PWM

The circuit is a variable duty cycle, constant frequency, astable multivibrator using the 555 timer. Diodes Dchg and Ddchg separates the charge and discharge paths respectively. They also maintain a symmetrical voltage loss along the two paths. Potentiometer Rdcadj adjust the duty cycle with slight effect on frequency. When the potentiometer is adjusted the resistance subtracted from one path is added to the other path. Thus the mark and space time of the output can be varied while maintaining their sum almost constant. This results in variable duty cycly and constant frequency output signal. For this configuration, the frequency is around 130-135kHz with a duty cycle that goes from 15%-95%. For the extreme cases, the frequency might go down as low as 115-120kHz. To change the frequency, change the value of Ct.