LC filter is one of the most basic filters in electroincs, which comprises of just one inductor and one capacitor in its simplest form. This circuit is widely used by FPV hobbyists to suppress noise in power created by ESC and motors.
What is an LC Filter?
An LC filter is made up of an inductor (L) and a capacitor (C). It’s that simple.
An inductor resists changes to the current flowing through it, while a capacitor resists changes to the voltage across it. See this post for a more sophisticated version of power filter.
It’s essentially a low pass filter, allowing lower frequencies to go through while attenuating higher frequencies.
Buy LC Filters
Looking to buy some LC filters? Here are some options I found that are suitable for RC.
- 16V / 1A: http://bit.ly/2tH1DaP | https://amzn.to/2lFhnYp
- 16V / 2A: https://amzn.to/2KkreNM | http://bit.ly/2Mqc36x
- 24V / 1.5A: http://bit.ly/2txsWox
Make sure the LC filters you are getting meet the voltage and current rating for your application.
How LC Filter Improves FPV Experience
In the quadcopter, Motors and ESC’s can generate noise which can affect your FPV video quality.
During flight, motor speed is constantly changing in order to stay airborne. The change of motor speed means change of current drawn by the motors, and voltage ripples (fluctuations in voltage level), which is basically noise.
The most common signs of noise are disturbing lines appearing across the screen when we are flying.
An LC filter can suppress the noise in the power getting to your FPV setup (e.g. your VTX and FPV camera). You might or might not need an LC filter. Some voltage regulators have some degree of power filtering ability, but an LC filter can enhance filtering.
LC Filter vs. Low ESR Capacitor
I often get asked which method should be used to combat noise in their mini quad, LC filter, or Low ESR capacitor. But I must say they serve very different purposes and applications.
LC filter is mainly used to stop noise getting to their FPV system, it does not reduce the amount of noise in the original power. On the other hand, by putting low ESR capacitors in your power, you should notice a reduction in the noise level.
Make sure to check out my tutorial about what low ESR capacitors are and how to use them in a mini quad.
LC filter has current rating due to the inductor’s current limit, and therefore it’s mainly used in low current circuits. You often see LC filters built into the 5V and 12V circuits on flight controllers or PDB for powering your FPV setup. And they are usually more effective than only using capacitor alone.
On the other hand, when using only low ESR capacitors, you only need to make sure the voltage rating is high enough.
Building an LC Filter
The value of capacitor and inductor changes the resonant frequency. This makes it possible to suppress noise more effectively if you know the noise frequency. However you will still get certain extent of filtering effect using random capacitor and inductors, so do not worry :)
If you know how to work out the exact capacitance and inductance you need for your application, good for you! But if you don’t it’s okay, we are not building a rocket and it’s going to work just fine :)
Here is how you connect the capacitor and inductor.
When choosing capacitor, ensure the voltage rating meets your requirement. or example if you are connecting the LC filter directly to your LiPo battery it should support your LiPo battery’s voltage. As for capacitance, the larger the better, I think 100uF to 2000uF is a good range.
You can buy inductors, or you can make one yourself with a Ferrite ring (toroid) and some wires. Choose the length and diameter of the wires carefully. Ideally you should be able to wrap around the ferrite ring at least half a dozen times. And the wire gauge should be large enough to support the current.
If you are making your own inductor, I find it work better with just the positive wire wound on the toroid. When winding the cable around the ferrite ring, DO NOT finish your winding back to the starting side, it should exit at the opposite side.
As for the capacitor, any electrolytic ones should do just fine. They are widely available from old radios or PC parts for example. Or just pick some up from eBay, they are very cheap. It’s best to use low ESR capacitors which are much better for noise filtering purposes.
Make sure to place the capacitor on the output, where it connects to the VTX and camera.
LC filters are great because they are so simple and cheap to make or buy. They can help attenuate or even eliminate “noise” completely if you are having “lines issues in your FPV feed”. If using only capacitors doesn’t seem to solve the issue, you might want to consider adding an LC filter for your FPV system :)
- Feb 2014 – Article created
- Jun 2018 – Article Updated with LC filter options