[GreenKeys] Loop Supply - Update

[Jack]

ZCZC

 

I received my parts from Mouser and immediately wired up

the transformer to see what voltages I would get. While the

circuit as drawn works fine, the loop voltage is around 200V

and that might make some of you nervous (although the TTY

will handle it OK). With the full-wave bridge rectifier, that is

the voltage I expected.

 

I tried a single diode rectifier (1N4007) with a 22 ufd capacitor 

across the full 120VAC secondary and was able to

obtain 60 milliamperes with a 2000 ohm 25W resistor at

125VDC (in fact it calculates to approx. 62.5 ma., the Bell standard).

 

The key element is the transformer. It will provide plenty of

capacity for multiple TTY's in the loop. If you use each half of the

center-tapped secondary, you could have two separate loops

at around 85VDC each. For local loop use, this works, too.

(In the old days, some of us used to use two D cells

for a "loop supply"! It's amazing what you can accomplish when you

have no idea what you're doing.)

 

I would suggest that anyone who has not wired such a circuit

be very careful with the AC and DC connections. Feel free to
substitute parts

such as the capacitor, diode(s) or resistor. But keep in mind that the

resistor has to dissipate power and it has to be at least a 25W

type. 

 

73,

Jack K0TTY

 

NNNN

 

 

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