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Drivers

Silicon controlled rectifier (SCR)

also known as Thyristor

Triac

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H-bridges

H-bridges built with transistors, MOSFETs or SSRs can turn on and off much more quickly than

mechanical relays. This allows them to control both speed and direction of rotation for DCmotors.

An H-bridge using four SPST relays is shown below to demonstrate direction control. Whenrelays A and D are closed at the same time, current passes through the motor causing CW

rotation. When relays B and C are closed at the same time, current passes through the motor in

the opposite direction causing CCW rotation. Closing relays A and B at the same time causes adead short across the motor to provide electrical braking. If all four relays are left open, the

motor can rotate freely.

Be careful using H-bridges. Accidentally closing relays A and C at the same time, or closingrelays B and D at the same time, causes a dead short across the battery. Additional logic

circuits are often used to prevent these catastrophic conditions.

Equivalent H-bridges using transistors and MOSFETs are shown below. Surge suppression

diodes should be connected across the transistors or MOSFETs to prevent damage caused bysudden motor stall or reversal. Amplifier chips (not shown) are often required to bias base or

gate control voltages high enough to activate the transistors or MOSFETs in the upper legs.

DC

motor

A

C

B

D

Battery + Battery +

Battery - Battery -

H-bridge CW rotation

DC

motor

A

C

B

D

Battery + Battery +

Battery - Battery -

H-bridge CCW rotation

DC

motor

A

C

B

D

Battery + Battery +

Battery - Battery -

Transistor H-bridge

Base A

Base C

Base B

Base D

DC

motor

A

C

B

D

Battery + Battery +

Battery - Battery -

MOSFET H-bridge

Gate A

Gate C

Gate B

Gate D

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SSRs are equivalent to SPST relays as shown below. They contain MOSFETs controlled by

internal LEDs to provide optical isolation between inputs and battery connections. Inputs can

range from 3 to 24 VDC and draw about 1 mA/V. Consequently, SSRs are particularly easy to

interface to TTL digital logic (5 VDC) and microprocessors. SSRs also usually contain internalprotection circuits for current limiting and surge suppression.

Motor speed is controlled by turning an H-bridge on and off rapidly at a fixed frequency but

using different duty cycles as shown below. This method of speed control is called PWM (pulse

width modulation). Motor speed is roughly proportional to duty cycle if the modulationfrequency is significantly higher than the motors frequency response. For CW rotation, H-

bridge legs A and D may be modulated simultaneously. Alternately, leg D may be turned on at

100% duty cycle while leg A is modulated.

DC

motor

A

C

B

D

Battery + Battery +

Battery - Battery -

SSR (solid state relay) H-bridge

Input A

Input C

Input B

Input D

OFF

Otime

70% duty cycle provides about 70% speed

OFF

Otime

20% duty cycle provides about 20% speed

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FOUR PHASE STEPPER MOTOR DRIVE SEQUENCE(Ref: Allegro Microsystems UCN5804 spec sheet)

Wave Drive Sequence (full steps)

Step A B C D

1 ON2 ON

3 ON

4 ON

5 ON

Two-Phase Drive Sequence (full steps)

Step A B C D

1 ON ON

2 ON ON

3 ON ON

4 ON ON5 ON ON

Half-Step Drive Sequence (half steps)

Step A B C D

1 ON

2 ON ON

3 ON

4 ON ON

5 ON

6 ON ON

7 ON

8 ON ON

9 ON

A

B

C

D