When power is supplied to a motor. current flows through the motors windings or coil, and the coil energies which produces inductance. When power is suddenly cut off or the during rotation direction changes, the energy stored in the coil does not have the path to the direction in which it was flowing and so suddenly energy flow direction is reversed and this reversal energy process in inductor produces a reversed voltage or potential difference which is commonly known as back e.m.f(electromotive force). You might be interested in how to build diy inductance meter with Arduino and comparator. This sudden sudden spike of energy or voltage can reach the transistor and then the pins of the h-bridge driver which are often IC chips such as a microcontroller board. This can cause damage to the transistor, microcontroller or part of the power supply.
So to protect the transistors, microcontroller or other IC when a motor or inductive actuator are used protection diodes are used. A standard rectifier diode like the 1N4001 is suitable for most relay coils and small H-bridge circuits using BJT transistors, provided the current does not exceed 1 amp. To safeguard the circuit, a diode should be installed between the motor terminal and the power source. When working with an H-bridge, four diodes are required—one for each motor terminal, connected to both the positive and negative power supply. The circuit diagram below shows H-bridge motor driver circuit built with four BJT transistors, two PNPs 2N2907 and two NPNs 2N2222.
Notice the four diodes which are connected across the transistors. If your H-bridge lacks built-in protection diodes, you can attach them directly to the motor terminals, as illustrated in picture below.
In this setup, Back-EMF protection diodes are directly soldered to the motor terminals, eliminating the necessity for diodes to be integrated into the motor controller.
Here bipolar transistors were used to build the h-bridge which is suitable for low current, voltage or power rating. For higher inductive loads, higher voltage and current the field effect transistors MOSFET and JFET are used. The construction and working principle of P-MOSFET and N-MOSFET were covered in H-bridge with P-channel and N-channel MOSFETs for Arduino Robotics and H-bridge with P-channel and N-channel MOSFETs with Inverters.
Another tip useful while working with motors and motor driver circuit is to use a capacitor between the terminals of the motor as shown in the circuit diagram below.
The picture below shows capacitor across dc motor when I was building arduino based wireless mobile controlled car and autonomous arduino robotic car.
The benefits of adding a capacitor across a motor are:
- Reduces Electrical Noise (EMI/RFI) – A capacitor helps suppress voltage spikes and electromagnetic interference, which is useful in circuits involving microcontrollers or sensitive electronics.
- Protects Switches and Transistors – When used with brushed DC motors, a capacitor can minimize arcing across switches or H-bridge transistors, extending their lifespan.
- Smooths Voltage Fluctuations – A capacitor can help stabilize voltage supply fluctuations, especially in battery-powered circuits.
The recommended capacitor types:
- Ceramic Capacitors (10nF to 100nF, typically 0.1µF): Commonly placed directly across the motor terminals to suppress noise.
- Electrolytic Capacitors (1µF to 100µF or more): Used for power smoothing, often placed between the motor's power and ground.
Where to Place Capacitors?
- Across Motor Terminals – Helps reduce noise and brush sparking.
- Between Each Terminal and Ground – Further reduces EMI, useful for motors in radio-controlled or wireless applications.
- Across Power Supply Rails – Helps stabilize voltage when motors draw large currents.
Do not use large capacitances directly across the terminals of a high-speed motor, as this may affect its response. If using PWM control for motor, ensure the capacitor does not interfere with the switching characteristics. Obviously this is because of the charging and discharging of capacitor, time constant at certain frequencies. Output Filter Capacitors are used as filters and used not only in motor control circuits but also in voltage regulators and power supply circuits.
In summary, back EMF is a major concern in H-bridge motor driver circuits, as it can damage transistors, microcontrollers, and power supplies. When power to a motor is cut off or its direction changes, the inductive coil generates a reversed voltage, which can spike into the circuit. To ensure circuit protection, diodes play a crucial role in redirecting this energy safely. A 1N4001 diode works well for small relay coils and BJT-based H-bridges up to 1A. In an H-bridge, four protection diodes should be placed between motor terminals and both power rails. If the H-bridge lacks built-in diodes, they can be soldered directly onto the motor terminals.
Capacitors also help mitigate back EMF effects. Placing a ceramic capacitor (10nF–100nF) across motor terminals reduces electrical noise (EMI/RFI) and protects transistors from voltage spikes. Electrolytic capacitors (1µF–100µF) smooth power fluctuations and enhance circuit stability. Capacitors can be installed across motor terminals, between each terminal and ground, or across power supply rails. However, excessive capacitance may interfere with PWM control due to charging time constants.
For higher inductive loads, MOSFET-based H-bridges offer better efficiency. Proper use of diodes and capacitors ensures transistor protection, circuit stability, and reduced electrical noise, making motor-driven systems more reliable.
