[GreenKeys] Current limiting

[Randy or Sherry Guttery]

Michael O'Day wrote:
> Randy said in his explanation:
> 
> "As noted above even with teletypes "designed" for low
> voltage keying - a certain "RiceGuy" has been known to
> replace the low voltage coils in his 35 and run it on
> a traditional loop (URA-8, etc.). Obviously - there is
> "wiggle room" here - particularly with the later
> machines where things weren't pushing the limits quite
> as hard as say the 15."
> Does that mean the 15 requires a higher voltage that
> the 33? I recently aquired a 15, do I need to build a
> HV (120+) supply for it, or can I run it on one of
> those Black Box RS232/Current Loop adaptors?

Yeah, I'm afraid that's what it means.  The "15 guys" will need to 
address this better - but IIRC the 15's selector has around 4Hy 
inductance - which would have a "raw" time constant of nearly 
60milliseconds - which is WAY over what the 15 can "range". We'd *like* 
to see a time constant less than 10% or better (i.e. less than 
2.2milliseconds) - which takes an additional loop resistance of um 
4/.003=1752 ohms (allowing for 66 ohms in the coil windings). So with a 
total resistance in the loop of 1752+66= 1818 * .06A = 110V (Ok 109.08V 
for you slip-stick jockeys). This give a really clean square wave on the 
magnets.  Of course this is all "theory" - real life likes to mock 
theory - and you - obviously - should be able to get away with less 
conservative numbers...

The CL50 can only handle 30V - however - there are two way to still use 
them (well, three actually) - 1) hang an external keyer transistor on it 
- there are several that have been recommended here that work well. 2) 
change out the keying transistors and the opto-isolator for higher rated 
ones - and disable the CL50s "source" parts. they won't work at the 
higher voltages. 3) use a relay (like a polar relay - or a (spike 
bypassed) reed relay.  Don is particularly fond of this method - as it 
is closer to "traditional" interfacing...

good luck - and best to ya'!
-- 
randy guttery

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