[GreenKeys] Loop Supply - Update

[Paul Heller]

Jack, could you post the parts list as ordered from Mouser? That might make it easier for those of us who don't have suitable extras hanging around and have to place an order.

How much heat is the resistor giving off? I was planning on putting mine in an enclosure, but need an idea of what a suitable enclosure would be.

Thanks for your postings!

73,
Paul
KD0DFR

On Aug 5, 2012, at 1:45 PM, Jack wrote:

> 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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