Electronic – common mode chokes and their selection

You've asked quite a few questions. Let me try to answer them in the order you asked them.

1) "What I read about common mode noise is that it is a noise generated on all signal lines and is common to all lines. How can that be?" You should re-read that again as your first link specifically states that it is in a common direction on all the "signal lines." Basically what that means is that the lines where you care about noise have the same noise present on them, and the noise is in the same phase with the noise on the other lines.

2)"Now, my doubt being, how is the common mode noise generated ? What components generate common mode noise ?" CM noise is generated by anything with a varying signal, which includes noise, that is close enough to couple that signal into the line in phase with each other. That sounds complicated but lets break it down a bit more so that it's simpler to understand. Everything has some variation to it, be it noise or signal or anything else. These variations are called AC waves. These can couple to other lines either through capacitive coupling or through inductive coupling. You have two line that you care about. If the AC on the line near by the two you care about couples in it's outside AC onto your signal in phase (going the same direction) on both lines then that is a source of common mode noise.

3)"Now, in the link1 document it says power sources generate common mode noise. Is this applicable to both ac and dc power ?" Yes. Even DC power has some AC components to it, and all AC can be considered a noise source. Any noise source can be coupled commonly into any other signal.

4)"This means We connect one end to Vcc and gnd and then the other side to a dc rail and GND ? Is that it ? So that choke will have a current rating sufficing enough for the load ?" This is where you start getting a little fuzzy. The picture you included has a great example of how to hook up a common mode choke. The Vcc should pass through one inductor of the choke, and the GND should pass through the other inductor of the choke (Don't short them together). This also means that YES, the choke must have a current rating high enough for the power source's supplied current. Your understanding is correct.

This is a thought experiment and something to think about rather than an answer to the direct questions. I believe that thinking about various scenarios like this is more helpful to the learning process than trying to focus on the specific questions raised.

So, if you had a small battery powered circuit that flashed an LED on and off a few times per second and then, with a wire, you connected the ground of that circuit to the live AC wire in your wall outlet what would you expect to see?

• Would you expect to see the LED fry?
• Would you expect to see the LED start flashing randomly?
• Would you expect to see the LED carry on as if nothing had happened?

For the 2nd experiment, using the same battery and LED flasher circuit, if you routed your AC live wire through the 0V connections on your flasher and out to a 60 watt lightbulb that connected back to neutral would the flasher continue to operate without a hitch?

In both experiments I have tried to demonstrate a common-mode (or probably better stated as a longitudanal interference) problem and the first experiment was the ability of the LED flasher to deal with a common mode voltage and the 2nd experiment was dealing with common mode currents passing through the 0V in the flasher (as well as the common mode voltage being present).

You should think about this. What would be the bad practises that would make the LED flasher either die/fry or run intermittantly? This last question is a bit more searching but at the heart of it is the ability of the flasher circuit to cope with electric field gradients and magnetic field induction. You don't need to worry about this last bit yet, just concentrate on the two scenarios. Hope this helps.