# DC Motor Speed Controller

Posted on Feb 7, 2014

This article aims to show you how to control a DC motor though an H-bridge driver chip using the PWM output from a PIC to achieve speed control. Although I am using a PIC microcontroller other controllers such as AVR`s, Basic Stamps and certain Picaxe`s can be used. Before we get too far along; PWM stands for pulse width modulation. For a given fr

equency the logic level of the output from the pic will be high (or on) for a certain percentage (duty) of the period (1/frequency in seconds). So with a duty of 50% the PWM signal would be high for 50% of the time. This can be used to create average voltages (i. e. with a PWM signal at 5V half of the time and 0V the rest the average voltage would be 2. 5V), by varying the voltage to the motors you can control the speed of the motor! The operation of the H-Bridge driver chip is fairly simple. With the enable pin you can turn the bridge on and off, the input pins you can change the direction of the voltage across the motor. So with the enable high and one input pin high and one low the motor will turn one way and with them the other way round the motor will go in the opposite direction. If they are both at the same level the and the enable is high then the motor will stall (or break) and with the enable low the motors will freewheel. There are also current sense pins which can be used to identify stalling and excessive current consumption when a resistor is connected to GND, and the voltage across the resistor will be follow V=IR so with a known R the current I can be found. The power rating of the resistor is very important, P = I 2*R so you must make sure your resistor can handle the stall current of your motors. The ones I am currently work with are rated at 2A stall current, making 4W with a 1Ohm Resistor or 0. 4W with a...

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