[GreenKeys] Loop power supplies

[John Nagle]

From: Jim Haynes
> I haven't tried it, but there are circuits that use the "constant current"
> properties of a transistor rather than a fixed resistor, and are said to
> give equivalent results with lower voltages such as 48V.  For instance
> QST, Jan 1972, p. 40.  Maybe someone who knows how can do a SPICE
> simulation to compare such a circuit with a plain resistor.

    At the output end of my loop driver, I have a current regulator.
It's a DN2625 depletion mode MOSFET in series with a 22 ohm current
sense resistor, and will deliver 60mA into loads from 220 ohms down to
a dead short. But the input is a capacitor charged to 120V and a 15V 
sustain supply, not a constant 120V supply.  You'd need a much larger 
MOSFET to replace a ballast resistor. With the typical Teletype loop 
supply, about 95% of the energy goes into heating up the ballast 
resistor.  If you're going to use a MOSFET for that job, you need
a big one.

    If you just want 120VDC, one option is

https://www.digikey.com/product-detail/en/cui-inc/PDM2-S5-S24-S/102-3007-5-ND/4006975

which is a 5V to 24V 83mA isolated DC-DC converter in a tiny
package. Price is 5 for $28.12. Put 5 of those in series and
you have a loop supply. You'll need a 10W 5V wall wart to
power them.  I've used those little DC-DC converters for other
purposes.

				John Nagle