[GreenKeys] M28 using 20ma

Paul Heller paul0926 at comcast.net
Mon Jul 29 04:33:43 EDT 2019 

That was supposed to  be “one simple post”, not “I simple post”. I’m not trying to take credit for anything here. 

Paul
W2TTY



> On Jul 28, 2019, at 6:22 PM, Paul Heller <paul0926 at comcast.net> wrote:
> 
> Unbelievable. I simple post started all this education. This is really great!
> 
> Thank you to the experts who weighed in and explained it all. You guys make Greenkeys a very special place.
> 
> I‘m motivated to capture this as a digest and post it on RTTY.COM.  It will be the start of a FAQ. I‘ll see if I can get a wiki running on RTTY.com
> 
> Does anyone who has posted on this topic have any concerns if I mention their names and call signs? If so, let me know and I‘ll anonymize your name and/or call sign.
> 
> Paul
> W2TTY
> 
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> 
>> On Jul 28, 2019, at 6:02 PM, Jordan Spencer Cunningham <js at teletype.net> wrote:
>> 
>> I'm no expert, but in my experimentation with my M15 and M19, it seemed that the magnets became unreliable at any less than around 50ma. 
>> 
>> -
>> Jordan Spencer Cunningham
>> teletype.net
>> 
>> 
>>> On Sun, Jul 28, 2019, at 4:30 PM, Robert Downs via GreenKeys wrote:
>>> I have never seen an explanation as to why the two currents are 60 mA and 20
>>> mA instead of 60 mA and 30 mA or 40 mA and 20 mA.  Last time that I had an
>>> M15 with holding magnets, it never occurred to me to try it on a local loop
>>> with the current turned down to 40 mA just to see how well it worked.  But I
>>> have always assumed that it was because 60 mA was the standard and there
>>> might be other things in the loop that had to have it.  And didn't hurt the
>>> magnets.  But they worked as well at 20 mA as they did at 30 mA so that
>>> became the other standard.
>>> 
>>> Robert Downs
>>> 
>>> -----Original Message-----
>>> From: greenkeys-bounces at mailman.qth.net
>>> [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Harold Hallikainen
>>> Sent: Sunday, July 28, 2019 14:49
>>> To: jim.w2jc at gmail.com
>>> Cc: greenkeys at mailman.qth.net
>>> Subject: Re: [GreenKeys] M28 using 20ma
>>> 
>>> Great explanation! In addition, inductors (the selector magnets) in series
>>> causes the inductance to add, while the inductance divides by two when the
>>> magnets are in parallel (ignoring, for the moment, mutual inductance).
>>> Considering mutual inductance, we could consider the selector magnets to
>>> be one inductor with twice as many turns. This causes the inductance to go
>>> up four times. Looking at the parallel magnets, the inductance drops to
>>> half if there is no mutual inductance between them. If there is full
>>> coupling, the inductance of the two in parallel is the same as one - It's
>>> like we just wound the magnets with thicker wire. Also, the inductance
>>> varies as the armature moves towards the selector magnet core. Then
>>> there's core saturation causing the inductance to drop. So, a lot of
>>> variables! Anyway, I always thought holding magnets were a very clever
>>> idea. We are only interested in what the loop current is at the middle of
>>> the bit time, so hold the armature until just before then, then see if it
>>> falls away.
>>> 
>>> Harold
>>> 
>>> 
>>> 
>>> 
>>> >
>>> > The 'pulling' magnets required the magnetic
>>> > field from the loop current to physically
>>> > attract the little plate against the selector
>>> > magnet ...  this took a lot of 'oomph' as well
>>> > as a few milliseconds, and also required the
>>> > loop current to KEEP the armature pulled
>>> > against the selector magnet.
>>> >
>>> > The 'holding' magnets have a cam that pushes
>>> > the armature up against the selector magnet,
>>> > doing mechanically what the pulling magnet
>>> > setup had to do with loop current ... once the
>>> > armature is pushed against the selector magnet,
>>> > it was a lot easier to 'hold' it there with a lower
>>> > loop current, such as the 20ma.
>>> >
>>> > otoh, as has been mentioned in some other responses, there
>>> > is also the LR time constant [actually L/R] consideration
>>> > since that affects the SHAPE of the square wave for each
>>> > data bit.  The selector magnet, being a magnetic device,
>>> > requires TIME for the magnetic field to be formed ...  how
>>> > fast this happens is dependent on the L
>>> > (inductance/sel.magnet) and the R (current limiting
>>> > resistance in the loop).  That is also a reason why the
>>> > 120vDC was used rather that some lower loop voltage
>>> > like 12v ...  going back to the L/R time constant formula,
>>> > you can't change the L of the selector magnet, so you
>>> > want a larger R for faster response.
>>> >
>>> >   http://www.learnabout-electronics.org/ac_theory/dc_ccts45.php
>>> >
>>> > So for a fixed voltage (120vDC) and a fixed L (selector
>>> > magnet) the controlling variable is the R ... so the higher
>>> > the R, the faster the selector magnet pulls in.  Who cares?
>>> > Because this affects the RANGE on the selector range
>>> > finder on the typing unit.  A perfect square wave in the loop
>>> > gives you a full range on the dial; a rounded square wave
>>> > (due to time it takes to build up the magnetic field in the
>>> > selector magnet) reduces your range, more and more toward
>>> > the center of the rangefinder scale.
>>> >
>>> > So why not raise the R in the loop even higher and use the
>>> > lower 20mA loop current?   Ah, because it is the CURRENT
>>> > that causes the magnetic field in the selector magnet to
>>> > build up!!   and that's why, for 20mA loops, they put the
>>> > coils of the selector magnet in series to create more
>>> > magnetic pull by adding the two coils; for 60mA operation of
>>> > that type of selector magnet, the coils are in parallel so
>>> > they don't get burned out from the high current ...
>>> >
>>> 
>>> 
>>> -- 
>>> FCC Rules Updated Daily at http://www.hallikainen.com
>>> Not sent from an iPhone.
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>> 
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