[GreenKeys] Why high loop voltages?

[Brooke Clarke]

Hi Joe:

The high voltage is needed so that a resistance can be added into the loop.
The time constant of an L R circuit is L/R and so by adding resistance you 
reduce the time constant (speed it up).

If you do a DC test on the selector magnets you'll find you need a small 
voltage and 60 ma to reliability activate the magnet.  But the time constant 
will be L/R where R is some very small number so the time constant will be much 
longer than the symbol time and so is too slow to work.

Have Fun,

Brooke Clarke
http://www.prc68.com

John Nagle wrote:
>     Something I don't get: why were such high loop voltages used?
> 
>     I just got a Model 15, and just finished degreasing, cleaning and
> re-oiling it, but don't have anything to connect it to yet.
> (It was mechanically in good shape, but out of use for years.)
> The magnets are wired in parallel and measure 55.3 ohms at the
> input to the printer.   This is a 60mA current loop device, so
> it should run on 3.3V.  3 D batteries in series will run the thing,
> drawing 75mA; I should put a 20 ohm resistor in to get the
> current right.
> 
>     So what is the rationale for huge loop voltages?  I realize
> the drive is supposed to be constant current, but making a
> current source by putting 120VDC in series with 2K seems like
> doing it the hard way.  Yes, you get the option of plugging in
> extra devices in series without adjusting the current.
> And you get some contact cleaning as higher voltages burn
> through oil and dirt.  But 120VDC?  I could see 12VDC;
> that's enough to get through automotive-type dirt on contacts.
> Thanks.
> 
> 				John Nagle
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