[GreenKeys] Just about all you need to know about Selector Magnet Drivers

[Jeffrey D Angus]

On 9/30/2012 1:58 PM, John Nagle wrote:
> They apply the full supply voltage through only
> a 27 ohm resistor (R11) and Q5, so for a brief period the
> selector magnet is almost directly connected to the power
> supply.  This would burn out the selector magnet if that
> state continued.
I think the deal here is they are using 40 VDC instead
of a 120VDC loop for the supply.

So it's only 162 mA max steady state current, and that
would be limited by the dt/di caused by the inductance
of the coils initially.

I remember doing "the math" back in the early '70s as to
what happens with the current through to selector magnets
on initial turn on. Using a 22 Hz square wave.

Using 120 VDC at first, with a series resistor to limit it to
60 mA, and then decreasing both the open circuit voltage
and series resistor down to zero and 13.2 VDC.

At the 120 volt loop voltage the waveform across the coil
looks good, down at 13.2 the rise time is such it that is
worthless.

I also did some experimenting with holding current vs pull-
n current. 120 volt loop supply with a larger series resistor
to limit the steady current to 20 mA followed by a shunt
capacitor to briefly supply as much current as needed to
snap the armature into position.

The most fun was using an SCR as a commutation switch
to put current into and suck current out of the coil on the
transitions.

Jeff-1.0
wa6fwi