[GreenKeys] Resistors

[Jeffrey D Angus]

Lee Mushel wrote:
> I really don't understand this on-going loop resistor thing.
> I was relieved when the behavior of the magnetic field in an
> inductor was finally explained on this reflector.
Somewhere around 1974 while playing with my first Model 15, I wondered
"Why do I need 120 VDC when it only takes 6V to pull in the selector?".

Well, just like trying to get a square wave through a series resistor 
and a shunt
capacitor, there's this thing called dv/dt for capacitors and di/dt for 
inductors.
(also known as time constant)

Fiddling around with a piece of chart paper and some values I drew up the
change across the coil with a series resistor of 200 ohms (the coil DC
resistance) and with a series resistor of 2000 ohms (requiring a higher
compliance to the current loop, ie,e more voltage)

The results were stunning. Oddly enough, a 10 to 1 improvement in the
shape of the square wave in that it looked like a square wave instead of
a ramped time constant greater than the signal time period.

But it's easy to see how this knowledge "went away". Back "then" it was
just "done that way, because we've always done it that way." Tubes and
B+ was readily available everywhere and even if the reasons for doing it
that way weren't clear, it worked.

Time marches on and things change. The reasons for doing things get
lost over time. The requirements of the systems change as well, such
as the model 28s and 35s as opposed to the model 20s and 15s.

Jeff-1.0
wa6fwi