Inductor's Parasitic DC Resistance Question

Discussion in 'Electronic Basics' started by JLD, Jul 22, 2004.

1. JLDGuest

Hello all. This is my first post.

I was wondering about the DC resistance of inductors. If you take an
inductor in hand and measure the resistance there will be some
resistance, somewhere around 10 - 30 ohms for a typical 100mH for
example. What I want to know is what is the bigger "cause" of that
resistance. Is it the resistance of the actual wire or is it something
else? Maybe cross coupled E/U fields? Does the core of the inductor
affect this DC resistance? Is this a result of the manufacturing of
the inductor (parasitic R)?
I know that an inductor in a circuit under steady state acts as a
short, so is that resistance is still there? Also if that resistance
is still there why is there such a small voltage drop across it?

tia

JJ

2. John LarkinGuest

It's just the copper (wire) resistance. A bigger 100 mH inductor,
wound with fatter wire, would have less resistance.
Yup. It's not a perfect short.

It follows Ohm's law. If the DC drop is low, it's because the DC
current is low.

Imagine your inductor as a perfect inductor with a resistor in series
with it. That's pretty much how it will behave.

John

3. John PopelishGuest

If measured with an ohm meter, you are seeing just the wire
resistance. It would be just the same if you unwound the inductor and
remeasured the wire resistance.

At operating frequency, the effective wire resistance goes up, because
each half cycle, the magnetic field reversal crowds the current to the
outside part of the wire. The higher the frequency, the less time the
magnetic field has ot penetrate the wire, to let the current density
in the wire core approach that at the surface. So at high enough
frequency, the wore acts as if it were a tube.
http://www.fact-index.com/s/sk/skin_effect.html
There are also exaggerations of this effect when turns are wrapped
around a gap in the core, where the core field fringes out through the
winding.

The core also consumes energy and may circulate a significant amount
of current that looks like a loaded secondary with the winding acting
as a primary. This looks from the ends of the winding like a
resistance in parallel with the inductor.

And we haven't even started into the effects of capacitance.

4. andyGuest

afaik, it's just the resistance of the wire. for a DC signal, that's all
it could be, because there is no varying electromagnetic field to create
any other effects.