20 Apr
2025
20 Apr
'25
8:43 p.m.
On Sun, Apr 20, 2025 at 7:36 PM Paul Koning via cctalk
<cctalk(a)classiccmp.org> wrote:
It also works well with inductive loads, which is why
you find it with Teletype machines. Early ones (the model 15 for example) used a 60 mA
loop current; the model 33 uses 20 mA. In either case, one would typically use a rather
high supply voltage with a correspondingly large series resistor to set the loop current.
If the resistance is substantially greater than the inductive impedance of the receiver
solenoid, that works well and the resulting waveforms are not badly distorted.
Another way to think about it :
It is well-known that the (voltage) time constant in an RC circuit is
given by R*C. It is less well-known that the (current) time constant
in an LR circuit is L/R. So by making R as large as possible you get a
low time constant, the current gets close to the final value more
quickly.
The receive 'magnet' (solenoid) in a Creed 7 teleprinter (more common
that most Teletypes over here) will normally operate on about 12V. But
if you try a 12V supply the signals are so distorted that the thing
wll not print reliably. We used to use 80V with a suitable series
resistor.
-tony