Model 33 wiring complexity

Paul Koning paulkoning at
Wed May 10 13:22:41 CDT 2017

> On May 10, 2017, at 2:06 PM, Tony Duell via cctalk <cctalk at> wrote:
> 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.

I'd put it this way:  For teletypes, the receiver is predominantly an inductive load.  And it wants a fairly short rise time, so you pick the time constant you need, which gives you R, which in turn gives you the needed drive voltage.  And that works out to be a fairly high voltage in a lot of cases, especially with the older machines with 60 mA loop current.  The needed loop R is almost entirely the series R; the receiver coil adds very little to the total.


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