[GreenKeys] New 60ma current loop driver board design

[John Nagle]

On 10/28/2016 08:50 AM, Jim Haynes wrote:
> That looks wonderful.  Maybe you could superimpose on the picture the
> result of using the standard 120V 2Kohm circuit to show that yours is
> about as good.
 > Maybe you could also show the effect of trying to drive
> two selector magnets in series, such as two printers.

     You can run LTSpice and try that yourself.

     This board is only designed to run one selector magnet pair.
Beyond that, you'd need active current control. That's
quite possible.  For that, I'd put an isolated current
sensor on the high side and feed it back to the low side,
using something like this: http://www.ti.com/lit/ug/slau521/slau521.pdf
I'd probably replace the 555 timer with an 8-bit CPU with
A/D inputs, probably one of the small PICs such as the
PIC12 in an 8-pin package, and give it a current feedback
input so it could adjust the boost supply timing for for 60mA
over a range of loads.

     That's more complexity than I wanted to deal with
in this initial design.  It's not like there's a big user
base for multiple-Teletype current loop drivers.

				John Nagle