[GreenKeys] Loop power supplies

[drlegendre .]

> I and others have tried it, and 24 volts and under does not work even on
> short loops ( like maybe 5 or 10 feet of wire)  with machines like the
model
> 15 printer.  It really needs more than 50 volts to do the job, close to
100
> or 150 volts to really work well.

The real irony, here, is that this "12V - 24V" idea was passed along to me
by another member of this list, a couple of years ago. I was simply passing
the comment along, I never actually made any point of testing it out.
Needless to say, when time came to build my converter box, I went with a
120V (at 60mA, resistive) supply as that's what is generally specified .

And yes, a voltage tripler doesn't provide for very good regulation, but as
has been pointed out, this just doesn't matter in our case. What's really
important is to have that initial high voltage to get current flowing in
the high loop + magnet inductance as quickly as possible. The simple
tripler works perfectly well in this capacity on my short loop, and I wager
would perform just as well on a very, very long loop. With a max. current
of only 60mA, the steady-state supply sag is almost non-existent. It does
use some fairly large cap values, though.. like 3x 2200uF 100V.

On Sat, Mar 3, 2018 at 5:05 PM, Sheldon Daitch via GreenKeys <
greenkeys at mailman.qth.net> wrote:

> Here is a link to the Northern Radio Company's loop current control manual:
>
> http://www.tmchistory.org/NorthernRadio/Manuals/ib_238-1-2_4_8_66.pdf
>
> 73
> Sheldon
>
>
> ------------------------------
> *From:* Jim Haynes <jhhaynes at earthlink.net>
> *To:* Ralph Mowery <rmowery28146 at earthlink.net>
> *Cc:* 'Green Keys' <greenkeys at mailman.qth.net>
> *Sent:* Saturday, March 3, 2018 11:36 AM
> *Subject:* Re: [GreenKeys] Loop power supplies
>
> On Sat, 3 Mar 2018, Ralph Mowery wrote:
>
>
> >
> > I and others have tried it, and 24 volts and under does not work even on
> > short loops ( like maybe 5 or 10 feet of wire)  with machines like the
> model
> > 15 printer.  It really needs more than 50 volts to do the job, close to
> 100
> > or 150 volts to really work well.
>
> >
> I haven't tried it, but there are circuits that use the "constant current"
> properties of a transistor rather than a fixed resistor, and are said to
> give equivalent results with lower voltages such as 48V.  For instance
> QST, Jan 1972, p. 40.  Maybe someone who knows how can do a SPICE
> simulation to compare such a circuit with a plain resistor.
>
>
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