[GreenKeys] Looking For Bits

[WA5CAB at cs.com]

Keelan,

You didn't say whether you are running a 60 or 20 mA loop.  And I have 
little experiance with M28's and none with M32's.  But 70 volts is near the 
minimum upstream voltage for acceptable range with two machines in a 60 mA loop. 
 This has been explained here many times before but the reason for the 
higher voltage than it takes to just pull in the selector magnets is something 
called inductance, not the simple resistance of long lines (which also have 
inductance).  It is the nature of inductance to resist any change in the 
current flowing through it.  Ideally, you want the current flowing in the 
selector magnets to be a square wave.  Either 60 (or 20) mA or 0 mA.  In a classic 
loop, this is achieved by running a supply whose output voltage is high 
enough to require  a series resistor whose resisistance is large compared to 
the inductive reactance of the selector magnet coils.  At around 60 volts with 
one M15 pulling magnet selector the current waveform through the coils is 
nearly a square wave.  And you will get around 90% of the range that you can 
get with that particular machine when running from a 130 volt supply.  What 
the magic number is for an M28 I don't recall.

As you jack in more machines, you increase the series inductance in the 
loop plus have to decrease the series resistance (because the coils do have 
resistance as well as inductance) in order to keep the marking current at 60 
mA.  If you monitor the loop current with an oscilloscope, you will see the 
departure from square.  The leading edge begins to round off and the trailing 
edge trails out before it returns to zero.  As you decrease the loop supply 
voltage (and decrease the series resistance to match) you reach a point 
where with althernate marks and spaces the loop current never reaches 60 nor 
returns to 0.  This point will obviously be reached sooner (at a higher 
voltage) as the baud rate goes up because the bit width decreases..  

In a message dated 08/27/2011 22:14:21 PM Central Daylight Time, 
keelan at mail.grenander.com writes: 
> This made me think of a more general question... My current loop supply 
> outputs somewhere around 70 VDC; is there any need to go to 135 VDC when I'm 
> running only local loops? As I understand it, the high voltage was needed 
> to overcome the adverse electrical characteristics of a long loop.
> 
> The only anecdotal evidence I can provide is that my M28 and M32 both seem 
> more than happy operating on a 70 volt loop.
> 
> - Keelan

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480