[GreenKeys] Model 15 remedial question

[Jim Cooper]

On 1 Jan 2022 at 11:14, Tom Hunter wrote:

> I use small 1:1 isolation
> transformers (from my junkbox) for 3
> of my main loops and for a local test
> loop in the cellar where I work on
> my model 15, 14 and 28 TD'S  and 28
> ROTR's . That gives me about 180
> volts. I start with a 2K, 10 Watt
> resistor then a 15K wirewound pot to
> set the current. 

Some newbies here might be wondering 
"why do i need such a high voltage?" ... 

after all, it is the current that is pulling up 
the selector magnet, so why can't I use a 
much lower voltage as long as the current 
is sufficient? 

The basic answer is TIME CONSTANT. 

The selector magnet(s) is a rather large 
inductance (L) being a coil wound with lots 
of wire ...  when voltage is applied to a coil, 
the current in the coil builds up SLOWLY, 
depending on two things:  the inductance (L) 
and the resistance (R) in the circuit. And 
the time is based on the fraction  L / R ... 

So you can see from simple math that to get 
a smaller time constant (faster rise of the 
current in the coil) you need a larger resistance. 

But if you have a large resistance in a series 
circuit, you need a large voltage to push the 
desired current ...  thus ever since "way back when" 
the standardized loop supply has used between 
100VDC and 120VDC with approximately 2000 ohms 
series resistance. 

Bottom line:  Baudot code at 60wpm has 22 millisecond 
'bits' ...  if your loop has a time constant of more than 
1 or 2 milliseconds, the 'range' of that 22 millisecond 
'bit length' that the tty can use is reduced -- which will 
show up on the RANGE FINDER dial as being less than 
0 to 100 ...  might go down to 40 to 60 which means the 
tty has a much shorter time to decide if the pulse is a 
mark or space ...  

So don't be chincy on your loop supply voltage !! 

w2jc