L293D

The L293D from STMicroelectronics is a classic quadruple half-H driver integrated circuit that has been a staple in hobbyist and educational motor control projects for decades. The device contains four push-pull output drivers arranged as two complete H-bridge circuits, each capable of driving a DC motor bidirectionally or a stepper motor winding. The D suffix variant includes internal flyback clamp diodes across each output transistor, eliminating the need for external suppression diodes on inductive loads such as motors and relays. Each half-H driver can supply 600mA continuous current and 1.2A peak (non-repetitive) current, making it suitable for small motors in robotics, toys, and educational projects. The motor supply voltage (VCC) can range from 4.5V to 36V, independent of the logic supply voltage (VSS, 4.5V to 7V), allowing the logic to operate at 5V while driving motors at higher voltages. Each H-bridge has a dedicated enable input that can be used for PWM speed control – driving the enable pin with a PWM signal from a microcontroller varies the average motor voltage and hence the speed. The two direction input pins per H-bridge control the motor direction by selecting which pair of output transistors conducts. The L293D is available in a 16-pin DIP package for easy prototyping on breadboards, and also in SOIC-20 surface-mount versions. The main limitation is the relatively high output saturation voltage (approximately 1.4V total at 600mA), which reduces efficiency and causes significant heat dissipation at higher currents.

The L293D operates as a dual H-bridge motor driver using bipolar Darlington output transistors. Key subsystems include: (1) H-Bridge Topology – each H-bridge consists of four output transistors (Q1-Q4) arranged in an H configuration with the motor connected between the two half-bridge outputs (Y1 and Y2); when Q1 and Q4 are on, current flows through the motor in one direction; when Q2 and Q3 are on, current flows in the reverse direction; (2) Darlington Output Stage – each output transistor is a Darlington pair (NPN+NPN), providing high current gain but with a saturation voltage of approximately 0.7V per transistor (1.4V total for both switches in the current path); this voltage drop causes significant power dissipation (P = Vsat x I = 1.4V x 0.6A = 0.84W per bridge at full load); (3) Logic Interface – TTL-compatible input pins (IN1/IN2 for bridge 1, IN3/IN4 for bridge 2) control the direction; each enable pin (EN1, EN2) turns the corresponding bridge on/off; PWM on the enable pin controls motor speed by varying the duty cycle; (4) Internal Clamp Diodes – four diodes per bridge (one across each output transistor) provide a path for inductive kickback current when the output transistors turn off; this protects the transistors from voltage spikes generated by the motor inductance; (5) Dual Supply – VCC (pin 8) powers the output stages and the motor; VSS (pin 16) powers the internal logic; this separation allows the motor supply to be higher than the logic supply.

• Dual DC motor robot: two H-bridges drive left and right motors independently with direction and PWM speed control
• Bipolar stepper motor: both H-bridges drive the two windings of a 4-wire stepper with full-step or half-step sequences
• Relay/solenoid driver: each half-bridge can independently drive a relay or solenoid up to 36V/600mA
• Educational projects: DIP package for breadboard prototyping, Arduino-compatible motor shield