LC Filter

[Coyote]

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 lower the value the better for FPV use as apposed to general use, i use 1000uf 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 couldnt 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 has to have the correct wire ( either positive or negative ) going to the correct side. The capacitor a thick stripe down one side of the metal can. In the thick stripe are marking like  - - - -  in it. This indicates the negative side of the capacitor so the negative wires go the the leg of the capacitor the stripe is one.

Here is an example of the stripe :



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.



The correct way round to have the filter is with the coil end to the supply ( lipo ) the capacitor end to the vtx / camera

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, a diode like this :

http://www.maplin.co.uk/module.aspx?moduleno=46385

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.Which way round the capacitor is in this circuit ie before or after the ring doesnt really matter.

Thats it, power it up and enjoy interference free flying :)

If you want ill put up circuit diagrams for all of them, there is another circuit that uses another two capacitors, a 47uf and 100uf with the diode as well, its slightly better than the basic if you need even more filtering :) 

[Badlands]

Neater than mine Ian - nice job.

[Pasty]

nice overview and build Ian... very useful.

Just one thing... it looks like you've wired my filter into your gear for the demo... would have been easier just sending it over :D

[Coyote]

Lol cheers Martin, Steve im doing yours with a 470uf capacitor, its the other end of the value range.

Then if you can can you report back any difference in the filter, there probably wont be any at all but if its even better then it would be handy to know :)

[mark1975]

Wow what can i say very useful and informative very detailed and good lay out ian  cheers

[electrotor]

Even closer to home than the crossovers in your speakers, you'll find lots of LC (tuned) circuits in both transmitters and receivers either filtering out unwanted frequencies or resonating at desired frequencies.
A lot of guys already fit ferrite rings to servo leads in order to dampen rf pickup.

Good thread coyote. :)

[Coyote]

Thanks, yes they are everywhere ! Normally just a ferrite will work on its own especially for rf interference only but other times you need a little more help :)

[electricmick]

Excellent thread, thanks.

[Pasty]

Got some caps in from maplins, gonna head down the workshop in a min and make me one of these puppies...

You can see my current interference when I fire up the motor on my recent Beevo video (see my beevo thread), lets see if this sorts it out!

[Pasty]

The capacitor has marking on its housing, the side with - - - -  on it is to go to the I`ve lead.

I'm presuming you mean live lead? :)

[Devonian]

What is an LC Filter ?

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

Ian,

Great article - BUT, I'd say that it DOES matter where the cap is for this simple filter.
The inductor should be nearest the input (battery/supply) and then the capacitor nearest the output.

There is a good explanation on RCG (and a long argument) regarding ideal components and placement in LC circuits...

Nigel.

[electrotor]

The capacitor has marking on its housing, the side with - - - -  on it is to go to the I`ve lead.

I'm presuming you mean live lead? :)

Nope. Both leads are live, one is +ve and the other is -ve. The dashes (- - - -) mean -ve side. Electrolytic capacitors are polarised and I have seen some marked with the +ve side.

[Pasty]

The capacitor has marking on its housing, the side with - - - -  on it is to go to the I`ve lead.

I'm presuming you mean live lead? :)

never assume :D

Wired it up and the cap went pop.... and thinking about the markings on the side of the cap, "- - - -" surely means negative? Lucky I bought more than one :D

When I got the pop/smoke/stink/weird liquid I feared the worst (as a leccy noob would) but only the cap went pop.

lets try this again...

[electrotor]

Hi Pasty, looks like you found out the hard way. You must have wired up and watched the smoke before I posted my answer.
Better luck next time.

BTW be careful how you wind the wire round the ferrite ring. Do it in the wrong direction and you will cause a catastrophic shift in the Earth' axis resulting in the destruction of all living things. Naw. just kidding.
;D

Even manufacturers sometime make boobs. I bought a transmitter once (an expensive upmarket one) and opened up the back to discover that some of the top panel switches had not been soldered onto the circuit boards. I finished the job myself with the manufacturers approval to avoid delay and postage costs. Mecifully no pop/smoke/smell ensued when I switched on.

[Pasty]

ehehe... thanks for the reply... but I move fast :)

I wouldnt have known what +/-ve means anyways! best to keep leccy terms as basic as poss for us no leccy chaps... thats simply positive and negative yeah?