[GreenKeys] Model 28 signal lines

[Jim Cooper]

On 16 Oct 2019 at 16:13, B M wrote:

> Can I use the 115v power coming into
> the teletype as the source for the loop
> current (after the full bridge
> rectifier to convert to DC and the
> power resistor to limit to 60ma)? 

I guess you are refering to the AC voltage 
coming into the cabinet ...  

If you use the 120VAC then yes, you will have 
to build (or provide) a rectifier to create the 
120VDC for the loop. 

In the 'old days' these rectifier supplies were 
really, really big and heavy (not sure just why, 
but they did not have the fancy components we 
have these days). 

A 1:1 isolation transformer WOULD be a good 
idea, just to keep the loop isolated from the 
house voltage.  I suppose these days there are 
not many "filament transformers" around, but 
when tubes were popular we used to use two 
filament transformers ....  120vAC to 6.3vAC 
from one transformer, them 6.3vAC to 120vAC 
through the second transformer ...  served the 
purpose and was fairly inexpensive. 

Not sure if you can find a cheap 1:1 isolation 
transformer for   120vAC : 120vAC these days. 
If so, a half amp rating on the 120v side should 
be safe enough. 

A full-wave rectifier circuit is not a bad idea; 
even a bridge circuit since diodes are so cheap 
these days.  You want lots of capacitance to make 
nice DC ...  the better the loop supply you provide, 
the better "range" you will see on your printer 
adjustment. 

A little bit of technical 'refresher' as to WHY it's 
so important to use the higher voltage ...  it has to 
do with the "time constant" related to the inductance 
of the selector magnet (L) ...  for a given inductance, 
the higher the series resistance is (R) the faster the 
square wave will go from zero current to full current ... 
and the faster the square wave switches, the more of 
the 'square' you can use for operating the selector 
magnet (this a wider Range adjustment on the printer). 

> Is an isolation transformer needed?

see above ... 

This turned out to be more of a dissertation than 
I had expected, so I'm going to copy the GreenKeys 
list with it ... 

Jim  W2JC