circuitlabduty cyclepwm
This is my first attempt to simulate a circuit in the time domain with CircuitLab. In order to ask a proper question about something similar driven with PWM, I'd like to add an adjustable duty cycle to the square wave, and then if possible ramp it.
How can I add variable duty cycle PWM to a CircuitLab simulation within a question?
simulate this circuit – Schematic created using CircuitLab
The first scheme is the astable configuration as descibed by the datasheet: it's a monostable multivibrator which triggers itself.
Note that the duty cycle is limited to 1/2.
The other two schemes have the advantage that the duty cycle can go from almost 0 to almost 100%, making them more suitable for PWM purposes.
The second schematic is very ugly, but basically you change the duty cycle varying the ratio of the resistances that charge and discharge the capacitor C1, turning the potentiometer R1.
For some reason, the charge and discharge of C1 is driven by the output Q, while the real output is through the DIS pin. The bad thing about this is that the output through the DIS pin is open collector, so it will be strongly driven low by the Dis pin but pulled up only by the 10k resistor.
Note that if you turn it fully to one of the rails, you are basically charging/discharging the capacitor through a diode and the output stage of the 555. This is not exactly a good idea.
The third one is based on the same concept, but a resistor is put to the OUTput to avoid the aforementioned problem, and the Vout pin is used properly.
If you need a PWM control, I would suggest looking at the datasheet, where you can find a good PWM modulator:
If the duty cycle itself does not matter and the requirement is for pulses of, say, 0.5ms, then you could send the output of your circuit to a monostable 555 circuit. Dual-555 chips are available: the 556.
Best Answer
This is an interesting question. I've never had to do it but this might work for you.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Two sawtooth oscillators and a comparator.
Figure 2. Note the DC offset in the sawtooth configuration.
Figure 3. The result.
I seem to have configured the amplitude incorrectly on the triangle waves. They should probably be 2.5 V with 2.5 V offset but perhaps the graph is clearer like this.