[GreenKeys] Loop Supply - Update

[COURYHOUSE at aol.com]

we need the diagram to!
 
 
In a message dated 8/5/2012 2:05:06 P.M. US Mountain Standard Time,  
paul0926 at comcast.net writes:

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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