[GreenKeys] Current limiting

Tue Jan 20 11:30:45 EST 2009

Randy,

Re: the 130 VDC and the voltage while "under load" i.e. passing thru the 
magnets...current flowing.

Would it be a good idea to have a large cap across the 130 VDC supply output 
to hold up the voltage during the keying of the magnet?  I would guess a 
"solid" 130 would be better for keying than the voltage excursions on the 
output side of the 2 Kohm resistor.

lar
----- Original Message ----- 
From: "Randy or Sherry Guttery" <comcents at bellsouth.net>
To: "Chris Elmquist" <chrise at pobox.com>
Cc: <greenkeys at mailman.qth.net>
Sent: Tuesday, January 20, 2009 11:15 AM
Subject: Re: [GreenKeys] Current limiting

> Chris Elmquist wrote:
>
>> but is the high voltage really neccessary if you are
>> not running a long distance with the loop?
>
> There isn't a simple "pat" answer to this - some of us have indeed been
> "playing" with "low voltage" loops - and the answer is:
> It depends.
>
>> In other words, could a modern constant current source using say, LM317
>> be built that provides a nice stable 20 or 60mA but with only 12, 24 or
>> 48v DC source?
>
> Again - that depends - and might actually be less successful than a more
> simple (passive vs. active) source.
>
>> Is there something in the older TTYs that requires a higher voltage on
>> the loop?
>
> Yes, but again it depends...
>
>>
>> My ASR-33 runs with 20mA sourced from 12v but that is pretty "modern"
>> compared to the 5-level stuff you guys are working with I am sure.
>
> Different critter, though there are possibilities here as well...
>
> So - let's talk about what's going on - and why it "depends"...
>
> Preface: I tired to keep this simple (some might think over-simplified)
> and some numbers are generalizations - but the principles are solid and
> can lead to successful implementations in real life.
>
> The selector in the older teletypes are made up of electromagnets, who
> (depending on wiring, etc.) are designed to "follow" either a 20ma. or
> 60ma. signal.  This "signal" is NOT a simple series current source, but
> delivered by a group of components which are optimized to deliver as
> faithful of a "signal" through the selector as possible. That signal is
> a series of intervals of current and no current which convey the
> "intelligence" of the signal.  In the case of the older machines - the
> intelligence is 7 - 8 intervals of current and no current - whose
> characteristic is a beginning interval of no current (start interval),
> then 5 intervals of current / no current - the pattern dictated by the
> "character" being conveyed, followed by 1 - 2 (or something in between)
> interval lengths (stop bit(s)).  The primary point of concern here is
> that word "faithful".
>
> Ideally - the keying circuit would merely have to make and brake the
> simple series circuit containing the selector magnet to accomplish the
> above info transfer... but several real life factors work against this
> simple solution.
>
> First is the selector magnet assembly itself. In most teletype machines
> (at least of the vintage we're talking about) the selector magnet is
> comprised of many turns of fine gauge wire wound on a bobbin, which then
> energizes pole piece(s) to alternately attract and release an armature -
> in step with the current in passing through the windings. In the case of
> the original model 28 - the two coils are wired in parallel (for 60ma
> circuits); and consisted of 3600 turns of #33 gauge magnet wire which
> results in (approximately) 132 ohms in each coil (i.e. 66 ohms in
> parallel; or 264 ohms in series). Both coils are "rated" at 115VDC (with
> a HIPOT A-C withstand rating of at least 500V). I don't have the
> inductance figures at hand for the 28 (IIRC the 15's coils are around
> 4Hy)- but with 3600 turns on an iron core - it's substantial. Point
> being - in addition to the resistance of the winding(s) you also have
> the inductance of them which tends to distort any square wave applied to
> them.  This distortion manifests itself as a phase displacement between
> the voltage applied to the selector magnet coils - and the resulting
> current through them.  If we assume for a moment - that our selector
> coils have no DC resistance - then when any source is applied to them -
> the voltage across the coils would "lead" the current through them by 90
> degrees. The higher the resistive components of the circuit - the less
> the current "lags" the voltage.  If we them add a significant resistance
> to the loop - say ten times the effective inductance - we minimize the
> phase lag (distortion) in our loop. However - this higher resistance
> requires higher voltage in order to achieve our desired steady-state
> current (say 60ma.). So with  say 1000 ohms added to the circuit - that
> requires an additional 60V just to "drive" that additional resistance -
> PLUS whatever voltage would be needed to drive the coil(s) (which if
> calculated as pure resistance - would be roughly 4V more).
>
> Then there are additional factors: how "accurate" is the keying device?
>  The contacts in a teletype's transmitter - being mechanical - are
> excellent.  Tubes (and transistor) circuits are good, long parallel
> wires start having an effect - and being a series circuit - everything
> contributes something. It's helpful if these do not contribute a serious
> amount of additional distortion, but sometimes that can't be avoided.
>
> In the early days of teletypes - these factors had been discovered and
> explored quite a bit.  Obviously adding resistance to a loop contributes
> quite a bit.  Another factor, of course, is how good in the mechanics of
> the keying system (tape readers, keyboard, etc.). All of these ideally
> don't "push" the envelope - but sometimes they do. To deal with some of
> these variables at the receiving end - virtually all machines have what
> is called a "ranging" adjustment. This adjustment accomplishes it's
> purpose in varying ways - but in simple terms - it allows the operator
> to manually advance or retard the "reading" of the incoming signal in
> relation to time - to physically displace in time the moment the
> selector mechanism samples the magnetic state of the coils (you could
> say "shifts" the *moment* it makes up it's mind - either advancing it -
> or retarding it).
>
>
> So... where does all of this bring us with regards to your question? Yes
> - some teletypes - depending on several factors coming together - can be
> run on embarrassingly low voltages powering it's current loop.
>
> In my case - I run my model 28 on 24VDC, with the loop "keyed" by a
> Blackbox CL50 RS-232C Serial to Current Loop converter box attached to a
> computer. The CL50's is rated to key a maximum of 30V; hence the use of
> a 24VDC supply. It works, quite well - within limits.
>
>  But several things have to come together to make it work:
>
> 1) The better condition the selector mechanism is in - the more tolerant
> of distortion it will be and still copy OK.  This means the armature
> spring and clearance adjustment(s) have to be as close as possible to
> "spot on". Also - my own tests show that the more modern armature styles
> (two button, quick release armature - similar to that supplied in kit
> 195634. There is also a replacement armature for the older style
> assemblies P/N 160180).
>
> 2) Speed- I can get consistently good results copying at 60 and 75 WPM -
> but things start getting "on the margin" at 100WPM. I don't like running
> at 100WPM anyway - (too frenetic compared to the "comfortable" 60WPM
> pace). With the higher "data rate" - it'd be better to have more
> resistance (with resulting higher voltage).
>
> 3) More in the loop: Obviously - with only 24Vdc - there is little room
> left to add more "stuff" - I wouldn't expect my URA-8A (CV-89A) to be
> happy keying such a loop...
>
>
> 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.
>
> Hope this enlightens more than confuses.
> -- 
> randy guttery
>
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