Helloe Daniel --

I have something close, but not exactly what you are looking for.  I have a TTY loop supply that runs from 12V DC.  It does have a boost converter to create a second voltage source of about 45 VDC.  When the TTY line goes marking the loop supply drives the magnets with 45 VDC and *no* loop series resistor (however there still is the resistance of the magnets, the fuse, the MOSFET switch and the current sense resistor).  With the 45 VDC applied to the magnets the current rises linearly up to 60 mA (takes about 3.5 ms) and then the microcontroller disables the MOSFET on the 45  V supply and then switches in the MOSFET for the 12V supply with a loop resistor of such a value as to cause 30 mA of current to flow through the magnets and stays at 30 mA until the line goes spacing.  When the line goes spacing the magnets are held active for another 2.5 ms to compensate for the 3.5 ms it took to get to 60 mA on the front end, and then the 12V supply is removed from the magnets.

If this is of interest I can send you the schematic.  The microcontroller needs to be changed to another part because of the world wide silicon shortage.  Have to move from a $1 MCU to a $2.25 MCU.  At least the $2.25 MCUs are still available.

Paul - ad7i