[GreenKeys] low Voltage Loop Supply

[Jim Cooper]

On 24 Mar 2022 at 16:20, Tom Hunter wrote:

> The purpose of a high voltage loop
> supply is not to overcome loop
> resistance but to build the current
> up quickly, in spite of the selector
> magnet inductance, until it reaches
> the value set by the current
> limiting resistor - 20 or 60 mils. 

Exactly .... the 'newbies' to this RTTY game 
forget, or just are not aware, of the effect 
of TIME CONSTANT on square wave current 
flow ...  bottom line is that the Time Constant 
depends on the simple fraction  L / R   .... 
inductance divided by resistance ...  thus for 
a fixed inductance (selector magnets) the larger 
the loop resistance is, the FASTER the current 
will change from zero to 60mA (or 20mA) ...  

In correlation, the higher the loop resistance is 
the higher the voltage (pressure) must be to get 
the required current flow (60mA or 20mA).  

Thus the reason original Teletype loops used 
120VDC and 2000 ohm resistance (approx.) ... 

It's tempting to want to use lower voltages, 
but the "Laws of Physics" don't change just 
because we want them to. 

w2jc