06-05-2011, 11:59 AM
Pulse-width modulation (PWM)or duty-cycle variation methodsare commonly used in speedcontrol of DC motors. The duty cycleis defined as the percentage of digital‘high’ to digital ‘low’ plus digital ‘high’pulse-width during a PWM period.Fig. 1 shows the 5V pulses with 0%through 50% duty cycle.The average DC voltage value for0% duty cycle is zero; with 25% dutycycle the average value is 1.25V (25%of 5V). With 50% duty cycle the averagevalue is 2.5V, and if the duty cycleis 75%, the average voltage is 3.75Vand so on. The maximum duty cyclecan be 100%, which is equivalent to aDC waveform. Thus by varying thepulse-width, we can vary the averagevoltage across a DC motor and henceits speed.The circuit of a simple speed controllerfor a mini DC motor, suchas that used in tape recorders andtoys, is shown in Fig. 2.Here N1 inverting Schmitt triggeris configured as an astablemultivibrator with constant periodbut variable dutycycle. Although thetotal in-circuit resistanceof VR1 duringa complete cycle is100 kilo-ohms, thepart used duringpositive and negativeperiods of each cyclecan be varied bychanging the positionof its wiper contactto obtain variablepulse-width. Schmittgate N2 simply actsas a buffer/driver todrive transistor T1 during positive incursionsat its base. Thus the averageamplitude of DC drive pulses or thespeed of motor M is proportional tothe setting of the wiper position of VR1potmeter. Capacitor C2 serves as aFig. 1: 5V pulses with 0% through 50% duty cyclestorage capacitor to providestable voltage to thecircuit.Thus, by varyingVR1 the duty cycle canbe changed from 0% to100% and the speed ofthe motor from ‘stopped’condition to ‘full speed’in an even and continuousway. The diodes effectivelyprovide differenttiming resistor values duringcharging and discharging of timing capacitorC1.The pulse or rest period is approximatelygiven by the following equation
ulse or Rest period ≈0.4 x C1(Farad) x VR1 (ohm) seconds.Here, use the in-circuit value ofVR1 during pulse or rest period as applicable.The frequency will remain constantand is given by the equation:Frequency ≈2.466/(VR1.C1) ≈250Hz (for VR1=100 kilo-ohms and C1=0.1μF)The recommended value of in-circuitresistance should be greater than50 kilo-ohms but less than 2 megaohms,while the capacitor value shouldbe greater than 100 pF but less than1 μF. _Download full report
http://pecworld.zxqAssets/SOURCE/PDF_fun..._Aug06.pdf
