[GreenKeys] Loop supply voltage

[Jones, Douglas W]

On Saturday, January 18, 2025 at 09:07:04 PM MST, steve bennett via GreenKeys <greenkeys at mailman.qth.net> wrote:

> But once the coil reaches steady state you are just left with the resistance in a DC circuit.
> So if the goal is to have 60mA at steady state 100v is way too much. What am I missing?
> The loop supply is DC correct? Maybe that is where I'm confused. Is the loop AC?

The loop supply must be a current-limited DC supply able to push 60mA in the DC steady state, but with sufficient voltage to overcome the coil impedance.  Remember, it's only DC when the line is in the steady marking state.  If someone is sending RYRYRYRYRY (or whatever), the line will be a square wave or close to a square wave depending on stop bits.

Using a dropping resistor with a high voltaage is the classic way to meet the supply requirements.  That's because current loop transmission is a century older than transistors.  In the modern age where we worry about energy efficiency, we'd use a switching current limiter.  Those are actually cheaper than dropping resistors in some cases because little ICs and switching transistors can be cheaper than high-wattage resistors.  See this tutorial on current limiting I wrote 30 years ago:
-- https://homepage.divms.uiowa.edu/~jones/step/current.html

            Doug Jones