Simple Transistor Based H-Bridge Motor Controller Circuit

A simple H-Bridge motor controller circuit that uses transistors is commonly used to control DC motors allowing them to spin both clockwise and counterclockwise.

This circuit helps manage the speed and direction of the motor by using easy switching techniques.

The name “H-Bridge” comes from the way the circuit looks, which is similar to the letter “H” and it has four switches that control how the motor works.

The circuit shown here is a typical four transistor H-Bridge.

This type of motor driver is really useful in areas like robotics, automation and many electronic projects that need motors to move in both directions.

Just remember, you should not use a motor that pulls more than 600mA of current.

The power supply (+Vcc) can be between 3 to 15V DC depending on what the motor requires.

Circuit Working:

Parts List:

The H-Bridge circuit is cleverly built with four transistors named Q1, Q2, Q3 and Q4 arranged in a shape that looks like the letter “H” around the motor.

The input signal A, B, C and D are used to manage the transistors, which help decide the direction of the motor, whether it should stop or if it needs to brake.

When the motor needs to spin forward the transistors Q1 PNP and Q4 NPN are turned on, allowing electricity to flow.

Meanwhile, transistors Q2 NPN and Q3 PNP are turned off stopping their conduction.

This setup lets current travel from the power source (Vcc) through Q1 into the motor and then through Q4 to the ground making the motor turn forward.

If the motor needs to spin in reverse Q2 NPN and Q3 PNP transistors are turned on, while Q1 PNP and Q4 NPN transistors are turned off.

This change allows current to flow from Vcc through Q3 into the motor, and then through Q2 to the ground causing the motor to rotate backward.

To stop the motor both pairs of diagonal transistors are turned off, which stops the current flow and brings the motor to a complete stop.

If quick braking is needed both transistors on one side of the circuit either Q1 and Q3 or Q2 and Q4 are turned on at the same time.

This creates a short circuit across the motor terminals, which helps slow down the motor quickly.

The circuit also includes diodes D1, D2, D3, D4 that protect the transistors from the back electromotive force EMF generated by the motor while it is running.

The H-Bridge circuit has four 1k resistors R1, R2, R3 and R4.

Each of these resistors is really important for managing the transistors.

They help keep the base current at a safe level, which protects the transistors from getting damaged by too much current.

Also, they allow the transistors to switch smoothly making it easier to control the direction of the motor.

Formulas with Calculations:

Below mentioned are the formulas with calculations for Simple Transistor Based H-Bridge Motor Controller Circuit:

Base Resistor Calculation:

The base current for the transistor is given by:

I_B = I_C / h_FE

where,

• I_C is the collector current of the Motor current
• h_FE is the DC current gain of the transistor.

The base resistor RB​ is calculated as:

R_B = (V_in – V_BE) / I_B

where,
V_in is the input voltage to the transistor base
V_BE is the base emitter voltage drop which is around 0.7V for BJTs

Example Calculation:

Assume:

• Motor current (I_C) = 500mA = 0.5A
• h_FE of 2N2222 = 100
• V_in = 5V

Calculate base current:

I_B = 0.5A / 100

I_B = 5mA

Calculate base resistor:

R_B = (5V – 0.7V) / 5mA

R_B = 4.3V / 0.005A

R_B = 860Ω

The nearest standard resistor value is 1kΩ.

Motor Current Calculation:

I_motor = (V_supply – V_CE) / R_motor

where,

• V_supply is the power supply voltage,
• V_CE ic the collector emitter voltage drop which is around 0.2V for a saturated transistor
• R_motor is the resistance of the motor.

Example Calculation:

Assume:

• V_supply = 12V
• V_CE = 0.2V
• R_motor = 24Ω

I_motor = (12V – 0.2V) / 24Ω

I_motor = 11.8V / 24Ω

I_motor = 0.49A (or 490mA)

Power Dissipation in Transistors:

Power dissipated in each transistor is:

P = V_CE × I_C

Example Calculation:

Using above values:

P = 0.2V × 0.49A

P = 0.098W

Since this is much lower than the power rating of 2N2222 (which is about 500mW) the transistor will operate safely.

How to Build:

To build a Simple Transistor Based H-Bridge Motor Controller Circuit below mentioned steps are required to follow to design your own circuit:

• Gather all the components as mentioned in the above circuit diagram
• Connect base of transistor Q1 to signal input of D through resistor R1
• Connect emitter of transistor Q1 to positive of +3V to 15V DC
• Connect collector of transistor Q1 to collector of transistor Q2
• Connect base of transistor Q2 to signal input of C through resistor R2
• Connect emitter of transistor Q2 to GND of the circuit
• Connect collector of transistor Q2 to collector of transistor Q1
• Connect base of transistor Q3 to signal input of B through resistor R3
• Connect emitter of transistor Q3 to positive of +3V to 15V DC
• Connect collector of transistor Q3 to collector of transistor Q4
• Connect base of transistor Q4 to signal input of A through resistor R4
• Connect emitter of transistor Q4 to GND of the circuit
• Connect collector of transistor Q4 to collector of transistor Q3
• Connect the diode D1 to D4 across the emitter and collector of each transistors Q1 to Q4

Conclusion:

The Simple Transistor Based H-Bridge Motor Controller Circuit is a smart way to control DC motors so they can go forward and backward.

It uses transistors like PNP and NPN, which makes the circuit simpler and cheaper to create.

Choosing the right transistors and resistors is important for it to work well.

This setup is perfect for robots, automation projects and controlling small motors.

References:

DC motor H Bridge with only NPN transistors?