H-Bridge Tutorial

Posted on Feb 6, 2014

The H-bridge is a circuit used in electronic control of high current devices, particularly where the device polarity may be reversed, e. g. DC motors. The name comes from the fact that the circuit typically looks like a letter `H`. The circuit shown has four switches and a motor. To apply a voltage across the motor a pair of diagonally opposite swi

H-Bridge Tutorial
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tches need to be turned on. Depending on which pair of switches are turned on the motor will turn one way or another. If both the top or both the bottom switches are turned on the motor will have no voltage difference across it so it won`t move at all. If the top and bottom switches on one side are turned on together by accident we will short out the supply, so you need to be careful if you`re building your own H-bridge. Most pre-packaged ones will have internal logic to prevent this. You can build an H-bridge like this out of relays instead of switches and control them with a lower current to drive big motors but most often the switches are replaced with transistors, a pair of PNP transistors (or p-type MOSFETS) at the top and a pair of NPN (or n-type MOSFETS) at the bottom. PWM is a method of digitally controlling an output with a variable equivalent voltage. Essentially if you take the average of the signal over time then it has a varying analog level, however in the short term it is digital. This makes it easy to generate and efficient as transistors are most efficient when on or off rather than partially conducting. Most modern microcontrollers have the ability to generate PWM built in, including the Arduino and derivatives. To control a plain H-bridge you need 4 signals to control the 4 transistors, to control it with PWM you need two PWM signals and two plain digital signals. You could theoretically use just the two...

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