[GreenKeys] Loop supply options

[John, W9DDD]

Most of the originals were basically an isolation transformer, a 
rectifier and capacitor.  All in a nice heavy duty box.  Some had 
fancier filtering with a choke and another capacitor. Many had multiple 
taps to adjust for higher or lower that normal AC mains voltages. Also 
some were large enough to also supply the punch magnet on a 19 keyboard.

So choices today are:

A packaged unit either switch mode or linear versions.  You can go 
Cadillac from Acopian at $240.00.  Mouser lists some at around $62.00. 
Those are both linear regulated supplies.  I'm sure searching for 
imports and surplus you may find a better bargain.  Unregulated supplies 
which would be less expensive seem to be a rare bird.

You can roll your own.

There is a Hammond transformer - 166G80 which has a 80 VAC secondary. 
That with a bridge rectifier would result in about 112 VDC depending on 
your line voltage.  I think it was under $30 at Mouser.  That might be 
the cleanest solution.

Triad used to make a 1:1 transformer.  (maybe still do).  This would 
result in more like 140 to 160 VDC output so you'd want want more like 
2,500 ohms instead of 2,000 for the current limit resistor.

An old trick was to use a couple of filament transformers back to back. 
The hard way to get an isolation transformer.  You get the 140 to 160 
VDC this way also.

Another trick is using a dual primary transformer.  You have 2 120 VAC 
primary windings which are either wired in parallel for 120 VAC supply 
or in series for 240 VAC supply.  If you ignore the secondary windings, 
you again have a 1:1 transformer to get you the 140-160 VDC by using one 
of the 120 VAC windings as the primary and the other as the secondary. 
(The original secondary windings might be of use at a lower current 
rating).



-- 
John, W9DDD
Ich habe dreimal abgeschnitten uns jetzt zu kurtz. (I cut it off 3 times 
and it's still too short.)