- Published: Tuesday, 01 January 2013
- Written by Jon Chandler
An EDN Design Idea
Aruna Prabath Rubasinghe, University of Moratuwa, Moratuwa, Sri Lanka; Edited by Martin Rowe and Fran Granville - February 3, 2011
Using an RC lowpass filter and an op amp, you can convert the PWM signal to a dc level that can adjust the LM317T’s voltage output. Varying the pulse width of the input signal lets you generate an analog voltage of 0 to 5V at the output of the lowpass filter. The op amp multiplies the voltage to achieve the desired voltage range.
For scenarios in which you must multiply the input voltage by two, the LM317T’s adjustment pin receives 0 to 10V. Its output-voltage range is 1.25 to 11.25V. The equation VOUT=VADJ+1.25V governs the LM3175T’s output voltage. You can change the op amp’s gain by choosing proper values for R4 and R2. You must be able to remove offset voltages from the op amp. Use an op amp, such as a National Semiconductor LM741, with null adjustment. The selection of values for the capacitor and resistor for the RC lowpass filter depends on the PWM signal’s frequency. This circuit uses values for a 1-kHz PWM signal.
You can improve the circuit by replacing the RC lowpass filter with an active filter and then feeding a feedback signal from the circuit’s output into the microcontroller for dynamic adjustments.
Comments to this article at EDN suggest that the LM741 op amp is not an ideal choice for this circuit; a better choice would be a rail-rail op amp.Discuss this article in the forums (2 replies).