[GreenKeys] Loop supplies, or how to drive a selector magnet.

[Jeffrey D Angus]

On 5/18/2016 5:23 PM, John Nagle wrote:
> The graph shows voltage across the selector magnet, and current
> through it.  The selector magnet has an inductance of 4 henries and a
> DC resistance of 55 ohms, which is standard for a Model 15 Teletype.
>
> http://www.aetherltd.com/public/misc/techdesigns/selectorcurrentsimple.png

Which is what I've been saying all along.
The reason the loop voltage is 120 VDC is to overcome the di/dt of the
coil inductance when the loop is switched on.

If you drop the loop voltage and decrease the R value accordingly to
limit the current to 60 mA, the amount of time for the current through
the coil to rise enough to pull in the selector magnet becomes excessive.

The graph shows that it takes roughly 5 mS to reach 90% of the
required coil current to operate properly.
The range adjustment "looks" at the selector magnet between 10 and
12 mS. If you drop the the loop voltage down to 60 VDC, the time it
will take for the selector magnet coil to reach sufficient current to pull
in will increase to 10 mS. At best, this makes the transition from Space
to Mark marginal. Add any additional distortion to loop and it becomes
unusable.

Additionally: If you add a second machine (selector magnet) in series
with the loop, it will double the effective inductance from 4 Hy to 8 Hy.
Again, doubling the amount of time before the current through the
coils is enough to operate the selector magnets.

If you put two machine selector magnets in parallel, each magnet will
only see 1/2 of the loop current. i.e. 30 ma which, again, is insufficient
to pull the selector magnet in.

This is basic electronics 101, it hasn't changed since 1930.

> What this tells us is that the efficient way to run a selector magnet
> is to charge up a capacitor to 120V, and on SPACE to MARK, dump
> the energy in the capacitor into the selector magnet to pull it in.
This has also been a standard technique to get solenoids to actuate
quickly and then only require a small holding current to keep them
in position. But the point here is that either way, it takes an initial
open loop voltage of 120 VDC to overcome the inductance of the
selector magnet to get the di/dt fast enough to have the selector
magnet in the correct position when the machine "looks" to see
what position it is in based on the range adjustment.

Also: Why 60 Ma? The short answer is the term "Ampere Turns."
Flux density (the magnetic field) of a coil is based on the product
of current through the coil times the number of turns. The 60 Ma
is required to have enough magnetic force to actuate the moving
part of the selector magnet (armature).

And lastly: why are we wasting all that power in a 2K resistor?
That's simple too. You're in a local loop and not having to deal
with the series resistance of several miles of wire between both
ends of the connection.



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

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