[GreenKeys] Of current loops and voltages.. (M15 20mA / 60mA)

[Alf Fisher]

drlegendre (name?)

To a large extent, it depends on the machine.
In most of the machines I have come across, it is required to drive a current through an electromagnet which, having a high inductance, will do its best to stop that current rising to the desired value hence the need for the high driving voltage to overcome the effect of the inductance.  There is a good explanation of this on RTTY dot com. 
In the Teletype model 33 (and others),  it is only necessary to drive a small current into some resistors to turn on a transistor and the work of driving the electromagnet is done by a driver transistor in the machine so that case, low voltages like 24 volts can be used.

73
Alf
G3WSD


From: drlegendre . 
Sent: Tuesday, September 15, 2015 1:48 AM
To: Green Keys 
Subject: [GreenKeys] Of current loops and voltages.. (M15 20mA / 60mA)

When I began my M15 project, I was under the impression that all of these machines required the 'standard' 120V DC / 60mA loop supply. In short (no pun..) 120VDC squeezed through a 1K8 - 2K resistor. 

Also was expecting to find a ~50R selector magnet coil. What I found was a dual-coil setup that - through the miracle of DPDT - allows the machine to run at 20mA or 60mA.

So if 120VDC / 2K was the expected supply for the 60mA setting, what open-circuit voltage / resistance was expected for the 20mA setup? 

Finally, did I just waste a lot of time building my RS-232 -to- 120VDC / 60mA converter? Could I have just as well used one of the recently rebuilt Camiacs Mdl. V RS232 -to- 20mA converters already in inventory?

IIRC, the Camiacs supply an o/c voltage of 24 VDC - seems low, in that the selector magnets are in series in the 20mA position, which means the inductances sum, and as such, should produce a lot more resistance to current rise across the windings? 

(In the real installation, the lines are very short, a matter of a few feet - so it's not like there are any long-line effects to worry about. But what's the theory?)


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