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Chapter 6: DC and Stepping Motors
Limiting Current with Series Resistors
It is often useful to add resistors in series with the coils in a stepping motor.
You may wish to power a six volt stepping motor from a 12 volt battery, either
for convenience or to improve the high speed torque of the motor (the higher
voltage helps overcome the increasing coil impedance due to its inductance). Or
you may wish to reduce the motor’s current draw to allow the Experimenter to
drive it safely within the 1 amp maximum driver current rating (circuits for
higher current drive are given in the next section).
Some of the motor supply voltage is lost in the drivers, about 1.2 volts. Since
an H-bridge configuration drives both high and low sides of the coils, about 2.4
volts is lost when driving bipolar motors.
Sometimes the motor’s label tells the coil resistance. Sometimes the label
specifies maximum coil voltage and current instead of resistance, so you can
Equation 6-1
calculate the coil resistance from Ohm’s Law (Equation
6-1). Or you can
measure the resistance with an ohmmeter.
Eq. 6-1:
Rcoil = Ecoil-max / Icoil-max
Rcoil
the internal resistance of the coil
Ecoil-max the maximum rated coil voltage
Icoil-max the maximum rated coil current
Now, using another form of Ohm’s law, we can calculate the additional
series resistance required:
Eq. 6-2:
Rseries = (Esupply - Edrop) / Ichosen - Rcoil
Rseries
Esupply
Edrop
Ichosen
Rcoil
the additional resistance required
the power supply to the driver.
the voltage drop across the driver, about 1.2 volts
unipolar, 2.4 volts bipolar.
the new coil drive current.
the internal resistance of the coil.
Now we need to calculate the power that will be dissipated in the resistors:
Eq. 6-3:
Presistor = (Ichosen)2 * Rseries
Presistor is the power dissipated in the resistor.
Ichosen
the new coil drive current.
Rseries
the additional resistance required
Choose the standard resistor closest to the value calculated in Equation 6-2
6-2.
A good rule of thumb is to get resistors rated for twice the power calculated in
Equation 6-3
6-3. Even so, they will get hot! You may also use series or parallel
combinations of resistors to get the desired values. Figure 6-7 shows a unipolar
stepping motor with series resistors directly connected to the Experimenter.
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