire wound resistor
> 

It also doesn't have to be adjustable from 0 to 2.5K - for example you can
use a high wattage fixed resistor like
http://www.digikey.com/product-detail/en/riedon/UAL10-2KF8/696-1492-ND/38866
24
or
http://www.digikey.com/product-detail/en/yageo/SQP10AJB-2K0/2.0KW-10-ND/1878
9
in series with a smaller variable resistor which at 500 ohms would dissipate
no more than 1.5W at 60mA.  May not provide sufficient adjustment range, but
it could at least offload the expensive part of the job to a cheaper fixed
resistor.

eric