[GreenKeys] 60 ma Loop Power Supply?

[Bill Horne, W1AC]

> Hi Everyone:
> 
> I have my 28ASR running now, but it needs a loop power 
> supply to stop the printer from chattering and to make it 
> respond to the keyboard.
> 
> The keyboard problem was resolved by simple cleaning and 
> lubrication, and the tape punch appears to be working now 
> (but hasn't been checked with tape in it).
> 
> What I need now is a simple 60 ma. loop power supply to 
> feed the selector magnets. Basically, for now, I need it 
> only to supply the loop current  so I can get the printer 
> to respond to the keyboard.
> 
> Does anyone have a cheap and simple way to do this?
> 
> Dave, VY2AC


Dave, 

There are several approaches you can take, depending on your tolerance
for risk and the equipment you have on hand already.

1. Use an existing Terminal Unit for the loop supply. Most TU's have
   loop supplies built in, and will drive the 28's magnets directly. 

2. Use the supply built-in to some 28's. Many Model 28's come with a
   loop supply already installed, and you can use that if it's
   available. Check the shelves below the typing unit.

3. Construct a "cheap and dirty" supply that connects directly to the
   120 VAC line. I know this isn't politically correct, but we've all
   done it: if you are _CERTAIN_ that the magnets (and keyboard, etc.)
   are _NOT_ connected to the chassis, _AND_ that their insulation
   won't fail in the near future, you can simply rectify, filter, and
   limit the line voltage for a power supply. 

4. Construct a transformer-isolated supply. Any isolation transformer
   will do, or (as someone else suggested) you can put a couple of
   beefy 120/12 volt transformers back-to-back, and get isolation that
   way. R=E/I, so 120 VDC/60 MA = 2K Ohms. You'll need at least 7.2
   watts of heat disapation, so a 10 watt wirewound resister will do
   fine. Remember to filter the output _and_ to use diodes that can
   withstand a lot of peak-inverse-voltage, since the magnets will
   generate a lot of kickback during operation. 

   This is, by the way, the design used in most Teletype loop
   current supplies: the higher voltage assures "snappy" performance
   by minimizing reactive delays, and you can plug in additional
   magnets (i.e., machines) without worrying about changing the loop
   current. 

HTH.

73,

Bill W1AC