[time-nuts] Sidereal timekeeping

David Martindale dave.martindale at gmail.com
Wed May 18 16:42:14 UTC 2011


The motor is essentially a permanent-magnet stepper motor.  The rotor
and stator have just 2 poles each, so the rotor has two stable
positions 180 degrees apart that provide holding torque.  Thus, the
motor holds position with no input current for most of each cycle.  To
move it, the drive applies a short current pulse to the motor.  The
pulses are alternating positive and negative polarity, so you'll need
something like an H-bridge to drive it.  Using a 3 volt supply instead
of the 1.5 that the motor was designed for would supply more power
than the motor needs if you keep the drive pulse the same width, but
you should be able to reduce the pulse width until the energy is about
the same as with 1.5 V drive and have the motor still operate.

Without having actually tried it, I think you should be able to select
a suitable tradeoff between reliable motor operation and power
consumption just by adjusting the "on" time of the drive pulse - no
voltage regulator or voltage dropping resistor needed.

There's only one stator coil to the motor, and one drive signal, so
you can't control motor direction.  The magnetic structures are
apparently deliberately asymmetric to ensure that the motor always
rotates in one direction when it receives a pulse of the appropriate
polarity.  (Get the pulse polarity wrong, and the motor just doesn't
rotate).

     Dave

On Wed, May 18, 2011 at 9:20 AM, Bob Camp <lists at rtty.us> wrote:
> The only obvious issue is to figure out the motor drive side and see what
> driving it straight rather than with a switcher does to the motor current.
> That's going to involve hacking up a wall clock. Since the clock is likely a
> single cell gizmo, the current probably will be a bit high.



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