# The design of a low pass filter

1. Feb 12, 2014

I am trying to design a 24V dc to 230V ac inverter operating at 50Hz. So far, by using multisim to simulate the circuit I have been able to achieve these parameters but my output is essentially a square wave.

I am using a variation of a circuit that I have found on line, and I am now trying to adjust the low pass filter to smooth the wave form of the output out. The components used for the filter are;

1uH inductor in series with a 1m Ohm resistor connected in parallel with a 100nF capacitor.

At 50 Hz this gives an overall impedance of 0.315 Ohms.

Using the following formulas I recalculated the component values,

Taking the resistor as 1k and frequency at 50Hz.

(1/c)=2(pi)(f)
L=(f/(2(pi)R))

This has given a much better approximation of a sine wave however the overall impedance has increased to approximately 500 Ohms, resulting in the voltage measured at the output dropping down to 88V.

Since the design of low pass filters (and general design work of electronic circuits) is new to me, could someone please lend a bit of a helping hand.

Regards.

N.B I have just seen the warning about posting homework on here. Please note, I am a student, but this is not homework, I am doing it just for a general appreciation into the design work involved for inverter circuits.

2. Feb 13, 2014

### Feldoh

It's actually kind of hard to go from square wave -> sine wave. What you really want isn't a low pass filter but a band pass filter about 50Hz. This is basically what you're building with the inductor, resistor in parallel with a cap.

You're impedance is gonna go down just because it's really hard to make a filter that perfectly passes a particular frequency. You can always amplify the voltage.

Last edited: Feb 13, 2014