Hey hey! I have a project in ORCAD in which I have to design a PWM with the following parameters: f=15000 Hz, amp. of PWM between 3 and 6 V and duty cycle between 40 and 70%. I am not very good with electronics, so I found a guy on youtube with a video with adjusting the duty cycle of a 555 timer without changing the frequency.I designed the circuit, entered the values he calculated to see how this works, run the simulation and I have nothing at output... I could use some advice about this.

 

You have the '555 output shorted to ground:
the battery negative should be connected ground;
Reset should be connected to VCC.

 

i am implementing a pwm using 555, and in circuit diagram, there are two diodes, can any one tell, why diodes are used, and whether this circuit can be made without using diodes in the first place.

 

It would be a GREAT help if you provides the schematic of the circuit you are building. As you have provided no information a wild guess is that one select the charge path and the other the discharge path.

Les.

 

i am implementing a pwm using 555, and in circuit diagram, there are two diodes, can any one tell, why diodes are used, and whether this circuit can be made without using diodes in the first place.

This old application note describes it: http://www.sophphx.caltech.edu/Physics_5/Data_sheets/555appnote.pdf

The whole note is interesting, but the steering diode discussion starts about page 4.

Yes, it can be made without diodes. In the usual configuration described in datasheets, that limits the duty cycle to more than 50%.

 

It would be a GREAT help if you provides the schematic of the circuit you are building. As you have provided no information a wild guess is that one select the charge path and the other the discharge path.

Les.

thank you, it was very helpful.

 

This old application note describes it: http://www.sophphx.caltech.edu/Physics_5/Data_sheets/555appnote.pdf

The whole note is interesting, but the steering diode discussion starts about page 4.

Yes, it can be made without diodes. In the usual configuration described in datasheets, that limits the duty cycle to more than 50%.

Thank you, i am reading that pdf, it's helpful.