# How to filter transformer output

Started by October 15, 2011
```Could some electronics guru please help ? I have a SPICE simulation of
a simple inverter(DC-> AC). The output of the transformer is a square
wave within some rails. If I pass it
through a smoothing filter (e.g., RC with cutoff at 60.0Hz),
the output is in the mV. In this case, what is the best way to
remove higher harmonics from the square wave output ?
```
```On Oct 15, 10:54=A0am, Daku <dakup...@gmail.com> wrote:
> a simple inverter(DC-> AC). The output of the transformer is a square
> wave within some rails. If I pass it
> through a smoothing filter (e.g., RC with cutoff at 60.0Hz),
> the output is in the mV. In this case, what is the best way to
> remove higher harmonics from the square wave output ?

Makes no sense to use an energy dissipating RC to filter a /power/
inverter output. See the advanced techniques section on harmonic
elimination through PWM techniques http://en.wikipedia.org/wiki/Inverter_(e=
lectrical)
```
```On a sunny day (Sat, 15 Oct 2011 07:54:38 -0700 (PDT)) it happened Daku
<dakupoto@gmail.com> wrote in

>a simple inverter(DC-> AC). The output of the transformer is a square
>wave within some rails. If I pass it
>through a smoothing filter (e.g., RC with cutoff at 60.0Hz),
>the output is in the mV. In this case, what is the best way to
>remove higher harmonics from the square wave output ?

Tune it.
```
```On Oct 15, 10:54=A0am, Daku <dakup...@gmail.com> wrote:
> a simple inverter(DC-> AC). The output of the transformer is a square
> wave within some rails. If I pass it
> through a smoothing filter (e.g., RC with cutoff at 60.0Hz),
> the output is in the mV. In this case, what is the best way to
> remove higher harmonics from the square wave output ?

Sounds like your spice simulation is broken. If you posted it in
LTspice someone might look at it.
But as Fred B. said, "Makes no sense..."

George H.
```
```Jan Panteltje wrote:
> On a sunny day (Sat, 15 Oct 2011 07:54:38 -0700 (PDT)) it happened
> Daku <dakupoto@gmail.com> wrote in
>
>> of a simple inverter(DC-> AC). The output of the transformer is a
>> square wave within some rails. If I pass it
>> through a smoothing filter (e.g., RC with cutoff at 60.0Hz),
>> the output is in the mV. In this case, what is the best way to
>> remove higher harmonics from the square wave output ?
>
> Tune it.

A square wave contains lots of odd harmonics.  Build your filter's cutoff to
be after the 2nd harmonic.
But remember that there's significant energy in the harmonics, so the filter
will be absorbing lots of energy.  Efficiency will be in the tank.
Better to look into a PWM or stepped square wave solution.

--
David
dgminala at mediacombb dot net

```
```Daku wrote:
> a simple inverter(DC-> AC). The output of the transformer is a square
> wave within some rails. If I pass it
> through a smoothing filter (e.g., RC with cutoff at 60.0Hz),
> the output is in the mV. In this case, what is the best way to
> remove higher harmonics from the square wave output ?
reduce saturation of the primary or increase the frequency.

Something along those lines.

Jamie

```
```Daku wrote:

> a simple inverter(DC-> AC). The output of the transformer is a square
> wave within some rails. If I pass it
> through a smoothing filter (e.g., RC with cutoff at 60.0Hz),
> the output is in the mV. In this case, what is the best way to
> remove higher harmonics from the square wave output ?
A series inductor and parallel capacitor will do fine.
But, be careful of SPICE simulations.  You can get great filters
by optimizing just one parameter like voltage, and then discover
that the currents are 1E12 Amps.  In other words, the simulation
is entirely mathematical, and not necessarily bound to the
real world.

Jon

```
```On 10/15/2011 7:54 AM, Daku wrote:
> a simple inverter(DC->  AC). The output of the transformer is a square
> wave within some rails. If I pass it
> through a smoothing filter (e.g., RC with cutoff at 60.0Hz),
> the output is in the mV. In this case, what is the best way to
> remove higher harmonics from the square wave output ?

1) Select how much power you want to lose. 5% 10% 20%?
You are stripping off power harmonics 3,5,7,9 etc and wasting it away as
heat.
See power contributions of the harmonics of a square wave.
2) Then you can select your cut off frequency and filter order.
3) Then select your filter topology. LC or RC sections.

```