[GreenKeys] Current Loop Question

[Jim Cooper]

On 27 Sep 2020 at 18:01, GodOfCows wrote:

> I understand the Teletype 32 uses a
> 20 mA current loop, does anyone know
> the voltage of this loop? 

standard practice is to use a 120vDC voltage 
source with the appropriate resistor to limit 
current to 20mA (be sure your resistor wattage 
is sufficient). 

the reason such a high voltage is used is because 
of the TIME CONSTANT with square wave keying; 
L/R    L being the selector magent coil, basically, 
R being the limiting resistor.  So the larger the 
resistor, the lower the time it takes to get from 
0 to max. current ... 

w2jc