[GreenKeys] Loop power supplies

[Jeffrey Angus]

On 3/5/18 6:55 AM, Bob kb8tq wrote:
> If I have a “normal” loop supply and the 60 ma magnets are up to
> the 30 ma point, I only get 1/2 the loop voltage on the magnets.
> They don’t “see” the full loop supply.
That's not how it works.
At initial turn on, the coil sees close to the full loop voltage
because of the reactive impedance of the coil. As soon as it starts
to draw current, the voltage drop across the resistor increases.
For a DC coil resistance of 100 ohms, this means 6 vdc at 30 mA
and 12 vdc at 60 mA.
> If I have a proper constant current loop supply, even at 55 ma,
> the magnets still get the full loop voltage applied. They “see” the
> supply right up to the point they are at full current.
Absolutely not.
The whole point of operation of a current source, is the compliance
voltage needed to make the current a set value over a wide range of
of resistance. In the case of a large inductance, it's a combination of
the DC coil resistance, which remains fixed, and the AC reactance,
which varies over time, dx/dt.
> The fact that full voltage is applied over a wider range of currents
> is why the constant current source is going to get the magnets
> energized faster.
Again, no.
The reason it is faster is due to the variable resistance of the current
source, rather than a fixed value. The lower initial source impedance
of the 120 v loop supply allows for a slightly faster rise time of the
current through the selector magnet.


-- 
"I am a river to my people."
Jeff-1.0
WA6FWI
www.foxsmercantile.com