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

Thu May 19 17:30:07 EDT 2016

I once used a 9V battery for a quick "loop" test...!
Sounds like we're getting into the "analysis paralysis" mode.

Let's talk about oil......

Jack K2TTY

-----Original Message-----
From: GreenKeys [mailto:greenkeys-bounces at mailman.qth.net] On
Behalf Of Bob Camp
Sent: Thursday, May 19, 2016 5:13 PM
To: Jeffrey D Angus <jdangus at att.net>
Cc: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Loop supplies, or how to drive a selector
magnet.

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/selectorcurrentsi
mpl
>> e.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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