[GreenKeys] Why high loop voltages?

[Bob McConnell]

Brooke Clarke wrote:
> Hi Joe:
> 
> The high voltage is needed so that a resistance can be added into the loop.
> The time constant of an L R circuit is L/R and so by adding resistance you 
> reduce the time constant (speed it up).
> 
> If you do a DC test on the selector magnets you'll find you need a small 
> voltage and 60 ma to reliability activate the magnet.  But the time constant 
> will be L/R where R is some very small number so the time constant will be much 
> longer than the symbol time and so is too slow to work.

If you want to try some calculations on that L/R circuit, IIRC when the 
two selector magnets in a model 28 printer were connected in parallel 
for 60 ma operation, the total inductance was 1 Henry. That's right, the 
whole thing. No milli or micro fractions here. They were half a Henry 
each, so the series connection for 20 ma resulted in 1/4 Henry.

Bob McConnell
N2SPP