# inductor "max dc resistance"

Inductor specs: please, what does "maximum DC resistance" mean?

It reflects how much current can be passed through the inductor. This is a result of the gauge of the wire used and the length of wire needed to obtain the desired induction. Two inductors can have the same induction value but have drastically different maximum current specification because of the wire size used.

Lefty

I notice inductors also have ratings for DC current. How is this different?

These are the inductors of interest in case some context helps... http://www.bourns.com/data/global/pdfs/RL622_series.pdf

Thanks.

I notice inductors also have ratings for DC current. How is this different?

This is the maximum current you can pass. Normally it has to do with heating.With only a resistance you can pass any current given enough voltage. The two combined give you the voltage you can use across it.

I'm not quite getting it yet. I've got "max DCR" is "how much current can be passed through the inductor" from Lefty. And I've got "DC current" is "maximum current you can pass" from Grumpy.

These sound like identical descriptions for two different specifications. I'm wondering if one of you hasn't been perfectly clear...

Is DCR perhaps the minimum load required to be connected in series?

Or perhaps Max DCR is the actual resistance measured across the inductor under DC (and max. is the mfg's guarantee that each unit will not exceed this). This was my first thought but a definition has been surprisingly hard to find, thus here I am.

"DC resistance" is a measure of the "lossyness" or inefficiency of an inductor; an important number if you're trying to make a 95% efficient DC/DC converter. It's controlled by the size and length of the wire winding.

"Max current" is usually limited by magnetic "saturation" of the core material. For a given substance, the magnetic energy you can put into it and have the field strength continue to increase is somewhat limited (this is probably vaguely related to why a permanent magnet of a particular material can only be so strong...) Once the magnetic field in the core material gets so strong as to "saturate" the core, the inductor stops behaving like an inductor (V = dI/dT) and starts behaving like a resistor (V = IR) (this is generally bad, but some circuits USE it.)

The two values are in conflict in practical inductors. Magnetic materials that have a high inductance-multiplying capability permit shorter wires (fewer turns) tend to have a lower saturation value. Air-core inductors (for example) don't saturate at all, but it takes a lot of turns of wire to make a given inductance (high-permeability magnetic materials (like ferrites) multiply inductance of a given coil by up to 15000, but they saturate at relatively low magnetic field strengths.)

Mitch, in the pdf file you linked to the column for "maximum DC resistance" is just what it says - this is the maxium resistance of the inductor when measured at DC voltage - no freq involved. I.E. if you took a wheatstone bridge (resistance too small for multimeter to measure) and measured the ohms resistance, that is the max value you would get.

I that is what you were looking for?

Ken H>

Once the magnetic field in the core material gets so strong as to "saturate" the core, the inductor stops behaving like an inductor (V = dI/dT) and starts behaving like a resistor (V = IR) (this is generally bad, but some circuits USE it.)

KenH,

Yes, much thanks. That clarity is what I was looking for.
I must say I’m surprised the resistance of a 47 mH inductor is a whole 99 Ohms!

I'm also surprised, but if you consider how many feet of very small wire there is in a 47,000 uH inductor.... but 99 ohms???

Ken H>