LC Filters

What is an LC Filter ?

An LC Filer is a circuit that removes frequencies from a power supply and smooths / cleans it so the equipment being powered gets a clean power source. In FPV sometimes we find our picture being effected by this dirty power supply.

There are many causes of the interference. If the interference is constant its normally rf based, ie your vtx or camera is providing the noise because its constant. If however the interference increases and decreases with the throttle this is down to the ESC. The speed ( frequency ) of the motor directly has an effect on the amount of interference seen.

ESC interference can sometimes be fixed by simply wrapping the servo lead from the ESC round a ferrite ring 5 or more times. In most cases that sorts it out. If not then its time for a LC filter.

What is one ?

An LC is one of the most basic filters going. It comprises of one or more coils and capacitors. What it does is remove frequencies from a voltage but high and low.

Are they new technology ?

Not at all, if you sat in your front room now chances are your looking at 2 or more. Your home speakers have them inside the speaker box, the filter in this case are used for the sound wave frequencies. They are in this case referred to as crossovers. The small speakers in your hifi cant cope well with bass, so an filter is used to remove the low frequencies ( bass ) so they only reproduce the high ones ( Treble. ) The same with the bass speakers, the higher frequencies are removed so it only produces the low bass frequencies. Where stereos crossovers let some selected frequencies through we use them to get rid of all of them.

They sound complicated

Not at all, with the most basic soldering skills and a near by maplins store your laughing.

There are a few different filters we could use for FPV. The first and most simple normally is all you would need. To make this all you need is :

1) Your power cable that’s going from your lipo to your transmitter / camera
2) Ferrite ring
3) Capacitor

The capacitor value can be anything from 220 up to 2300 uf ( microfarads ) The higher the value the better, i use 100uf ones. The capacitor is available through maplins

You want the VH52G ( 0.74p ) you could go for the VH50E but i recommend the higher voltage one so you can connect to any lipo cell pack without a worry.

A ferrite ring off ebay, get a pack of them, you can never have enough ferrite rings when Fpving

To make it it couldn’t be simpler. Take your components :

You need to wrap your wire around the ferrite ring. 5 times or more is fine, the red wire only needs to go round, but doing both has no ill effects so its up to you, using both will look like this :

I choose to only use the red myself so mine looks like this :

Next take your wire and strip both leads a little, this is to solder the capacitor too. depending on how you want the capacitor to sit depends on how far away from the wrapped ferrite ring you strip at.

The picture below shows heat shrink I`ve put on the wires either side of the ring, i did this to keep the red wire tight on the ring and for neatness only :

The capacitor we are using need the correct wire to go to the correct side of the capacitor. The capacitor has marking on its housing, the side with – - – -  on it is to go to the I`ve lead. Make sure this is correct or the capacitor will blow up when you power up.

Once you have soldered the capacitor legs to the power wires you then need to insulate them so they cant short out, a bit of heat-shrink does the job fine :

There you go, one LC filter made, no all you need to do is solder / connect the lipo balance lead to one end and the other to the vtx / camera power leads.

Which way round the capacitor is in this circuit ie before or after the ring doesn’t really matter.

That’s it, power it up and enjoy interference free flying

There are a few variations on this circuit. Lets say you have a very long lead to your vtx, for instance you  have to have the vtx on the tail of the plane, in that case the only extra you really need is to add a diode before the circuit on the +ve wire

That removes half the noise before it even gets to the rest of the circuit.
Simply cut the red wire before the coil and solder the diode in between with the end marked with the bar on the diode towards the coil. Again heat shrink over it to insulate.