[GreenKeys] M28 using 20ma

Sun Jul 28 20:02:10 EDT 2019

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
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