[GreenKeys] Loop power supplies

[Bob kb8tq]

Hi

I built my first transistor based constant current loop driver back in the early
1970’s. Was it before or after the QST article ….I simply don’t remember. 

You can put a scope on a constant current loop and watch the loop current 
vs time. It is not really hard to do. You can pretty quickly demonstrate that even
with a 15 and few other items in the loop, it does at least as well as a resistor 
based loop. 

I always ran mine up in the 60 to 80V range. That was simply because of the
transformers I had back then. I haven’t bothered to compare the two approaches
to each other since then. 

====

If you really want to get creative: 

You only need the high voltage when you are trying to pull in the magnets. It’s 
not very hard to delay data by one bit time. Running a supply that boosts up 
“on demand” is hardly rocket science these days. Let it drop back to whatever 
your magnets drop at 60 ma as long as nothing is going on …..

Bob

> On Mar 3, 2018, at 11:35 AM, Jim Haynes <jhhaynes at earthlink.net> wrote:
> 
> 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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