Analysing RTFQ's Common Mode Filter

[DVM]

HandsOn has some interesting LC filters for sale here:

http://www.multi-rotor.co.uk/index.php?action=classifieds;sa=view;id=356

One of the more interesting claims made by RTFQ is "One advantage of this filter is that both the negative and positive leads are filtered (common mode)."

Well, I took a look at the pictures and it';s really obvious that the GND line passes straight through. Common mode filters have an inductor in both the +Ve and GND. Busted!

https://en.wikipedia.org/wiki/Choke_(electronics)#Common-mode_choke

But what';s with the eight pin IC? The clue is that four of the pins are tied together and another 3 are too. You can find similar chips in ESCs - they are MOSFETS. Also, if I squint I can make out the logo of International Rectifier, a well known manufacturer of MOSFETS.

Turns out the 8 pin IC is a P-MOSFET, it';s being used for reverse polarity protection. See Hackaday:

http://hackaday.com/2011/12/06/reverse-voltage-protection-with-a-p-fet/

That';s quite a cool feature, if you wire up the filter the wrong way then the capacitor won';t explode! (Electrolytic caps explode if you wire them the wrong way round). It only protects the input to the filter though, if you wire up the output the wrong way round you will be shopping for a new vTx

In the middle of the PCB is a small black device labelled 3A. Thats a fuse that will limit the current to 3A. Of course, your vTx will be on fire by then and you cant easily replace the fuse but, hey, its better than nothing 

There are also ceramic caps on the input and output, this is a Good Thing (TM). When designing filters you want several different capacitors since each one is most effective at a particular frequency. I can';t tell the values but having several different ones is a good sign.

There is also an LED to show the device is powered - you can';t have too many LEDs, right?

Finally there is the main event, the LC filter. The inductor is labelled 101, that translates as 100uH. The capacitor is 1000uF at 16V working. These are big numbers folks, not much noise is going to get thru (especially combined with ceramics I mentioned above). The 16V working is a bit disappointing, when designing with electrolytic caps you should always specify a capacitor with a working voltage higher than the part will actually see. I like a factor of 2.

TL;DR

Pros:

• Big ass L & C (plus smaller ceramic Cs) will knock down the noise
• Reverse polarity input protection
• 3A fuse
• LED (I hope it';s blue!)

Cons:

• NOT a common mode filter - if the noise is on the GND line then this won';t help
• The Cap is under rated - using 4S batteries will significantly shorten the life of the capacitor. Not a big deal, the frame will likely be trash before the capacitor goes pop (at least the way I fly anyway  )

Note: I don';t for one moment think HandsOn is anything other than 100% honest, he';s only quoting the blurb from RTFQ. RTFQ are responsible for falsely advertising the common mode filtering.

I will certainly consider one of these when I resurrect my Micro Thug to replace the hand knitted LC filter I currently have fitted

[kilby]

Considering RTFQ/Witespy are notorious for so many reasons it';s always a gamble as to which claims they make actually reflect reality.

[teslahed]

What kind of situations might produce noise on the ground wire? I am using a RTF filter and still have some diagonal lines on my video.

[Gandhi]

I';ve been using a couple of these in various craft and they work quite well. Little bit more expensive - whether it';s genuinely common mode or not, I can';t tell from looking at the track on the bottom of the  board?

http://www.ebay.co.uk/itm/LC-Common-Mode-Power-Filter-FPV-LC-filter-No-leads-/251991242864?hash=item3aabd93870

they also do a 4s version but that';s £10!

I get the feeling the currency might be quite strong in Norway?

[DVM]

What kind of situations might produce noise on the ground wire? I am using a RTF filter and still have some diagonal lines on my video.

Longs wires. The noise is very often caused by large changes in current (motors) in an inductor (long wires to the battery). That means 0V at the battery is not the same as 0V at the PDB. Worse, the difference between the two will jump around randomly. This means the voltage supplied to the vTx and camera will be jumping around too - it';s the difference between +V and GND - even if you filter the +V line with a good LC filter

For best results try connecting you vTx and camera to the balance plug.

I';ve been using a couple of these in various craft and they work quite well. Little bit more expensive - whether it';s genuinely common mode or not, I can';t tell from looking at the track on the bottom of the  board?

http://www.ebay.co.uk/itm/LC-Common-Mode-Power-Filter-FPV-LC-filter-No-leads-/251991242864?hash=item3aabd93870

Yes, that';s a true common mode filter. Both the GND and +V lines are going through the black component (it _should_ be two inductors in one package)

There';s nothing special about these chokes tho. You can make your own with a ferrite ring, a few turns of the +V wire as normal and do the same with the GND wire.