inductor
I want to buy a 1mH inductor. I am just making a basic LC filter and I see that there are "common mode" inductors.
I found one here with a data sheet:
http://www.mouser.com/ds/2/355/DS_EV-EHseries_20110616_web19-8801.pdf
The inductor is 1mH and "common mode". Can I use this inductor to give me 1mH?(it meets my current rating too)
There are four legs on it as well. Will only using two of them give me half the inductance?
Your assumptions are spot on.
Another thing to bear in mind is that any leakage inductance between the common-mode windings manifests itself as a parasitic differential mode inductor - this is why it's common to see toroidal common-mode chokes with the windings opposite each other (not interleaved) to maximize leakage inductance and get that 'free' differential mode filtering.
If you are winding your own, you can use bifilar wire which when not twisted on a toroid is pretty nearly perfect. Otherwise you can expect to see the effect of a fraction of a turn; if you use one turn at your maximum current to specify your toroid you will be safe. Wind in bifilar fashion in any case for the best results. If you wind one winding over the other the results will not be as good.
You can measure the results directly with a third (temporary) coil of fine wire, short out the output and drive the input with some AC.
Best Answer
Since it's a common mode inductor, it's intended to be used for a signal pair. For signals that are common to both conductors in the pair, it presents 1mH of inductance. Since the windings are counter-wound, for signals that are differential in nature (source on one conductor, return on the other) it presents (ideally) zero inductance. So if you wire it such that both windings are in series in the correct direction, you can use it as a 1mH inductor.
However, common mode chokes are typically designed to be lossy, so that the noise that they are filtering is dissipated as heat from hysteresis losses. Therefore it might not make a good single-ended inductor for your purposes.
Why not just get or wind a 1mH inductor that meets your requirements?