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I am in the planning stages of building a motorized altazimuth telescope/antenna mount. I will be using two large stepper motors that operate at 1.75 Volts / 12.7 Amps. I would like to make the mount portable by powering it with a 12 Volt car battery.

What would be the most appropriate circuit and component values to power these stepper motors within spec?

Note: I am not needing details on the stepper motor wiring or switching, just a means of dropping the voltage/current to the appropriate amounts.

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    \$\begingroup\$ Can you provide a link or a technical document for these motors? These specs seems weird to me... Anyway you want to search for a dc-dc step down converter. \$\endgroup\$ – Vladimir Cravero Jun 10 '14 at 19:04
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You don't need to drop the voltage externally if you use a chopper type stepper driver. The controller will automatically chop the incoming voltage to a suitable on/off ratio to generate the 12.7A current. You'll get the ability to start with a relatively high inertial load, which you would not get if you tried to run the motor off of 1.75V. The motor inductance will smooth the current so it's relatively constant during each step.

The higher voltage allows the current to rise quickly in the motor winding despite the inherent inductance of the winding. That's why the steppers are made with such low voltage windings- you are expected to run the controller from 12V, 24V or even 36V.

If you don't need full holding torque, I would suggest buying a controller that has the ability to scale back the holding current when it is not stepping to prevent undue heating- at full current the motors will be burning 22 watts per winding each just sitting there (could be as much as 88W total).

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