A Quarterly Publication of
The Smith-Kettlewell Eye Research Institute’s
Rehabilitation Engineering Research Center
William Gerrey, Editor
Original support provided by:
The Smith-Kettlewell Eye Research Institute
and the National Institute on Disability and Rehabilitation Research
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TABLE OF CONTENTS
Stepper motors are brushless devices whose shafts can
be rotated in discrete "steps." Common applications for
these motors are: driving the head carriage in many disk
drives, as well as positioners in printers and
numerically controlled milling machines.
"Variable-reluctance" and "hybrid" types are discussed
Surplus stores have bins full of stepper motors,
some affixed with gears and others with lead screws.
You'll know a permanent-magnet stepper motor by the way
the shaft feels as you turn it; the shaft wants to move
in jumps as the permanent magnets seek pole pieces of the
stators. (There is room for some confusion here, since a
simple DC motor with a permanent-magnet field can also
feel like this, but steppers will usually have more leads
than a simple DC motor.)
The main design parameters worth considering are:
the "resolution" (the number of degrees per step) and the
static torque (will it be powerful enough to stay where
you put it?). Many steppers come with gear boxes, making
the attainable resolution quite fine. When toying with
surplus ones, you can guess the resolution by noting how
many times the rotor magnets "home in" on stator poles.
In general, stepper motors have more than one
stator. (There is no electrical connection to the
rotor.) Each stator is a ring of iron surrounding a coil
of wire--the "stator winding." "Pole pairs" are created
by attaching L-shaped pieces either side of the ring.
This creates interleaving fingers on the inside of the
circle; half of these fingers (of one magnetic polarity)
point in one direction, and the other set, positioned in
between the first set of fingers, points the other way
(these being of the opposite magnetic polarity). Thus,
when energized, the inner circumference of each stator
has alternating magnetic poles--a north followed by a
south, then north, and so on. Having multiple stators,
we will designate them by letters A, B, C, and so on.
Unlike motors with "brushes," it is the business
of driver electronics to be the "commutator." The
simplest driving system is to energize one stator (stator
A), then energize stator B, then C (if there is a C),
etc. Depending on the type, this is not always done,
because energizing two stators simultaneously can
increase the available torque.
The Variable-Reluctance Type
Typically, these motors have three stators. The rotor
has "teeth" of soft iron with the same "pitch" (called
the "pole pitch") as pole pairs of the stator. When a
stator is energized, the rotor moves to the nearest
position that minimizes the air gap--the position that
establishes the best magnetic circuit.
Possibly to isolate the magnetic circuits, the
rotors may have multiple stacks of rotor teeth, one for
each stator. Such a motor was described in a textbook as
a 3-phase variable-reluctance stepper made by Warner
Electric. (Published by the Institute of Electrical
Engineering, London and New York, the book is "Stepping
Motors" by P.P. Acarnley, c1982.)
We dismantled an old 8-inch disk drive and
retrieved the motor used to drive the head; this looks
identical to the one in the book by Acarnley. With the
brand name being Applied Motion Products, this is a
3-stator 24-volt device. One lead of all three stators
goes to a common (black) lead; the free ends of the
stators come out as brown, red and orange leads (colors
representing 1, 2, and 3, respectively). The winding
resistance is listed on the motor as being 19 ohms (per
Two driving schemes are discussed in Acarnley:
The simplest is the "full-step" system; first A is
energized, then A is turned off and B is energized, then
C (by itself) is energized, then A again. With our
cannibalized motor, this worked very well, and it was
observed that 24 steps complete a revolution. However,
as described in Acarnley's book, there is a "half-step"
driving scheme which goes as follows:
First A is energized by itself; this causes poles
of the rotor to "home in" on poles of A. Then B is
added--causing the rotor to make a "half step" forward
(in the direction of B). Then A is turned off; this
causes the rotor to home in on B. Then C is added,
causing the rotor to advance a half step toward C. When
B is turned off, the rotor advances a half step to home
in on C. When A is next energized, the rotor advances
(in the direction it has been going) a half step away
from C--teeth in the rotor are about to align with poles
of A, but shifted by one period of the pole pitch. When
C is turned off, poles in the rotor home in on A, except
that the shaft has made one-eighth of a revolution; 48
steps will be required for each complete revolution. Of
course, any such sequence can be run in reverse to change
the direction of rotation.
For some reason, our liberated disk-head motor
does not make the half steps properly. This may be due
to alignment of stacks in the rotor, or this motor may
just not be made for it.
The driving circuit for a "whole-step" process can
be very simple: A "one-of-three" up-down counter could
drive power transistors in sequence--the counter
advancing in accordance with a clock signal.
The Hybrid Type
So-called "hybrid stepper motors" have a "single
stack" of permanent magnets on their rotors. These often
have two sets of stators with "N" number of pole pairs.
Their rotors have a corresponding number of radially
mounted permanent magnets with alternating polarity--a
south pole followed by a space, then a north pole
followed by a space, then another south pole, and so on.
The amount of rotation for each step is one-fourth the
"pole pitch"; the rotor can be moved to any of four
positions for any stator pole pair.
The two sets of stators are displaced from one
another; the poles of one set are aligned with spaces
between poles of the other. For analysis, let us call
these stators "A" and "B."
The traditional driving scheme is a four-step
switching process. A graph of the drive signals
indicates that two flip-flops generate square waves which
are out of phase by 90 degrees; these flip-flops are
driving the two stators. We can go through the process
"step by step" as follows:
When a stator is energized, its pole pieces take
on alternating magnetic polarity--north followed by a
space, south followed by a space, another north pole,
etc. Suppose we energize both stators with the same
polarity; there will be two north poles adjacent to one
another, followed by two adjacent south poles, and so on.
(By adjacent, I mean "across the aisle," from one stator
to the other.) Let us consider the current north pole of
stator A, the starting place of our analysis.
Magnets of the rotor will align so that each
permanent-magnet south pole is attracted to adjacent
stator north poles. Thus, the rotor "starts" by being
displaced from the pole pieces of either stator.
If we change the polarity of the drive to stator A
(and proceed around the stator from where we started), we
have a south pole, followed by two adjacent north poles,
followed by adjacent south poles; i.e., coincident poles
of the same polarity have been shifted in the direction
of stator B. Now, by changing the polarity of stator B
and noting polarity from our starting position we have:
two adjacent south poles, followed by adjacent north
poles, followed by adjacent south poles, etc.
Changing A again, we polarize stator A as we
started with. We now have north, followed by adjacent
south poles, then two norths, etc. Finally, we change B
back to the polarity with which we began; establishing
two norths, two souths, which is the order we started
Each time we change one polarity, we shift the
location of "like poles" of the two stators, and magnets
of the rotor are attracted to these. Of course, this
sequence can be reversed, giving us the ability to run
the motor in either direction. Note that skipping steps
can get you into trouble; for example, if you change
polarity of both stators, the rotor will be in a quandary
as to which way to go and the torque is theoretically
If each stator has twelve pole pairs, the rotor
will move four steps per pole pair, making 48 steps per
revolution (rotating 7.5 degrees per step).
Two winding configurations are available: The
stator windings in "bipolar" stepper motors have two
leads per stator. Bipolar stepper motors with two
stators have four leads; the winding of one stator is fed
by one pair of leads, while the winding of the other is
fed by the other pair. The "unipolar" version has
center-tapped windings; the connections come out as three
leads per stator--a motor with two stators has six leads
Bipolar motors require more complicated switching
than unipolar ones, since polarity of the voltages
applied to the windings must be reversed; this requires
eight driver transistors. In driving unipolar motors,
the polarity of the electromagnets is changed by using
one-half of the winding or the other; twice as much
magnet wire is required to get the same number of
ampere-turns in the motor, but only four driver
transistors are needed.
The literature discusses timing issues of driver
circuits; these discussions have been put to rest by the
emergence of driver chips which do the work for us. The
simplest driver circuit, used for the unipolar style, is
as follows: The center taps of both stator windings go
to the positive supply; the ends of the windings go to
open-collector outputs of flip-flops, a flip-flop for
each stator. The bipolar circuit requires polarity
reversals: Two flip-flops, one for each stator, drive
complementary transistor pairs whose outputs can either
pull to the positive supply or to ground. The stators,
which only have two leads each, can thus be powered in
either direction. (One of the timing issues is that
without safeguards, both transistors in a complementary
pair might turn on at the same time and burn themselves
Comparing performance of the two winding styles,
choosing motors of identical physical size, unipolar
motors have about 30% less torque at low-to-moderate
speeds. This is due to the unavoidable requirement of
reducing wire size in order to fit as many turns each
side of the center tap as can fit on the whole winding of
a bipolar unit. Torque of the two types is equivalent at
Inductance of the windings causes a decrease in
torque as speed increases. You can compensate for this
by increasing the power supply voltage by a factor of
four and placing a resistor in series with each stator
winding. They recommend a resistor value of three times
the DC resistance of the winding. (This is called the
"L/4R circuit," counting the winding resistance.)
Naturally, this will increase power drain, and the power
rating of the resistors will have to be 2 times I squared
times R, using carbon power resistors.
In the unipolar circuit, a resistor is placed in
series with the center tap; since only half the winding
is powered at any given time, resistors on each end are
not required. The winding resistances listed in
specifications for unipolar motors are for half the
winding--measured either side of center tap.
We cannibalized a Seagate hard drive and found two
stepper motors. Oddly enough, the rotor that spins the
two hard disks is a very crude stepper capable of four
steps per turn. It has two 3-ohm stators; three leads
connect to these, making them look like a center-tapped
Of much more interest is the stepper that drives
the head carriage; this is capable of a resolution too
high to count by hand. It has two bipolar stators of
27.5 ohms each.
Lying about the place, we found a head motor of
exactly the same physical dimensions--also a
Seagate--which has five stators. This appears to have a
similar resolution, although I don't relish the task of
driving it. The winding resistance, all identical, is 19
ohms for each stator.
Now that IC's to drive stepper motors can be
gotten, they become an attractive electromagnetic device.
The signals required for the driver are pulses, which
can come from a variable-frequency clock, and a
direction-determining signal. If you couple this with a
counter, you can tell the system how far to go and when
to stop the motor.
Addendum Based on New
I just came back from our major electronics show of
the West Coast, WESCON. I saw some stepper motors of the
types described here, all right, but there are new kinds.
It seems that they are able to get about 20 times
the position resolution using lasers to cut pole pieces
of the stators. A company called Semix had a unit
capable of 800 steps per turn in the whole-step
mode--1600 steps per revolution in the half-step mode.
Another exhibitor, Motion Science, Inc., has an
eighteen-inch long table full of permanent-magnet poles,
with a shuttle containing stators riding atop an air
bearing (air escaping from holes underneath the shuttle
to make it nearly frictionless). He was claiming a
movement of only 2 microns per step.
Well, it looks like we can look forward to some
very fine control with these new kinds. They were all
bipolar types using permanent magnets.
A SURVEY OF PHILIPS/AIRPAX STEPPER MOTORS
Once known as Airpax and now a division of Philips,
the products listed here are but a few representative
samples--all of the "hybrid" type (using permanent-magnet
rotors). The Philips motors all come in 5-volt and
12-volt models, as well as bipolar and unipolar. Often,
there are related models with different resolutions, 24
versus 48 steps per turn.
Series K82600 and K82800--
These models have gear boxes with 1/4th-inch output
shafts. Their motor bodies are 2.187 inches in diameter.
The over-all length--up to the mounting surface from
which the shaft emerges--is 2.48 inches. The shaft
protrudes one inch, but for some of this distance (a bit
less than half an inch), there is a bushing around the
shaft that serves as a bearing. (There is a "flat" on
the shaft for a setscrew to rest against.) On opposite
corners of the gear boxes are 8-32 mounting studs;
mounting holes on the remaining corners presumably accept
All in this series are unipolar; they have six
leads and the stator windings are center-tapped. The
K82600 units have 24 steps per revolution (one step moves
the rotor 15 degrees); the K82800 ones have 48 (one step
moves the rotor 7.5 degrees). Speaking for the basic
motors (before their gear boxes), the holding torque of
the K82800 units is 13 inch-ounces, while the K82600 ones
have a holding torque of 10 inch-ounces.
They carry a hyphenated suffix; -P1 means 5 volts,
while -P2 means 12 volts. The winding resistance of 5V
units is approximately 7.5 ohms, and that of 12V units is
about 45 ohms. The inductances (for 5V and 12V units,
respectively) are 7mH and 32mH for 82600 ones; for the
82800 series, inductances are 8.5mH and 44mH,
Gear boxes are specified by the last two digits;
for example, K82616 and K82816 each have gear reductions
of 5 to 1. Available gear ratios are as follows:
The K82601 and K82801 units have no gear boxes;
the ratio is listed as 1 to 1. Remembering that a 6 or 8
specifies 24 or 48 steps: 11 means 2 to 1, 16 is 5 to 1,
21 is 10 to 1, 24 is 15 to 1, 27 is 20 to 1, 30 is 25 to
1, 31 is 30 to 1, 36 is 50 to 1, 37 is 60 to 1, 39 is 75
to 1, and 45 means 150 to 1.
Now, remember that the gear reduction affects both
the torque and the angle of rotation per step. For
example, a 10 to 1 gear box--such as that of the
K82821--creates a holding torque of 130 inch-ounces, and
the rotation per step is 0.75 degrees. The K82845-P2 is
a 12V unit whose steps rotate the shaft 0.05dg.
B Series--These also have gear boxes. They are 1.86
inches deep--including the gear box. The gear box is egg
shaped (2.44 inches in its longest direction). A
1/8-inch diameter shaft emerges nearest the narrow end.
Ears either side of the gear box are provided for
They are available in unipolar and bipolar
configurations--either available as 5- or 12-volt units.
Three step angles are available--18dg (20 steps per
revolution), 15dg (24 steps), and 7.5dg (48 steps).
(Remember, torque and resolution specs are for the motor
before the gear box.)
For the unipolar ones, the 18dg, 15dg, and 7.5dg
units have holding torques of 1.9, 2.4, and 2.6
ounce-inches, respectively. As you would expect, the
holding torques for bipolar ones are higher--2.3, 2.8,
and 3.5 ounce-inches, respectively.
The 35M020B has 20 steps per revolution of the
rotor, the 35M024B has 24 steps per revolution, and the
35M048B has 48 steps. A 1 or a 2 following the B means
5V or 12V, respectively. Following that designation, a U
or B specifies unipolar or bipolar. Thus, a 35020B1B
unit is a 5-volt bipolar 20-step motor.
Gear ratios are another suffix--a hyphen X,
followed by a number: 24 is 15 to 1, 27 is 20 to 1, 31
is 30 to 1, 37 is 60 to 1, 39 is 75 to 1, 45 is 150 to 1,
52 is 300 to 1, and 64 is 1350 to 1. Thus, a
35M020B1B-x31 is a 5V bipolar 20-step motor carrying a 30
to 1 gear box. (Note that this would give you 600 steps
per revolution. Do you sense a clock in the works?)
For both winding styles, 5V units have 12.5-ohm
resistances, while the 12V units are 72 ohms.
Inductances vary through the list of combinations.
Rounding off, unipolar 5V motors in this series present 6
or 7 millihenries, while 12V unipolar ones range from 30
to 36 millihenries. Bipolar ones have about twice the
inductance--14mH for 5V motors and about 75mH for 12V
The L82400 Basic Stepper Motor--
This has a 1.6-inch diameter mounting flange at the
output end. It is about 0.92 inches deep and the motor
body is 1.654 inches in diameter. The shaft diameter is
1/8 inch. The holding torque of the unipolar version is
10.4 ounce-inches; for the bipolar version, this torque
is 12.4. It has 48 steps per revolution.
The L82401 is unipolar; the L82402 is bipolar.
The suffix -P1 means 5 volts; -P2 means 12 volts.
The winding resistances for both bipolar and
unipolar ones are 9.1 ohms for the 5V one and 52.4 ohms
for the 12V one. Inductances for unipolar ones are 7.5mH
and 46.6mH for 5- and 12-volt units, respectively. For
bipolar, inductances are 14.3mH and 77.9mH.
The Basic L82701 Stepper--
Slightly more than an inch deep, this model has two
mounting holes on a roughly diamond-shaped flange. The
diameter of the body is 2.28 inches. Its shaft diameter
is 1/4 inch.
This motor has 48 steps per revolution. It only
comes in a uni-polar version. The holding torque is 15
For 5V and 12V versions (specified by -P1 and -P2
suffixes), the winding resistances are 6.3 ohms and 36
ohms, respectively. The corresponding inductances are
7mH and 38mH.
These are the cutest little darlings! They look like
a small can with mounting ears at one end. Running
through the center is a leadscrew--a threaded shaft with
a rather coarse pitch to the thread. The rotor has no
shaft; it is tapped to match the thread of the leadscrew.
The leadscrew is secured to something which is to
be moved to and fro; i.e., the idea is to keep it from
turning. When the rotor inside the device turns, it
operates the leadscrew so as to make it move in or
The 9200 Series of Linear Actuators--
The diameter of these devices is 1.06 inches, and the
body is 1.05 inches deep. The body has a bushing which
protrudes beyond the mounting face by about 0.6 inches.
The output end of the leadscrew has a 4-40
thread--terminated by a shoulder--so that it can be
screwed to the piece being moved.
The part number is preceded either by K or L; the
K has a maximum travel of 1/2 inch, while the L model
(provided with a longer leadscrew) has a maximum travel
of 1-7/8 inch.
Available units are unipolar; this is shown by the
last digit in the number, which is always 1. Then, the
third digit in the number determines the distance
traveled per step: 4 means 0.004 inches per step, 2
means 0.002 inches, and 1 means 0.001 inches per step.
Thus, an L9211 is a unipolar linear actuator whose range
is 1-7/8 inches (specified by the L), and whose
resolution is one-thousandth of an inch per step. A
K9241 is a short-range (1/2 inch) unit whose resolution
is four-thousandths per step.
Finally, 5V and 12V actuators are available,
specified by the suffix -P1 and -P2, respectively.
The coil resistances are 15 ohms for the 5V
version and 84 ohms for the 12V one. Winding inductances
are 5mH and 29mH, respectively.
The catalogue lists two addresses as follows:
- Philips Technologies, Airpax Mechatronics
Group (International Division), 604 West Johnson Ave., PO
Box 590, Cheshire, CT 06410; Phone: (203) 271-6000.
- Airpax, 2001 Gateway Place, San
Jose, CA 95110; Phone: (408) 453-7373.
A SURVEY OF STEPPER-MOTOR DRIVER CHIPS
Three Motorola chips, as well as a couple of
experimenters' kits, are presented here. It seems that
currently available devices are designed to drive 2-phase
bipolar hybrid motors, examples of which are listed in
the previous article.
The Motorola MC3479P
For about $4, this is available from at least two
sources. The Jameco Cat. No. is 25216. It can also be
gotten from JDR Micro Devices under the catalog number of
its own part designation--MC3479P.
This is said to be appropriate for driving 6- or
12-volt motors. Actually, by associating a resistor with
a bias pin, the outputs act like current sources, so they
can drive 5-volt stators of units listed in the previous
article. With this particular chip, the maximum drive
current available is 350mA.
This is a 16-pin chip. Pins 4, 5, 12 and 13 are
connected to the substrate; a heat sink can be soldered
to these, or soldering these pins to a ground plane on
the board can aid in dissipating the heat. These pins
are the negative supply terminal on the chip and can be
considered as ground.
Provision is made so that the outputs can be diode
protected; the four outputs go to diodes internal to the
chip (the anodes of these diodes being at the output
pins). The cathodes of these four clamping diodes go to
a common pin (pin 1), and pin 1 is connected to the VCC
(pin 16) through a zener diode, a regular diode, or even
a jumper. If a 3-volt zener is used (such as a 1N5221),
its cathode goes to pin 1 and its anode goes to pin 16.
Outputs 2 and 3 go to one stator--the leads called
L2 and L1, respectively. Outputs 14 and 15 go to the
other stator--these leads are called L4 and L3,
This chip is capable of supplying two kinds of
drive signals. The conventional scheme is to always
supply both stators simultaneously. The Motorola
literature calls this the "whole-step" drive. The
advantage is that the "holding torque" is consistent for
each step. This is true because magnets of the rotor are
always attracted to like poles of both stators. Four
steps per pole pair are made (see the previous article).
The so-called "half-step" drive alternates
energizing both stators with powering one or the other
singly. This causes the rotor to move eight steps per
pole pair of a stator. The disadvantage is that, when
only one stator is energized, the holding torque is not
as high as when both are powered.
There is an open-collector output, pin 11, that
when the output drivers are in the "phase A" state, the
collector brings this pin low. Phase A is defined as
Connections L1 and L3, pins 3 and 15, are high.
L2 and L4, pins 2 and 14, are low. In other words, the
coil between pins 2 and 3 will see negative on pin 2 and
positive on pin 3. The coil between pins 14 and 15 will
see negative on pin 14 and positive on pin 15.
Motorola defines "clockwise" as first changing the
polarity of the L1-L2 stator, then reversing the current
through the L3-L4 stator. Thus, in the whole-step mode,
the first clock pulse causes pin 2 (L2) to go high and
pin 3 to go low. In the half-step mode, this clock pulse
would de-energize the L1-L2 stator, and a subsequent
pulse would cause its reversal.
There is a pin (pin 6) which has three functions:
First, it is a "bias pin." This pin determines
the base currents of the NPN transistors which sink
current at the negative ends of the coils; these
transistors are current sources. Normally, this pin is
taken to ground through a resistor, the value of which
determines the drive current.
Pin 6 is also called the "set pin"; if less than 5
microamps is drawn from it--if it is open-circuited--the
internal logic presets to the "Phase A" condition. When
pin 6 is open, pin 11 (the "phase-A output detection pin)
goes low, and all other inputs are ignored until current
is drawn from pin 6 again.
Finally, opening pin 6 causes the outputs to
assume a high-impedance condition (simultaneously setting
the chip for the "phase A" state).
Unlike the other inputs, pin 6 is not to be
operated at the 5V logic level. Unless it is permanently
connected to ground through a resistor, pin 6 should go
through that resistor to an open-collector output. This
could be an open-collector gate or just an NPN transistor
Pin 6 goes through a resistor to the collector of
a 2N2222 whose emitter is grounded. The base of the 2222
is then operated through a 10K resistor. (A small
capacitor in parallel with this 10K resistor will make
the transistor turn off faster.)
The output sink current is determined by: IBS
(the current in microamps drawn from pin 6, the "bias/set
pin") equals 0.86 times the output current in milliamps.
Pin 6 wants to see a current sink of greater than 5uA.
In normal operation, the voltage on pin 6 will be VM (the
main supply) minus 0.7V. Thus, the appropriate RB (the
resistor from pin 6 to ground) will be the quantity VM
minus 0.7V, divided by IBS.
Suppose I wish to drive a 2-phase bipolar stepper
motor with drive currents of 200mA, and suppose I supply
the circuit with a healthy 9V battery. This 200mA times
0.86 equals 172uA. Next, I want the value of a
resistor--from pin 6 to ground--which will draw 172
microamps: The quantity 9 minus 0.7 volts, or 8.3 volts,
divided by 1.72 times 10 to the minus fourth, equals 43K.
One suggested use of pin 6 is to reduce current
drain when the motor is not being operated. If the
stepper is positioning something which does not require
the full holding torque to keep the mechanical assembly
in place, the current drawn from pin 6 can be reduced
while the system is dormant.
Besides the "bias/set" pin, this chip has four
inputs. They are intended to be operated with 5-volt
logic; the maximum allowable voltage is 5.5V. They are
CMOS compatible, however, with the maximum hysteresis
The "clock input" (pin 7) advances the logic that
controls the output drivers, and it does so on positive
transitions. When the "set pin" is open-circuited,
pulses of the clock--as well as all other input
signals--have no effect.
The direction of the logic sequence is determined
by pin 10. When high, driver outputs operate in a
sequence defined as "clockwise"; when pin 10 is low, the
sequence is defined as "counterclockwise." (The actual
direction of rotation also depends on how the leads to
the stators are wired to the chip.)
Pin 9 selects the whole-step vs. half-step mode.
When low, the motor will move four steps per pole pair.
When pin 9 is high, the motor can make twice as many
steps because there are times when only one stator is
energized. (In the half-step mode, the holding torque
will not be uniform, since the rotor, during these
"half-steps," will be attracted to one stator only.)
Pin 8 is called the "output impedance control."
Although it always should be committed to 5V or ground,
the setting of pin 8 is only relevant when driving motors
in the half-step mode. Its operation gives you a choice:
In the half-step mode, one or the other of the
stators will not be energized one-fourth of the time.
When pin 8 is low, the two outputs of a non-energized
coil will be in a high-impedance state. When pin 8 is at
logic 1, the PNP transistors will be turned on; the
outputs will have a low impedance with respect to VM.
(The importance of this selection has eluded your editor
The SAA1042 and SAA1042A
These are different from the above 3479 in two
respects. First, they have mounting ears by which they
can be attached to heat sinks. Second, they are not
quite as versatile; they have fewer features. However,
since they are capable of supplying 500mA, they are worth
The SAA1042 is for 6V and 12V motors; the SAA1042A
can run 24V motors. Running 5V motors is no problem,
since they, too, have current-sourcing outputs. Their
pin assignments are the same; the 1042A just has higher
Their pin arrangements are slightly different from
the 3479: they have a separate supply pin for the logic.
Coil-output drivers are on pins 1 and 3, 14 and 16.
This means that the external jumper or diode for clamping
the output voltages goes between pins 2 and 15, a small
Pin 6, the "bias/set" pin is the same, except that
the relationship between bias current and drive current
is not such a nice linear one as it is with the 3479.
Some sample values for RB off pin 6 were gotten from a
graph in the literature, and the corresponding drive
currents are listed as follows:
- For 6V:
- For 12V:
- For 24V (using the SAA1042A):
For over-voltage clamping of the outputs,
they recommend a 3.9V zener between pins 2 and 15
(cathode at pin 2). Appropriate part numbers would be:
1N5520, 1N4622, 1N4730, or 1N748.
As with the MC3479, pin 8 determines half-step
versus whole-step mode; bringing it high puts the chip in
the "half-step" mode--eight steps per pole pair. Pin 10
is the clockwise versus counterclockwise pin; bringing it
high means that the outputs work in the "clockwise"
direction in response to the clock. ("Clockwise" means
that the L1-L2, pins 1 and 3, change first; they reverse
polarity in the "whole-step" mode, or de-energize the
coil in the "half-step" mode.)
If pin 6 is open-circuited, the chips (both 1042
and 1042A) are set to "state A"; pin 3 will be high and
pin 1 will be low, while pin 16 will be high and pin 14
will be low. (Note that pin 3 is L1, pin 1 is L2; pin 16
is L3 and pin 14 is L4.)
An advantage in these chips is that the logic need
not be restricted to 5V. The logic VCC pin, pin 11, can
be up to 20 volts. (This restriction of 20V applies to
the "logic supply" of both chips.) The SAA1042A allows a
"VM" (on pin 15) of 30V, while the SAA1042 allows an
absolute maximum VM of 20V.
As with any driver system, power dissipation is an
issue. The SAA chips list a maximum power dissipation of
2 watts; it is the editor's impression that this is
without a heat sink.
The literature for the MC3479P does not give a
maximum package dissipation directly. They say that
dissipation should be such as not to exceed junction
temperatures of 150 degrees C. They then say that
"thermal conductivity," junction to ambient, is 45
degrees per watt without a heat sink (hmmmmmmm).
Calculating the power dissipated by the chip is a
bit of a trick, since output transistors needn't be
saturated. However, if you know the motor coil
resistances and the currents you intend to run them with,
you know the voltages across the windings; subtracting a
winding's voltage from the full supply and multiplying
the difference by the winding current, you will know how
much each driver output pair will dissipate while that
winding is energized. In the "whole-step" mode, both
coil drivers are always on simultaneously, so the power
dissipated by the whole package will be twice the winding
current times voltage dropped across the driver
transistors. We are only partly finished; power
dissipation must now be linked with junction temperature.
Suppose our power calculations lead us to a figure
of 2 watts of dissipation. Multiplying 2 watts by 45dg
per watt, we get 90dg C. They just add an arbitrary 25dg
C. to this--an assumed figure for the ambient
temperature--thus obtaining a figure of 115dg C. With
the 3479 in mind, this is less than the 150dg C. which
they allow without a heat sink.
- MC3479P Specs:
- Recommended Operating Voltage--7.2 to
- Maximum Voltage--18V.
- Clamp Voltage (at diode
cathodes)--At VM, or up to VM plus 4.5V.
- Drive Current--Up to 350mA.
- Maximum Logic Input Voltage--5.5V.
- Input Hysteresis--0.4V max.
- Clock--Positive-edge triggered.
- MC3479P (without heat-sink mounting):
- 4, 5, 12, 13--Ground
- 16--VM/VCC (for both logic and
- 2--Output L2
- 3--Output L1
- 14--Output L4
- 15--Output L3
- 11--Phase A Indicator (open
collector brings pin 11 low when outputs are in "phase
- 6--Bias and Set
- 7--Clock (positive-edge triggered)
- 9--NOT Whole/Half Step (when low,
chip operates in "full-step" mode, moving the motor four
steps per pole pair)
- 10--Clockwise vs. Counterclockwise
(also called "NOT Counterclockwise," logic high is said
to make operation clockwise)
- 8--Output Impedance Control (in
half-step mode, when pin 8 is low, non-energized outputs
will be in tristate; when pin 8 is high, non-energized
outputs will be low-impedance with respect to pin 16)
SAA1042 and SAA1042A:
- 4, 5, 12, 13--Nonexistent
(coinciding with ears for heat sink)
- 9--Ground (along with heat-sink
- 11--Logic VCC
- 15--VM (V with a subscript M) for
- 1--L2 Output
- 3--L1 Output
- 16--L3 Output
- 14--L4 Output
- 2--Clamp (to be clamped at pin 15)
- 7--Clock (positive-edge triggered)
(high for clockwise)
- 8--Whole/Half step
Exemplary Circuits Using the MC3479Pand a Bipolar
These should get you started. We cannibalized a
Seagate hard drive, and we were able to run its head to
and fro across the disk just for fun.
The basic driver here makes it just a variable
speed motor with selectable direction. An embellishment
is then added to make the shaft oscillate (rotate to and
fro). This is just the sort of hookup you could use to
make a mechanical monkey beat a drum and tap his foot.
For those who remember the fabulous window
displays set up by F.A.O. Schwartz at Christmas time,
compare how much easier it is to build this monkey
without gears and a reciprocating mechanism. With that
in mind, why not design a mobile window display for your
business or something.
Just a clock and the driver chip are required to
run the motor, as long as the direction terminal, pin 10
of the 3479, is committed to VCC or ground.
The oscillating shaft requires a primitive scheme
for starting the mechanism at an initial position. That
way, the monkey won't poke himself in the eye with his
drumstick or put his foot through your work bench.
Basic Variable-Speed Motor--
A supply of 9V or 12V is used. As you might expect,
VCC must be bypassed heavily because of hash from the
motor--parallel combinations of 220uF and 0.1uF at either
ends of the rails will do (negatives of the electrolytics
at ground). This supply does not have to be well
Pin 1 of a 555 is grounded; pins 4 and 8 go to
VCC. Located close to the chip is 0.1uF between pins 1
and 8. Also, pin 5, the voltage-control pin, is bypassed
to ground by 0.1uF.
Pins 2 and 6 are tied together and go through
0.01uF to ground. Pins 2 and 6 also go through 100K to
pin 7. Pin 7 goes through 47K, in series with a 500K
rheostat to VCC.
The 555's output, pin 3, goes to the clock input,
pin 7, of the MC3479P motor driver chip. This clock
input is positive-edge triggered.
On the MC3479P, pin 4 is grounded. Pin 16 goes to
VCC--heavily bypassed nearby. Pin 6 goes through 47K to
ground. Pin 8 goes to VCC (although in "full-step mode,"
pin 8 can be committed anywhere). Pin 9 is grounded;
this selects the "whole-step mode." Finally, committing
pin 10 to ground, or to VCC, selects direction of
rotation (which is also affected by hookup of the stator
Pin 1 goes to the anode of a 1N4001 diode; the
cathode goes to pin 16. One stator goes between pins 2
and 3, while the other stator goes between pins 14 and
15. Experimentation will determine which way the motor
runs when the chip runs "clockwise" (with pin 10
committed to VCC).
Circuit for Making the Shaft Oscillate--
The motor is made to run in one direction until a
switch is tripped; thereafter, the excursions are
determined by selecting outputs from a counter. A
flip-flop sets the initial direction by bringing pin 10
of the driver high; closure of the switch when the
mechanism arrives changes the direction and enables a
4040 12-bit counter. (The switch can be a simple spring
leaf, or an opto-isolator.)
A 4011 quad NAND gate is used to create a
NOT-R/NOT-S flip-flop. Pin 7 of the 4011 is grounded,
while pin 14 goes to VCC. Pin 3, the output of one gate,
goes to pin 5, an inpinput of the second. Pin 4, the
output of the second gate, goes to pin 2, an input of the
first. Pin 1 is NOT-Reset and pin 6 is NOT-Set.
One side of the switch is grounded. The other
side goes to pin 1 of the 4011. Pin 1 also goes through
100K to VCC. Pin 6 of the 4011 goes through 910K to VCC,
as well as going through 0.1uF to ground.
The clear terminal of the 4040 counter, pin 11,
goes to pin 4 of the 4011. Pin 3 of the 4011 goes to its
own pin 12, another gate input. Pin 11, the output of
this gate, goes to pin 10 of the 3479 driver chip.
The pin 13 input of this gate receives inverted
squarewave pulses from the desired 4040 output. Thus,
pin 13 of the 4011 goes to its own pin 10. Its pins 8
and 9 are tied together and are taken to an output of the
4040--pin 13 or pin 12 being likely choices.
Pin 3 of the 555 goes to pin 10, the clock input,
of the 4040 counter (positive-edge triggering for this
pin). Pin 8 of the 4040 is grounded; pin 16 goes to VCC.
Located close to the chip, pin 16 is bypassed to pin 8
The amount of rotation back and forth is
determined by the choice of output on the 4040 12-bit
counter. Pin 12 causes the shaft of our disk-drive motor
to go approximately one full turn. The following are the
divider outputs of the 4040 in order: Pin 9 divides the
clock by two--the shaft would go two steps in either
direction. Next come: 7, 6, 5, 3, 2, 4, 13, 12, 14, 15
The All Electronics SMKIT-2 Stepper Motor
A company called All Electronics, an outfit that sells
various oddments, has a stepper-motor kit. Called the
SMKIT-2, it comes with a motor, a driver (which must be
assembled on a PC board), and a power supply--all of this
selling for about $25. The driver allows you to run the
motor step-by-step or at variable speed.
We have not purchased this kit, so we have no
verbal description for its assembly. But, it sure sounds
like a screamin' deal.
- All Electronics, PO Box 567, Van Nuys, CA
91408; Phone: (818) 904-0524; (800) 826-5432.
- Jameco Electronics, 1355 Shoreway
Road, Belmont, CA 94002; Tel: (415) 592-8097
- JDR Micro Devices: 2233 Samaritan
Drive, San Jose, CA 95124; Phone: (800) 538-5000, or