[GreenKeys] [External] Re: low Voltage Loop Supply

[Jones, Douglas W]

From: Jim Haynes [jhhaynes at earthlink.net] -- Thursday, March 24, 2022 5:31 PM

> I've seen some published circuits for a selector magnet driver that
> takes advantage of the constant-current characteristics of a transistor
> to make the loop current rise faster than it would with a simple
> resistor. 

See here:
-- https://homepage.divms.uiowa.edu/~jones/step/current.html

I wrote that on the subject of current limiting for motors, but the theory is exactly the same with current loop supplies.  Figure 4.2 shows the impact of different voltages on the rise and fall time in the motor (or loop) current with a series resistor used as a current limiter.  Figure 4.3 shows a simple saturating bipolar transistor current limiter, and Figure 4.4 shows the waveform you can get with it.

For a TTY current loop, I wouldn't bother with a switching limiter, either open loop or closed loop, but it could be done.

         Doug Jones
         jones at cs.uiowa.edu