[GreenKeys] Loop supply

[Peter Gottlieb]

One of our members asked about the loop supply I built and I sent him a 
diagram, but then thought others might be interested as well.  I put it 
at http://petergottlieb.com/TTY_loop.jpg
if anyone wants to take a look.

Note, I use a pretty high loop voltage to get very crisp operation.  
Don't touch during operation!  The snubber across the selector magnet is 
also optimized for very crisp and fast operation.  This is a classic 
current source, keep the comp diode in there to track temperature.  All 
I did was add a 2N2222 to switch the current source.  If you look at 
current on a scope, this circuit is very clean and due to the high 
voltage has a very fast rise time even with large inductances.  I used 
an old CPU heatsink, about 1.5 inches square, for the MJE13005.  I have 
the heatsink on a perfboard, it is "hot" as I did not want the added 
thermal resistance of insulation.  The power resistors are not critical, 
just get close.  Every part in this was from my junkbox.  Opto couplers, 
big capacitor, diodes, resistors, from a computer power supply, other 
stuff whatever I had laying around.  This puts out 60 ma, if you want 20 
ma you will need a lot more resistance in the emitter of the MJE to 
ground.  I didn't finish the data path from the loop out but it is easy 
and there are many designs out there to use if you don't feel like 
experimenting.

I get rock solid copy on my 2B if my signal is halfway decent.

Peter