The H-Bridge Circuit: Widely Used in Robotics

The H-Bridge circuit is essential to modern applications that span from robotics to electric cars to simple stepper motors. In my first Arduino project, I personally used it in IAADL: an Infrared-Activated Automatic Door Lock. The topology has three popular variations: one with four NPN BJTs, one with two NPN BJTs and two PNP BJTs, and another using four PNP BJTs. But before discussing the circuit, we have to start with the basics, the transistor.

Silicon Doping

The transistor is a three-terminal semiconductor device. It can act as a switch and or amplifier. Its general chemistry consists of silicon doped with phosphorus and boron.

If you're familiar with silicon, you know it has four/eight valence electrons. Silicon can be doped with boron, an element with three valence electrons, causing electron holes to form (the absence of an electron). Silicon can also be doped with phosphorus, an element with five valence electrons, causing a surplus of electrons. These two regions are denoted as p and n regions, p for positive and n for negative. Although the electron to proton ratio in each of these regions is one, we can think of the n and p regions as charged.

The Transistor

After discussing doping, this diagram of a transistor should make more sense. With the case of the NPN transistor, a NP junction, two regions put together, is combined with another N region. When the P region, the base region, has a positive voltage relative to the emitter, it is forward biased and current can flow through this junction. For current to flow through the entire device, the collector region must be reversed biased relative to the base (the collector must have a positive voltage applied to it). The PNP transistor has similar biasing for current to flow. The emitter-base junction must be forward biased, and the collector-base junction must be reverse biased. When these conditions are met, the BJT is said to be in forward active region.

H-Bridge

The H-Bridge circuit's main purpose is to control the spin direction of a motor. The image above models an H-Bridge with two NPN and two PNP BJTs. The H-Bridge we will discuss uses four NPN transistors, set in an H-like formation, hence the name. To assemble the H-Bridge, the GND node needs to be connected to the emitters of two of the four NPN transistors. The VCC node needs to be connected to the collectors of the other two of four NPN transistors. Now, each collector and emitter pair that haven't been touched need to be shorted together. Make the connections seen above so that each end of the motor is shorted to these pairs. Now, connect each base to separate variable voltage sources. In order to change the spin direction, apply positive base voltages to Q1 and Q4 to spin the motor one direction and to Q2 and Q3 to spin the motor the other direction. If you follow the current from GND to VCC, you can see that the current changes direction, which spins the motor in different directions. The physics to describe why this works could be for another article entirely, so I won't get into it.

Discover:

Check out my other projects and how I used an H-Bridge circuit in IAADL: an Infrared-Activated Automatic Door Lock on my dashboard.