[GreenKeys] Model 28 Drive Voltage

[WA5CAB at cs.com]

Bob,

I thought someone else would jump in here and explain this but it does not 
matter what you use for a switch.  Or whether you're running pulling magnets, 
holding magnets, 20MA or 60MA loop, etc.  You can't operate an M14/15/19/28 
selector off of voltage just high enough to get the required steady state current 
through the resistance of the magnet wire.  24VDC is plenty high enough to get 
60MA through any of the selector magnets.  But if you try using a 24VDC loop 
supply, your error range even on a local loop will be zero.

Selector magnets have inductance.  Inductance impedes the rise and fall of 
current.  For successful operation, the current rise and fall in the selector 
magnets should be as nearly a square wave as practical.  To achieve this you use 
a current source, not a voltage source.  The ideal current source consists of 
an infinite voltage in series with a variable resistor.  The resistor swamps 
the inductive reactance and makes the circuit essentially resistive (i.e., 
fast current rise time).  In practice, with Teletype Corp selector magnets, 
115VDC in series with 0-2500 ohms is a practical compromise.  The vacuum tube just 
replaces the resistor.  It's only practical advantage over the variable 
resistor is that it is connected as a current regulator and within limits, does not 
require manual adjustment if you jack another machine into the loop.  "Within 
limits" means that you can't jack an infinite number of machines into the 
loop.  

I have a dim recollection that the loop supply with some of the vacuum tube 
regulators may be lower that 115VDC, taking advantage of the fact that the 
current regulator's response time is short WRT one selector pulse width.  But you 
have to build a lot of loop supplies to save any money on the difference.


In a message dated 5/23/2004 9:43:07 PM Central Daylight Time, ham at cq.nu 
writes: 
> Each of the 28's I have sitting here have a cute little box in them. 
> The box has a high voltage supply and a vacuum tube to switch the 
> magnets. I had more or less assumed that it was a standard part of a 28 

Robert & Susan Downs - Houston
<http://www.wa5cab.com> (Web Store)
<wa5cab at cs.com> (Primary email)
<wa5cab at houston.rr.com> (Backup email)