[GreenKeys] Of current loops and voltages.. (M15 20mA / 60mA)

[Jeffrey D Angus]

It all comes down to the key variable in transformer design.
Ampere Turns.

And simple electronic theory.
1. It takes an infinite amount of current to force the voltage to rise 
instantly
across a capacitor.
2. It takes an infinite amount of voltage to force the current to rise 
instantly
through an inductor.

(Tangentially related to the CRT question)
Typically, early 'scopes used a simple RC time constant for the sweep
oscillator, this resulted in a curved wave form rather than a linear saw-
tooth. Using a constant current source allows for a linear sawtooth.

Back to getting coils to move mechanical parts fast.
The whole point of the 120 VDC loop supply, with it's attendant 2K ohm
or so total resistance, is to provide enough voltage compliance to force a
fast rise time through the selector magnets.

You can see how this works by playing with a circuit simulator like LTSpice
and monitoring the current rise time through a 4 Hy inductor and varying
the square wave source driving it from 12 vdc @ 200 ohms to 120 vdc @
2000 ohms. Look at the time delay required to rise to 80% or 48 Ma.

Most telephone exchanges had no shortage of 48 vdc to run things. It
should be obvious that they went through to additional expense of using
120 vdc for good reason.


-- 
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com