common mode choke help

[milen007]

hi

can someone help me make an intelligent guess on how much current can this common mode choke passes?

outer diameter is: 30mm
height is: 20mm
wire diameter is: 1.5mm

i saw in ebay the one thats almost the same with this has 0.5mH 20A

i wanna use this choke for less than 5A of +-35vdc. is this good for it?

thanks in adv
erwin

[jfitz57]

Looks like it will handle 5 amps no problem. Why do you want to use a common mode choke for DC?

[sawreyrw]

As a rough rule of thumb current density for wire in magnetic devices should not exceed 450 amps/cm^2. Using this your wire is good for 8 amps RMS.

Your core needs to be a low permeability core that will not saturate under DC current.

[Elvee]

Originally Posted by sawreyrw

Your core needs to be a low permeability core that will not saturate under DC current.

For a CMC, this is not required. Quite the opposite in fact.

Small magnetic components having a few turns in direct contact with the ambient air can tolerate current densities in excess of 8A/mm², so the 20 amp rating is probably realistic.

[abraxalito]

Originally Posted by sawreyrw

Your core needs to be a low permeability core that will not saturate under DC current.

As Elvee points out, saturation isn't an issue for CMCs because they're designed to run with very low net flux. The magnetization of the two windings cancels out leaving a very low differential-mode inductance and a much higher common-mode one. Saturation only becomes an issue if the two currents become unbalanced. Low permeability cores though are very useful for filtering out higher frequencies (say above a few MHz) where the higher mu cores give up.

[milen007]

thanks guys for all the replies.

i can conclude that i can use these choke for the 5A circuit. thanks

jfitz57, i dont know to be honest. i saw few schematic using it and i would like to try it as it incur less losses in voltage. any insight? idea or suggestion? enlightenment?

erwin

[DF96]

You use a common-mode choke when you have a common-mode noise/interference problem. Otherwise, you are simply adding some impedance (mainly winding resistance) to your PSU output impedance. Worth doing when necessary.

[Andersonix]

Can one use this type of common mode choke in the CLC section of a linear +/- PS? They're so much cheaper and smaller than an air core inductor....

F.ex. D-K has a Triad Magnetics 2mH 7.5A CMC for under $5: 237-1230-ND

[DF96]

Not, a common mode choke is for choking common mode noise. It needs the two currents through it to be equal and opposite. It will not act as the L in a CLC filter.

[abraxalito]

Originally Posted by Andersonix

F.ex. D-K has a Triad Magnetics 2mH 7.5A CMC for under $5: 237-1230-ND

I think what you'll find is that the 2mH is either a single coil's inductance or perhaps the common-mode inductance. It won't give you 2mH at 7.5A because as DF96 says its designed to work with the two coils in opposition. Try to use it as a normal choke (either with just one winding or with both windings in parallel) and it'll saturate long before you get anywhere near 7.5A. That explains why its so much cheaper than an air-cored choke. If you want to do CLC filtering I suggest looking for powdered iron cores - they have a useful increase in permeability over air and saturate at fairly high currents. Ferrites aren't a good choice unless the currents are fairly low.