[GreenKeys] Loop currents

[Ralph Irish]

​I've not been following the thread on loop currents closely, but after reading a few of the later things, 
it appears that some think that by just changing the series resistance in their loop they can make 
things work at either 20ma or 60ma. 

It is whether the selector magnets are in SERIES or PARALLEL that determines the need for 
either 20ma or 60ma. I'm not going to lobby for one situation or the other, just that all of my ham 
experience in RTTY was using 60ma for selector magnets wired in series. 

As someone said recently the higher current tends to keep the keyboard contacts clean with just 
a small bit of arcing as the contacts make and break. 

Another GreenKeys participant was looking for a 'regulated power supply', maybe thinking that 
starting with that would be best. I pointed out to him that the standard loop supply in many Model 
28 machines was a simple transformer rectifier and a capacitor to 'smooth out the bumps' created 
from rectifying AC. I've never measured the output voltage of these supplies, but they always 
worked well for me in the "LOCAL" situation when I wanted to use the machine 'off line'. 

- - - - 

Back in the 'good old days', Irv Hoff, a prolific designer of TUs, experimented with very high 
voltage loop supplies. As high as 250 volts! All he needed to add was enough series resistance to 
limit the flow of loop current in the selector magnets to 60ma. Theoretically, this much higher 
voltage made the sides of the 'square waves' created as MARKS and SPACES were created in 
the decoding of an RTTY signal, steeper. The steeper the sides, the less error. But, this had a price, 
as well. TOO MUCH voltage on those keyboard contacts caused erosion and eventual failure 
of the contacts. 

Loop supplies of 120VDC to 150VDC seemed to be what most manufacturers used in the commercial 
TUs on the market. 

The built in supply in most Model 19s is more than adequate for the operation of several sets of 
selector magnets. Some Model 19s came with a 'shelf' over the TD. This shelf held a model 14 
reperf for making tape from in incoming signal. There was some 'switching' somewhere that allowed 
the loop current to flow in both sets of selector magnets. Sometimes making a tape without bothering 
to make page copy was required and this arrangement allowed tape to be made without wasting 
paper in the process. In this situation, the loop current flowed only in the selector magnets of the 
reperf. 

Determining whether the magnets are in series or parallel determines the amount of needed series 
resistance to supply 20ma (series) or 60ma (parallel) to the loop. 

Ralph - W8ROI 


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