[GreenKeys] Current limiting

[WA5CAB at cs.com]

Chris,

There was a lengthy thread on this subject I think early last year.  But the 
short answer is no.  It has nothing directly to do with length of line.  And 
everything to do with the ratio of XL to R in the loop.  It is the nature of an 
inductor (coil, i.e., selector magnet) to resist or retard a change in the 
magnitude of current flowing through the inductor.  So, when the circuit changes 
from Mark to Space, the inductance keeps the current flowing for a time 
determined by the ratio of L to R.  And retards the current magnitude rise in Space 
to Mark.  The smaller the ratio (the larger the R or smaller the L), the 
faster the current will drop or rise to the other value.    

As a practical matter, if you put a 'scope across a small current sense 
resistor in the loop, you can see this at various loop supply voltages.  With a 
Model 15 with a pulling magnet selector at around 24 VDC loop supply the current 
pulse through the magnets (if you drive with alternating one unit Mark and 
Space) goes to a triangle and the selector range goes to zero.  As you increase 
the loop supply voltage (holding the current fixed at 60 mA) the pulses begin 
to square up and the machine range increases in response.  The minimum 
practical loop supply voltage for an M15 with pulling magnets in a local loop is 
somewhere around 60 VDC.  

As an aside, besides the fact that it worked and didn't cost significantly 
more, the reason for settling on a nominal 115 VDC was probably that in the 30's 
there were a lot of systems around where the main source of electricity was 
115 VDC.  Especially in portable (military) and ship-board systems plus some 
commercial distribution hanging on.  The motors and relays were also 115 VDC.

In a message dated 1/20/2009 7:33:34 AM Central Standard Time, 
chrise at pobox.com writes: 
> I'm a little green on current loop fundamentals-- having grown up in
> the RS232 age...  but is the high voltage really neccessary if you are
> not running a long distance with the loop?
> 
> In other words, could a modern constant current source using say, LM317
> be built that provides a nice stable 20 or 60mA but with only 12, 24 or
> 48v DC source?
> 
> Is there something in the older TTYs that requires a higher voltage on
> the loop?
> 
> My ASR-33 runs with 20mA sourced from 12v but that is pretty "modern"
> compared to the 5-level stuff you guys are working with I am sure.
> 

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