# AC Wire Resistance

I stumbled across some unexpected data points when measuring the AC resistance of magnet wire. I varied the frequency of the AC voltage and measured the corresponding impedance of half a meter of 32 gage magnet wire. As expected, due to eddy currents the AC resistance of the wire is greater than the DC resistance for 1 kHZ - 10 MHz. However, the AC resistance of the wire drops below the DC resistance from 12 MHz to 20 MHz.

This is a hard trend to justify, since increasing the frequency of the voltage would increase the magnitudes of the eddy currents in the wire. Has anyone experienced a sudden drop in the resistance of a wire above a certain frequency?

Thanks guys!

• You mean above the self-resonant frequency of the wire? – Ignacio Vazquez-Abrams Jul 15 '15 at 3:17
• How was your measurement setup? – Lorenzo Donati Jul 15 '15 at 3:33
• It seems more likely you're getting a transmission line effect which you're measuring as a decreased impedance. – Samuel Jul 15 '15 at 3:43
• What instruments are you using to measure resistance at 20 MHz? And how are you compensating for the inductance of the wire under test? – tomnexus Jul 15 '15 at 4:56
• Bifilar-wind the length of wire (fold it in half along itself) and re-run the test. That should all but nullify it's self-inductance. – rdtsc Jul 15 '15 at 5:18

It not hard to justify and is a very logical problem during AC operation the impedance(no resistance) is dependent on the capacitance and inductance of the wire. Where as in DC operation these elements play no role.

So to say Your capacitive impedance Xc ~ 1/f ie,inversely proportional to the frequency,hence it decreases as the frequency rises. Similarly your inductive resistance is proportional to frequency X~ f hence your inductive reactance increase with the frequency.

Are we Done?

Thats what most people would have thought.

But what you forgot was very important phenomena of skin Depth Skin depth tends to decrease as frequency increases hence conductor penetration decrease and current flow on surface,rather than like Dc where current flow through whole wire.

New resistance after skin effect calculation is given as R=Rdc * k* sqrt(f). Also how much your material is affected by skin effect depends on what you are using.