[GreenKeys] Loop driver question

Wed Apr 24 11:50:40 EDT 2013

I have not tried to prove this, but I suspect the original high voltage,
constant current, design choice was determined by the need to drive the
machine from a central office, down a long copper pair.  And I should note
that on very long circuits, the CO loop would drive a polar relay in the
machine rather than a selector magnet directly.

Lester B Veenstra  MØYCM K1YCM W8YCM

lester at veenstras.com

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From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Keith Lueck
Sent: Wednesday, April 24, 2013 12:42 AM
To: GreenKeys at mailman.qth.net; Ken Schwieker
Subject: Re: [GreenKeys] Loop driver question

Ken - 

If you want to go w/ a lower voltage, and not compromise current rise time,
you can go w/ a constant current source.  Using that with a 50 V supply, you
can achieve the same rise times (about 5 msec) that you can get with the
traditional 120V / 2KOhm series resistor.  If you're interested, I can send
you some LT Spice simulation files of a suitable circuit...  A bonus is that
it's pretty easy to drive the current source w/ a junkbox optocoupler like a
4n25...

cheers,

Keith

--- On Tue, 4/23/13, Ken Schwieker <ksweek at mindspring.com> wrote:

From: Ken Schwieker <ksweek at mindspring.com>
Subject: Re: [GreenKeys] Loop driver question
To: GreenKeys at mailman.qth.net
Date: Tuesday, April 23, 2013, 8:39 PM

Thanks Eric for the web site and the reply. Now I need to figure out 
if I can squeeze sufficient ma from the Tempest Dovetron output as is 
or do I need to add a transistor stage inside. I presently don't have 
the Dovetron or any paperwork at this location.

The previously mentioned transformer has a center tap so I could run 
the PS at 50 volts DC out but am thinking that might start to get 
close to marginal on range if used on radio reception. Might be an 
interesting test to see if there is any range contraction in going to 
50 volts from 100 volts (each with the appropriate series resistance).

Ken S

At 05:44 PM 4/23/2013, you wrote:
>On Tue, Apr 23, 2013 at 10:34:33AM -0500, Ken Schwieker wrote:
> >
> > Wanted to find out if anyone has had experience with the CEL
> > PS7141E-1A-A optocoupler used as a loop driver. Found it in a list
> > on the following website.
> >
http://heepy.net/mediawiki/index.php/Optocouplers_for_teletype_current_loop
> > Do not know who produced this website but thanks. Found that the
> > PS7141 is available from Mouser at $1.56 each. Some of the others
>
>That's me :)
>
>The PS7141E is what I shipped with almost all of the 100+ little demod
boards
>I was selling on here a while back. They work fine at normal TTY speeds.
They
>would not be suitable for "modern" telecom speeds - the turn-on time 
>is hundreds
>of microseconds to milliseconds depending on LED drive current. 10 mA gets
you
>under 1 ms which is sufficient for teletype. under 5mA they get very 
>very slow.
>One nice side effect of them being slow is that that limits the creation of
>inductive spikes :)
>
>eric
>
>
>
> > optocouplers are somewhat more difficult to source including one
> > that was a special order with a 100 min.  I plan to build an
> > external loop driver for a Tempest Dovetron but will have to see if
> > the Dovetron can output around the 10 ma necessary for the input to
> > this optocoupler.
> >
> > Any inputs appreciated.
> >
> > Ken S
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > =
> >
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