Motor Drivers and Protection

A controller pin cannot power a motor directly. A motor driver translates logic commands into controlled current while protecting the electronics from inrush, inductive voltage, overheating, short circuits, and wiring mistakes.

Learning Objectives

By the end of this lesson, you should be able to choose a driver topology, understand current limiting, add flyback or recirculation paths, size protection parts, and verify driver temperature and fault behavior.

Driver Types

Driver	Best for	Key checks
Low-side MOSFET	one-direction DC loads	flyback path, ground switching limits
H-bridge	reversible DC motors	shoot-through prevention, current rating
Stepper chopper driver	bipolar steppers	current limit, decay mode, cooling
Servo drive	closed-loop motors	feedback, following error, brake handling
Relay	slow on/off loads	contact rating, coil suppression

H-Bridge Basics

flowchart LR MCU["PWM and direction"] --> DRV["H-bridge driver"] --> M["DC motor"] SENSE["Current sense"] --> DRV FAULT["Fault output"] --> MCU

An H-bridge reverses motor polarity. Never turn on both high-side and low-side switches in the same leg at once; that creates shoot-through. Good drivers include dead time, current sensing, and thermal shutdown.

Inductive Energy

A motor winding resists sudden current change:

$$
v=L\frac{di}{dt}
$$

When switching, the driver must provide a recirculation path through MOSFET body diodes, synchronous MOSFETs, flyback diodes, or clamps. Without that path, voltage spikes can destroy the driver.

Current Limit and Heat

Motor startup or stall current can be much higher than running current:

$$
I_\text{stall}=\frac{V}{R_\text{winding}}
$$

Driver conduction loss is commonly estimated from MOSFET resistance:

$$
P\approx I^2R_{DS(on)}
$$

Stepper drivers use current regulation because winding resistance alone is not the operating limit. Set current from the motor rating and verify case temperature.

Protection Checklist

Fuse or resettable protection at the power entry.
Reverse-polarity protection when connectors can be miswired.
TVS diode for long leads or harsh supplies.
Flyback or clamp path for coils.
Current limit and thermal shutdown.
Fault signal routed to firmware.
Emergency stop or safe torque off where motion can injure or damage.

Layout and Wiring

Keep motor current loops short and wide. Separate motor returns from sensor references. Twist motor leads where practical, add bulk capacitance near the driver, and place current-sense routing away from switching nodes.

Common Mistakes

Selecting by average current instead of stall or peak current.
Forgetting heat sinking and copper area.
No flyback path for a relay, solenoid, or brushed motor.
Driving both H-bridge legs incorrectly in firmware.
Sharing motor ground current with ADC sensor ground.

Summary

A motor driver is both a power stage and a protection boundary. Choose the topology for the motor, size it for peak current and heat, provide inductive current paths, and verify fault behavior before connecting the final mechanism.

Mind Map

mindmap root((Motor Drivers)) Core concept Logic controls current Driver protects switches Motor is inductive Formulas v equals L di dt Istall equals V over Rwinding Ploss approx I squared RDSon Applications DC motor Stepper Relay Servo axis Design rules Size peak current Add flyback path Prevent shoot through Route high current loops Practical checks Stall test briefly Driver temperature Fault pin Supply droop Common mistakes Average current only No clamp Poor cooling No ground separation