PWM - Pulse Width Modulation for DC Motor Speed and LED Brightness 2

Posted on Feb 7, 2014

To understand how the 74AC14 PWM circuit works, let`s zoom in on the section of the schematic containing the trimpot, diodes, capacitor, and the first inverter logic gate. The capacitor is empty (discharged) when the power is first applied to the circuit. Because the capacitor is low, and because the capacitor is connected to the input pin of the

PWM - Pulse Width Modulation for DC Motor Speed and LED Brightness 2

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inverter gate, the input is low. The conventional current flows from the output of the inverter gate through the left half of the potentiometer (R2) and then through diode D1. The current cannot flow through the right half of the variable resistor (R2) because diode D2 blocks the path (notice the direction of the diode arrow). Diodes are one-way valves. The current flows into the capacitor (C2); charging the capacitor. If the potentiometer is dialed to the far left, the resistance will be small and the capacitor will charge quickly. If the potentiometer is dialed to the far right, the resistance will be large and the capacitor will charge slowly. Eventually, the capacitor fills up (is charged). Because the capacitor is now high, and because the capacitor is connected to the input pin of the inverter gate, the input is now high. The conventional current flows from the capacitor (C2), through diode D2, and then through the right half of the potentiometer (R2). If the potentiometer is dialed to the far right, the resistance will be small and the capacitor will discharge quickly. If the potentiometer is dialed to the far left, the resistance will be large and the capacitor will charge slowly. The repeated charging and discharging of the capacitor occurs because the inverter gate output flips to the opposite of the state of the capacitor. When the capacitor is empty, the inverter output charges the capacitor. When the capacitor...

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