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

[Cory Heisterkamp]

Still haven't seen enough designs here utilizing vacuum tubes...

> On May 19, 2016, at 1:23 PM, Jeffrey D Angus <jdangus at att.net> wrote:
> 
>> 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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