regarding to the design of iron-cored inductors (chokes)

[metebalci]

Hi,

Is it possible to design a choke working in dc current (lets say 100mA) -to say in another way, to design a choke with dc flux- without an air gap ?

Reading Radiatron Designers Handbook Chapter 5-Section 6-Page 242-Iron Cored Inductors, what I understand is:
1) First design the choke assuming an initial permeability
2) Check if your assumption is correct
3) If it is correct, no problem. If it is not correct, try to correct it by changing number of turns.
4) If you can correct it by changing the number of turns, it is ok. If you cannot, try to put an air gap to reduce flux densities in the core.

I am asking this because I have contacted to a local transformer design company to design a choke. They have said they can design the choke with and without the air gap. Is it possible ? If it is possible, can I assume that the one without air gap will have more turns than the one with air gap ?

The AC voltage across the core is the harmonics of rectified AC, isnt it ? (that is about 0.544Vinrms according to Morgan Jones ?)

Thanks in advance...

MB

[EC8010]

In practice, all wound components using E and I laminations have an air gap, even if it's unintentional. Transformers alternate the Es and Is, but there is still a slight gap. Toroids don't have a gap because the core is wound from a continuous strip.

The more turns you have, the more Ampere Turns of magnetising force you apply, but an air gap is effectively a magnetic resistance and reduces the resulting flux density. An ideal choke design juggles air gap against turns and core size for a set current to give lowest cost, or reduced resistance. Decide what you want. Bear in mind that if you request 20H at 100mA with DC resistance of 10R, it can be done, but it will be enormous (and expensive).

Assuming that you are following the choke with a capacitor down to 0V, then the AC voltage across the choke will simply be the AC harmonics leaving the rectifier.

[Sch3mat1c]

You can go without a gap in some circumstances, low or no DC current for example; but to keep it within ratings over a wider range, especially with say a choke input, you need a gap to reduce the flux. This reduces mu and requires a few more turns, but you handle more signal in the process.

Almost zero chokes designed to handle DC have no air gap.

Tim

[jackinnj]

go to National Semi's site:

http://www.national.com/onlinesemina...magnetics.html

Sanjaya has developed an algorithm which is very helpful for dealing with "the gap"