L293D Motor Driver IC: Sizing, Wiring, and Load Limits

The L293D Motor Driver IC: What It Actually Does

The L293D is a dual H-bridge motor driver IC capable of driving two bidirectional DC motors or one 4-wire bipolar stepper motor. It handles up to 600mA continuous current (1.2A peak) per channel across a 4.5V to 36V operating range. If you are building a basic robotics chassis or an automated pan-tilt rig, this chip is the default interface between low-voltage microcontroller GPIO pins (which can only source ~20mA) and high-current inductive loads.

Internally, the L293D uses Bipolar Junction Transistor (BJT) Darlington pairs to switch the motor phases. While this architecture is rugged and forgiving of minor wiring mistakes, it introduces a significant voltage drop—typically 1.4V to 2.0V across the H-bridge. This means if you feed the chip 5V, your motor only sees about 3.5V. Understanding this voltage drop and the chip's thermal limits is the difference between a reliable build and a frustrating afternoon of debugging stalled motors.

Motor Type Matchup: Which Load Fits the L293D?

Not all motors demand an H-bridge. Treating a stepper and a servo as interchangeable is a common beginner mistake that leads to fried components. Here is how the L293D maps to common hobbyist motor types.

Wiring and Terminal Identification

The L293D comes in a 16-pin DIP package. The 'D' in the part number signifies built-in flyback snubber diodes, which protect your circuit from inductive voltage spikes when the motor coils de-energize. If you are using the base L293 (without the D), you must wire external 1N4007 diodes across every motor terminal.

Sizing Rule of Thumb and Worked Load Example

The datasheet claims a 600mA continuous current limit per channel. However, this assumes an infinite PCB copper heatsink and a 25°C ambient environment. In a real-world breadboard or perfboard setup with free air convection, the thermal resistance ($\theta_{JA}$) of the DIP package is roughly 80°C/W.

The Sizing Rule of Thumb: Never run the L293D above 400mA continuous in free air without active cooling.

Worked Load Example: 6V TT Gearmotor

Let's size a driver for a standard yellow TT gearmotor used in 2WD robot chassis.

• Motor Specs: 6V nominal, 200mA no-load current, 800mA stall current.
• Supply: 4x AA battery pack (6V nominal).
• Voltage Drop: The L293D BJT H-bridge drops ~1.8V under load.

The Math:
If you supply 6V to the VS pin, the voltage reaching the motor is $6V - 1.8V = 4.2V$. Because DC motor speed is proportional to voltage, your robot will run roughly 30% slower than expected. Furthermore, if the robot hits a wall and the motor stalls, it will attempt to draw 800mA. This exceeds the 600mA continuous rating, and the $800mA \times 1.8V = 1.44W$ of heat dissipated will spike the junction temperature by over 115°C, triggering thermal shutdown in milliseconds.

The Fix: Bump your supply voltage to a 2S LiPo (7.4V nominal). Accounting for the 1.8V drop, the motor sees 5.6V—right in its sweet spot. Ensure your mechanical design prevents prolonged stalling to keep the current under the 400mA safe continuous threshold. Note: Always use a fire-safe LiPo charging bag and a dedicated balance charger when working with 2S LiPo packs.

Failure Signatures: Hum, Overheat, and Stall

When an L293D circuit fails, it rarely just 'breaks.' It exhibits specific physical signatures that point directly to the root cause.

• The 'Hum' or Whine: If the motor emits an audible high-pitched whine but doesn't spin, your PWM frequency is likely too low (under 1kHz), or the motor is mechanically jammed while the driver is still passing current. If it's a stepper motor, a hum without rotation usually means the step pulse rate is too high for the rotor inertia to catch, causing it to stall in place.
• Thermal Overheat (The 3-Second Cycle): The L293D features internal thermal shutdown that triggers at 150°C. If your motor runs for exactly 2 to 4 seconds, stops, coasts for a few seconds, and then starts again in a loop, the IC is overheating and resetting. You are exceeding the continuous current limit. You must either reduce the mechanical load, add a heatsink, or upgrade the driver.
• Brownout Stall: The motor starts, the microcontroller resets, and the motor stops. This happens because the motor's initial inrush (stall) current causes the shared power rail to sag below the microcontroller's minimum operating voltage (usually ~3.3V or 4.5V). Fix this by adding a 470µF to 1000µF electrolytic decoupling capacitor directly across the VS and GND pins on the L293D to supply the instantaneous inrush current.

Decision Tree: Stick with the L293D or Upgrade?

The L293D was designed in the 1980s. While it remains a staple in educational kits due to its ruggedness and low cost, modern MOSFET-based drivers offer vastly superior efficiency. Use the decision matrix below to finalize your component selection.