Low pass filter inductor and capacitor values unknown?

[JAMES IRVINE]

I am doing a power electronics project for my HND second year Graded unit. I have chosen the DC to AC Inverter to build. I have come across one or two circuit diagrams which are not too complex and may be able to work.
My project is for a true sine wave inverter but the circuits I have in mind to use have a square wave output.
I was hoping someone would be able to let me know for a 50 Hz frequency which value of Inductor and Capacitor would I need to filter out the unwanted harmonics with the low pass filter arrangment. A series inductor followed by a capacitor in parallel.
Any Ideas on any aspect of this project I have would be really appreciated especially if I could receive help on understanding what the circuit I will finally use actually does component by component if at all possible.

 

[erosennin]

See this attachment on this forum, it was from a thread about getting a sine from a 555 and it progressed slightly... If you have simulation software, try changing the values of R1=R2 and C1=C3, make sure they're the same, and see which values get you 50Hz... I'm sure there is a more legitimate way of doing this...

 

[audioguru]

A square-wave inverter has a peak output voltage the same as the RMS voltage of a sine-wave so the power in a heater and in an incandescent light is the same. If you could somehow filter out the harmonics then the voltage will be too low.

An inductor and a capacitor to filter out harmonics above 50Hz will be huge and expensive. The inductor will have such a high resistance that hardly any current will flow.

Modern sine-wave inverters stepup the battery voltage with a small high frequency circuit and small transformer then use a high frequency pulse-width-modulation IC to make a switched waveform with many steps in it. Then a small LC filter removes the high frequency which smooths the steps into a sine-wave.

 

[crutschow]

The design you use depends upon the efficiency and sine wave distortion you can tolerate.

A modified square wave technique used on some inexpensive converters produces square waves with some dead spots between positive and negative half-cycles to more closely approximate a sine-wave which can be more easily filtered.

The best technique uses a pulse-width-moduated circuit to generate a series of square-waves whose average value is the desired sine-wave. This can be smoothed with a small amount of filtering to remove the square-waves. This is similar to a PMW Class-D (switching) audio IC circuit such as built by TI among others.

 

[JAMES IRVINE]

so do you have any circuit diagrams for this pulse width modulated circuit

 

[crutschow]

so do you have any circuit diagrams for this pulse width modulated circuit

Afraid I don't. But Texas Instruments, among others, builds PWB (Class D) audio amplifers and you could perhaps use one of those to drive the inverter output transistors. The type you want accepts an analog input (50Hz sinewave in your case) and outputs a PWM digital signal.

 

[chosen_1]

Hi. Why dont u use bipolar PWM so that you can filter high frequency harmonics? Square wave would mean bulky LPF

 

[crutschow]

You do realize this thread is over a year old?