38 Elektor Electronics 12/99

K.S.M. WalravenThe control is a compact design for driving bipolar stepper motors.Driver stages IC2 and IC3 are modern types from ST Microelec-tronics. Because of the combination of CMOS logic with D-MOSpower transistors, these devices need few external components.Also, compared with the previous generation of bipolar devicessuch as the L298, the D-MOS transistors drop lower voltages sothat the internal dissipation is smaller.

The input stage, IC1, enables the motor to proceed one stepfor each pulse at its input pin 18. The level at pin 17 (CW/CCW)

determines whether the motorrotates clockwise or anticlock-wise. The level at pin 19 decideswhether the motor moves wholeor half steps for each pulse atpin 18. In normal operation, pin20 (reset), pin 11 (control), andpin 10 (enable) should be linkedto the +5 V supply. Pin 1 (sync)is an output used when severalL297s are driven in tandem andshould be left open in the pre-sent design. Pin 3 (home) is anoutput that indicates when out-puts A, B, C, and D, assume thebinary code 0101, and is notused in the present design. Theother pins are of less impor-tance and will in most cases notbe used at all – further informa-tion may be obtained fromInternet addresshttp://www.us.st.com

Since the current throughthe motor coils must not only beswitched on and off, but also bereversed, the driver ICs containa complete bridge formed byfour D-MOSFETs. The uppertwo need to be driven by apotential that is higher than thesupply voltage, and this isobtained with the aid of a boot-

HALF/FULL

CONTROL

CW/CCW

ENABLE

SENS1

SENS2

RESET

L297

IC1

VREF

INH1

INH2SYNC

HOME

STEP

OSC

17

18

19

20

10

15

14

13

12

16

11

4A

6B

7C

9D

5

81

3

2

ENABLE

BOOT1

SENSE

L6203BOOT2

IC2

OUT1

OUT2

VREF

IN1

IN2

11

10

3

47

1

2

6

5

8

9

ENABLE

BOOT1

SENSE

L6203BOOT2

IC3

OUT1

OUT2

VREF

IN1

IN2

11

10

3

47

1

2

6

5

8

9

K1

10

1

2

3

4

5

6

7

8

9

R2

3k

9

R3

1k

R1

22

k

C1

3n3

R7

0Ω

5 C3

220n

R6

0Ω

5 C4

220n

C10

100n

C9

100n

C5

15n

C6

15n

C7

15n

C8

15n

C11

22n

R4

10

Ω

C12

22n

R5

10

Ω

C2

100n

L1

L2

C13

10µ63V

5V5V

5V

5V

994065 - 11

(C) Segment 994065-1C1

C2

C3

C4

C5

C6

C7

C8

C9

C10

C11C12

C13

H1 H2

H3H4

IC1

IC2IC3

K1

R1

R2

R3

R4R5

R6

R7

0

+

L2 L1

994065-1

(C) Segment 994065-1

009stepper motor control

39Elektor Electronics 12/99

strap circuit (C5, C6). Network R4-C11 suppress voltage peaksacross the motor terminals. Most of the other capacitors in the dia-

gram are decoupling (bypass) elements.The driver ICs can handle currents of up to 4 A at voltages up

to 42 V. For safety’s sake, it is better for the voltage to remain wellbelow 42 V; the current is internally limited to 4 A. Any tendencyof the current to rise above this level is sensed by resistors R6 andR7, whereupon the IC is disabled. The value, R, of these resistorsmust, therefore, be in line with Im, the motor current: R=1/Im.

The driver ICs are provided with internal thermal protection,but when the dissipation is large, it is advisable to mount them ona suitable heat sink. They do not get damaged by heat, but they doswitch off the motor when the temperature rises above the maxi-mum specified temperature.

Finally, it should be noted that IC1 operates from a +5 V sup-ply (via K1) from which it draws a current of about 50 mA. Thevoltage at pins 0 and + is intended for the stepper motor andshould be equal to, or a little higher than, the rated motor voltage.

[994065]

Parts list

Resistors:R1 = 22 kΩR2 = 3.9 kΩR3 = 1 kΩR4, R5 = 10 ΩR6, R7 = 0.5 Ω, 3 W (see

text)

Capacitors:C1 = 3.3 nFC2, C9, C10 = 100 nFC3, C4 = 220 nFC5–C8 = 15 nF

C11, C12 = 22 nFC13 = 10 µF, 63 V, radial

Semiconductors:IC1 = L297

(ST Microelectronics)IC2, IC3 = L6203

(ST Microelectronics)

Miscellaneous:K1 = 10-way headerPC1–PC6 = PCB pinsL1, L2 = bipolar stepper

motorPCB Order no. 994065-I

Design by V. Mitrovic

The LM3914 is a truly versatile component. Besides LEDs, only afew other components are needed to make the ‘bidirectional’ bar-graph voltmeter shown here. The circuit is similar to a conven-tional bar display, but it offers a possibility to change the directionin which the LEDs are switched on. This may be useful, for exam-ple, when positive and negative voltages are measured. For a pos-itive input voltage, the LEDs are switched on in the usual manner,that is, from D3 to D12, while for negative voltages, the LEDs areswitched on in the opposite direction, from D12 to D3. Obviously,the negative voltage must be ‘rectified’, i.e. inverted, before themeasurement. A suitable circuit for this purpose is presented inthe article ‘Absolute-value meter with polarity detector’ (referencenumber 994020-1) elsewhere in this issue. A set of transistor switches (MOSFETs) controls the direction inwhich the LEDs light. When the control voltage is high (+6V,according to the schematics, but any voltage that is at least 3V

higher than reference voltagewill do), T1 and T4 are switchedon, while the other two MOS-FETs are off. In this way, theLM3194 is configured in theusual manner with the top endof the resistor network con-nected to the internal voltagereference and the low end con-nected to ground. As the inputvoltage rises, the comparatorsinside the LM3914 will causethe indicator LEDs to beswitched on one by one, startingwith D3. When the control voltage islower than about –3V, T2 andT3 are switched on while T1and T4 are off. Consequently,the ends of the resistor networkare connected the other wayaround: the top end goes toground and the low end, to thereference voltage. The first

LED to be switched on will then be D12; i.e., the LEDs that formsthe bargraph display light in the opposite direction. Although notdocumented by the manufacturer of the LM3914, this optionworks well, but only in bar mode (in dot mode, internal logic dis-ables any lower-numbered LEDs when a higher-numbered LEDis on, which obviously conflicts with our purposes).To achieve good symmetry, an adjustable resistor is added to thevoltage divider in the LM3914. Using a DVM, adjust the presetuntil the voltage across P1+R4 equals 1/11th part of Urefout.Sensitivity is determined with the ratio of resistors R5 and P2. If, forexample, the reference voltage is set to 2.2 V by means of P2, therewill be a voltage drop of 200 mV per resistor in the ladder network(including R4-P1). So, the first LED will switch on when the inputvoltage exceeds 200 mV, the second, at 400 mV, and so on, and thewhole display will be on at 2 V.The circuit draws about 100 mA when all LEDs are switched on.

(994012-1)

R5

2k

2

R2

10

0k

R1

10

0k

R3

10k

R4

820Ω

4k7

P2

1k

P1

T4

BS170

T1

BS170

T3

BS250

T2

BS250

D1

6V2

D2

D10

D9

D8

D7

D6

D5

D4

D3

D12

D11

REFOUT

REFADJ

LM3914

IC1MODE

SIG

RHI

RLO

L10

17

16

15

14

13

12

11

10

L9

L8

L7

L6

L5

L4

L3

L1

18L2

9

5

8

4

6

7

3

2

1

D13

1N4001

C2

100n

C1

100µ 25V

500mW

2x

UIN

+/– 6V

control

994012 - 11

9V

010+/– voltage on bargraph display