[GreenKeys] Current Loop Question

[Keith Lueck]

 I don't mean to nit pick here, but in the case of a 32/33, you're looking into the selector magnet driver card which is transistorized, so, there's no magnet inductance to worry about.  In fact, the 101C modem has only a +/- 20V DC power supply.  I'm sure that for a unit without a modem (for leased / private line applications), either there's more circuitry in front of the SMD (maybe just a resistor?), or a different SMD card is used so that a "standard" (high) loop voltage can be used, but I'm not sure.  I'd be curious to know.  Wayne???  ;-)
Keith
    On Sunday, September 27, 2020, 7:02:56 PM CDT, Jim Cooper <jim.w2jc at gmail.com> wrote:  
 
 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 



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