Model 33 wiring complexity

Tony Duell ard.p850ug1 at
Wed May 10 13:06:02 CDT 2017

On Wed, May 10, 2017 at 6:22 PM, Shoppa, Tim via cctalk
<cctalk at> wrote:

> I usually also found a useful current loop supply in the base. Sometimes I would find
> bipolar relays and 20mA/60mA conversion supplies. I was a little surprised at some
> of the current loop supplies - some of them weighed 20 pounds and were obviously
> made for driving exceedingly long lines (open-circuit voltage way over 100VDC).
> Seemed odd these were in there considering the units had modems just a foot
> away from magnet and keyboard switch.

Not necessarily for long lines.

Some teleprinters had the receiving magnet directly in the current loop. The
resistance of this was too small to limit the current to 20mA (or whatever) so
a series resistor was added.

Now the time constant of an LR series circuit is L/R. In other words a higher
resistor gives a shorter time constant. You get more rapid operation with a
higher voltage supply as you then have a higher resistance circuit.


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