The MIT License (MIT)
This circuit can supply either a 3.3 or 5 volt logic level PWM signal depending on the reference voltage used. The voltage control input can also be driven with either a 3.3 or 5 volt logic level signal depending on the how the circuit is calibrated.
The PWM frequency can be adjusted in the range of 25K Hertz.
Spice files for testing the circuit viability.
KiCAD files of the schematic and PCB.
- +12 and -12 Volt bench power supply.
- +5 or +3.3 Volt bench power supply.
- Variable voltage bench supply (0V-3.3V or 0V-5V).
- Multimeter
- Oscilloscope
- Frequency Counter (Optional--Can use the scope)
First, be sure your dual voltage power supply is as close to +12 and -12 volts as possible. This will also be true in the actual environment that this PWM will call home. If there is a slight offset between the absolute + and -voltages there will also be a slight offset between the internal zero crossing point of the triangle wave. Resistor R6 (460) can be adjusted to compensate for this. The two trimmers R9 and R11 can also affect a slightly off zero crossing by adjusting the complete on or off of the PWM, that is where a 0 control voltage causes the output PWM to be 0 volts and the max 3.3/5 control voltage causes the PWM to be 3.3/5 volts.
- The R3 trimmer is used to adjust the internal triangle reference voltage. This should nominally be 10 volts peak-to-peak with the trimmer set to approximately 250K. Put your scope probes between TP2 (Triangle Wave) and TP0 (GND) then adjust R3.
- The R1 trimmer is used to adjust the frequency of the triangle wave. This should nominally be 25K Hertz with the trimmer set to approximately 24K. Put your scope or frequency counter probes between TP2 (Triangle Wave) and TP0 (GND) then adjust R1.
- The R9 trimmer is used to adjust the symmetry of the sweep voltage. This is the Control Voltage that adjusts the modulation of the PWM frequency. Adjust the variable power supply to 50% of 3.3V (1.65V) or 5V (2.5V) depending on your drive voltage then attach the supply to J3 and attach the multimeter probes to TP3 (Sweep) and TP0 (GND). Double check that the supply voltage is at 50% of your driving voltage then adjust R9 so the voltage on the multimeter is as close to 0 (zero) volts as possible.
- The R11 trimmer is used to set the peak-to-peak of the sweep voltage. Set the variable power supply to your drive voltage 5V or 3.3V and attach it to J3 (Voltage Control Input). Attach the multimeter probes to TP3 (Sweep) and TP0 (GND). Adjust R11 so that the multimeter shows plus 5V. Adjust the supply to 0 (zero) volts (or short the two pins on J3) and check that the voltage on the multimeter is -5V. See note 1 below.
- Check the PWM. Adjust the supply voltage to 50% as was done in step 3 above and check that the PWM is 50%, then adjust the supply to 0 (zero) (or just short the two pins on J3) Check that the PWM is 0 (zero) volts.
Notes:
- In order to get 0 (full off) and 5/3.3 Volts (full on), on the PWM the sweep voltage may need to be slightly more than 10V peak-to-peak (R11) assuming the maximum triangle wave voltage is still 10V peak-to-peak. Adjusting the zero crossing with R9 may also be necessary.
- TP1 provides a square wave output, however it is not used in any of the calibrations.
- On Version 1.0 of the PCB the silk screen for the TPs (test points) are rather small and difficult to read. This will be fixed in version 2.0 of the PCB. The photos below should help with identifying which TP is which.
I want to give attribution to Zero999 for the oscillator part of my circuit.
Feel free to contact me at: carl dot nobile at gmail.com