Jordan Dimitrov This Design Idea describes a new approach to producing a variabledutycycle waveform from a 555based freerunning oscillator. The circuit's wide modulation range, highly linear control over a wide range of dutycycle values, and excellent linearity make it ideal for PWM (pulsewidthmodulation)based control applications. Figure 1 shows the basic circuit, which works as follows: When IC_{1}'s output goes high, switch S_{1} closes, and IC_{1}'s internal discharge, switch S_{2}, opens. Capacitor C_{1} charges through R_{1} and R_{2}. When IC_{1}'s output goes low, S_{1} opens, and S_{2} closes, discharging C_{1} through R_{2} and R_{3}.
The generic configuration works well for producing a fixedvalue duty cycle. To obtain a continuously variable duty cycle, Figure 2 shows how to connect potentiometer R_{4} to the common junction of R_{1}, R_{2}, and R_{3}. The output waveform's duty cycle, D_{T}C, follows the equation: D_{T}C=(R_{1}+R_{2}+R_{VAR})/(R_{1}+2R_{2}+R_{3}+R_{POT}) where R_{POT} is the potentiometer's endtoend resistance, and R_{VAR} is the fraction of R_{POT} between the rotor and R_{1}. As the equation shows, D_{T}C depends linearly on R_{VAR}. Switch S_{1} comprises one section of a 4066 CMOS quad bilateral SPST switch, IC_{2}.
You can use the circuit in Figure 3 to evaluate dutycycle linearity. A rotary switch and a tapped series string of 16kΩ resistors provide a 10kHz signal with nine discrete, equally spaced dutycycle values ranging from 2 to 98%. For accurate results, use a 5½digit multimeter to match the values of resistors R_{4} through R_{11} and a Tektronix 3012 oscilloscope or equivalent to gather D_{T}C data.
Microsoft's Excelspreadsheet software includes a linearity analysis that returns the following trend line for the dutycycle measurements: D_{T}C=0.7565×R_{VAR}+2.1548; R_{2}=1. The value of 1 for R_{2} as Excel calculates shows that the transfer function is perfectly linear. Switch S_{1}'s onresistance and particularly its leakage current slightly affect the D_{T}CversusR_{VAR} equation's slope and intercept, but the equation remains strictly linear. Using only one of IC_{2}'s four switches eliminates leakage effects and crosstalk that would occur if other circuits used the remaining switches. In addition, using moderately low values for the resistor network further reduces leakagecurrent effects on circuit performance. PCB Pricing: https://jlcpcb.com/quote
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