Voltage Controlled PWM Generator

Posted on Feb 7, 2014

Circuits that generates PWM pulses. Most of them will translate a resistor value into duty cycle change. Although that is handy and easy, sometimes a voltage controlled PWM generator is needed. There are of course those micro-controllers that could make almost any kind of PWM signal translation, but i preferred to study and create a circuit without the use of such chips. I tried to keep it as easy as can be,

Voltage Controlled PWM Generator
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but without this reflecting to the accuracy and integrity of the output pulses. The operation is similar to the digital signal transmission using PWM signals. The DC input level voltage is compared to the current voltage of the triangular waveform. Every time those two levels have the same value, the output will change state. If the triangular waveform was on the rising edge, the output will go HIGH, otherwise the output will go LOW. The green waveforms are the triangular signals. The brown line is the DC voltage level. Finally, the red pulses are the output PWM. You can see how these pulses changes the duty cycle as the DC input level changes. You can find complete theory, details and drawing for the triangle wave oscillator in our relevant circuit. I have used the first circuit and i added the same transistor with a slightly changed resistors. My goal was to amplify the circuit but shift it a little bit, to help the circuit achieve duty cycles from 0 to 100%. You could as well use another circuit for generating a triangular waveform. The oscillation frequency in our circuit is around 1. 5 KHz at the time, but this is not a critical value. You should keep in mind thought that the frequency of this triangle waveform will determine the PWM frequency. To test the circuit, i needed a DC reference level. I used a simple 5K potentiometer implementing a voltage divider and i added a 1. 5K resistor as shown in the schematic. This...

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