AN155
The center tap of the motor winding is connected to the
positive voltage supply. Each coil can be energized in
either direction by turning on the appropriate transistor.
The center-tapped winding acts as a transformer. So the
voltage on the unused switch will be twice the supply
voltage.
A clamped unipolar drive circuit is shown in Figure 2.
When Q1 is turned on, current will flow from the +12V
supply, through the A winding, through Q1 to ground.
When Q1 is turned off, the current will tend to continue
to flow through the winding inductance. The drain
voltage of Q1 will rise above the supply voltage. The
center-tapped winding acts as a transformer. Thus,
when the voltage on A+ reaches 24 V, the voltage on
the A- terminal will go below ground and be clamped by
the internal diode of Q2. There is also a considerable
amount of uncoupled inductance in each winding. This
will cause an additional overshoot voltage when the
transistor is turned off. The four diodes D1-D4 and the
clamp zener D5 form an effective clamp circuit to limit
the overshoot voltage. The zener voltage should be
slightly higher than twice the maximum supply voltage.
A bipolar stepper motor drive uses eight transistors to
drive the two phases as shown in Figure 3. A stepper
motor with four wires or six wires may be used with a
bipolar drive. A four-wire motor can only be used with a
bipolar drive. The four-wire motor might be marginally
less expensive in high volume applications. The bipolar
stepper motor drive uses twice as many transistors as
the unipolar stepper motor drive. The four lower
transistors can be usually be driven directly from the
microcontroller. The upper transistors require a more
expensive high-side drive. The bipolar drive transistors
only need to withstand the motor supply voltage. The
bipolar drive does not require a clamp circuit like the
unipolar drive.
D1
D2
D3
D4
D5
+12V
A+
A-
B+
B-
Q1
Q2
Q3
Q4
Figure 2. Unipolar Stepper Motor Drive with Zener Clamp
4
Rev. 1.0
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