[GreenKeys] M28 using 20ma

[Jim Cooper]

On 28 Jul 2019 at 9:28, Harold Hallikainen wrote:

> My long ago experience with model 15
> and 14 (typing reperf) was that they
> were available with either pulling
> or holding magnets. Pulling magnets
> were in series for 60 mA. Holding
> magnets were in parallel for 60 mA and
> series for 20 mA. 
 
> Do I remember 50 years ago right?

Indeed, you remember correctly !! 
(It has been a long time....) 

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 ... 

 
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