> From:         Colin Constant[SMTP:colinpiclist@HOTMAIL.COM]
> Sent:         Friday, February 27, 2004 11:44 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE:] Stepper motor voltage question

> Hi,

> I've got a stepper motor that's rated at 9.2V (240mA).  I'm just wondering
> how picky these things are about the voltage.  Could I run it off a 9V wall
> wart? Unregulated? Should I maybe add an electrolytic?

> I'm using an MC3479 to drive the motor.

> At the moment I'm using a 15V wall wart into an LM317 set to 9.2V, which
> works OK except that the 317 runs pretty warm.

> Thanks guys,
> Colin

The voltage enters in two ways. First, the voltage determines how fast the
current builds up in each phase as it is turned on. This matters only for
high speed switching, in which the rise time of the coil inductance and
winding resistance becomes comparable to the "on" time of the phase.
Second, the steady state current reached once the switching stops must
be limited to the maximum designed for the motor. Too little voltage
means not enough torque (holding or running). A compromise which will
reduce power consumption and also help out the LM317 is to raise the
voltage to a high level when switching (moving the motor), and lower it
once the motor stops.
The manufacturer's voltage specs for the motor serve to limit the
naximum coil current on the one hand, and to guarantee the
published torque and speed numbers on the other. Stay below the
maximum, but adapt lower values to suit your application.

John Power

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