On Sun, Apr 20, 2025 at 7:36 PM Paul Koning via cctalk <[email protected]> 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
