Specifications

CIRCUIT IDEAS
S.C. DWIVEDI
STEPPER MOTOR DRIVER
CIRCUIT IDEAS
PIYUSH P. TAILOR
S
tepper motors are widely used
where precision and accuracy are
the primary considerations during
rotation or positioning. Microprocessors or
microcontrollers are often employed for
controlling
their operation.
But it may not
always be con-
venient or nec-
essary to use
microcon-
trollers, as it
would make the
gadget unnec-
essarily cost-
lier.
Here is a
simple and low-
cost circuit to
drive a stepper
motor on full
power for any
number of
whole steps.
The present cir-
cuit is intended
to drive four-winding stepper motors, but
one can easily modify it for other types.
The popular decade counter CD4017
(IC1) with decoded outputs is used here
as a sequence generator (similar to the
running-light effect). As we need only four
outputs, the fifth output (pin 10) is con-
nected to the RESET pin (pin 15). The
four outputs, in conjunction with four npn
power transistors, function as half-power
full-step drivers. In order to get full power,
eight diodes (8 x 1N4148) are used. Truth
Table I depicts the half-power operation,
while truth Table II depicts the full-power
operation.
The use of hex inverter IC2 (CD4069)
gives two benefits:
1. The inversion through NOT gates
allows the use of pnp power transistors
(4 x BD140), which make it possible to
ground the common terminal of the mo-
tor. This is useful in many applications.
2. The two unused inverter gates (N1
and N2) are handy to use as clock gen-
erator, in conjunction with preset VR1 and
capacitor C1. Varying the preset allows
the change in clock frequency and hence
the speed of the motor.
If one does not know the sequence of
motor terminals to be connected to termi-
nals A through D of the circuit, then first
connect any one terminal of motor to ter-
minal A of the circuit and connect com-
mon terminal to the ground. Now give
the lowest possible supply (3V) and check
for correct sequence of the remaining
three terminals, using trial-and-error
method for the maximum six combina-
tions/possibilities. At correct sequence, the
motor would rotate in either clockwise or
anti-clockwise direction.
To use external clock pulses, simply
disconnect pin 14 of CD4017B from pin 8
of CD4069B and then connect external
clock pulses to pin 14 of CD4017B. Each
pulse drives the motor by one step, which
may normally be 1.8
o
or 3.6
o
, as shown on
the label plate of the motor.
To reverse the direction of rotation,
one should interchange terminals A with
B and C with D simultaneously.
The colours of motor terminal wires,
shown in the diagram, are those of the
stepper motor used in head-drive of a
1.2MB floppy disk drive unit, operating
on 12V with a 3.6
o
/step, which the author
has used in his prototype.
Notes: 1. Heat-sinks may not be re-
quired for the power transistors.
2. Cost of the circuit is less than
Rs 100.
3. Supply voltage for the circuit is
equal to the operating voltage of the mo-
tor (i.e. between 3V and 12V).
4. RPM of motor =
, where
f is the frequency of clock pulses and d
the angular displacement in degrees per
step.
TABLE II
Full-Power Operation
Step Supply to coils
number A B C D
1 On On Off Off
2 Off On On Off
3 Off Off On On
4 On Off Off On
5 Repetition On On Off Off
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TABLE I
Half-Power Operation
Step Supply to coils
number A B C D
1 On Off Off Off
2 Off On Off Off
3 Off Off On Off
4 Off Off Off On
5 Repetition On Off Off Off
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