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

[Nick England]

For the archives, here's my earlier reply to Bill (a.k.a. Adrien-Marie
Legendre?)

---------- Forwarded message ----------
From: Nick England <navy.radio at gmail.com>
Date: Mon, Sep 14, 2015 at 10:04 PM
Subject: Re: [GreenKeys] Of current loops and voltages.. (M15 20mA / 60mA)
To: "drlegendre ." <drlegendre at gmail.com>


Approx 120VDC / 6K for the 20ma circuit. Compared to the 120v/60 ma
circuit, L is 4X and R is 3X so L/R is a little longer time constant
meaning a little less timing range.

The problem with a passive 24v/20ma circuit is indeed the L/R time
constant - Compared to the 120v/60 ma circuit, L is 4X and R is approx
1/2 so L/R is 8X. High voltage yields crisp snappy selector magnet
action, giving far more timing range and tolerance to signal
distortion. More analysis starting at page 26 of the RTTY handbook
http://www.navy-radio.com/manuals/tty/RTTY_Handbook_1982.pdf

UNLESS - your 24v gadget really is an active current source that
places the full 24v supply across the coils to get them electrons
flowing asap. Teletype's selector magnet drivers did this with a 40v
supply but 24v might work.
http://www.navy-radio.com/manuals/tty/sel_mag-15-19-20-28.pdf

But I think 20ma RS-232 converters like that were designed to drive a
20ma signal loop where the M33 teletypes always had internal selector
magnet drivers. The loop was essentially resistive so switching times
weren't an issue.

IIRC one of the greenkeys guys has made a 5v converter with a charge
pump feeding a HV cap to get the armature moving and then providing
sustaining current from the 5v

I recommend 120v/60mA for simple reliable operation.
Good luck
Nick England K4NYW
www.navy-radio.com