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

[Bob Camp]

Hi

If you really want to go crazy, all the supply needs to do is to put enough
voltage on the magnets to get the current going. It’s no big surprise that an 
expotentialy decreasing waveform will do this quite adequately. 

The gotcha is that you double the needed voltage when you add a second set
of magnets. That really messes up some parts of the design ….

Bob

> On May 19, 2016, at 2:23 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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