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Vintage threads from the first ten years

eddy currents

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11/22/2005 9:27 PM
Roscoe	eddy currents How do you check a pickup for eddy currents?

11/23/2005 8:28 AM

On 11/23/2005 4:27 AM, Roscoe said:

quote:
"How do you check a pickup for eddy currents?"

If there is any metal nearby (other than the coil itself), there will be eddy currents in all nearby metal. So there is nothing to check - the answer is always Yes.

What are you trying to do? Why do you ask?

11/23/2005 9:57 AM
moocow	There's something on the Bill Lawrence site about using an eddy current test to find shorted turns in a pickup. Maybe that's what he's asking?

11/23/2005 10:48 AM
Roscoe	Yep, Moocow got it, I was reading on the Bill Lawrence site and wondered how you can test for eddy currents. I can't think of a way, but that's because I'm an idiot. ;]

11/23/2005 12:12 PM

On 11/23/2005 4:57 PM, moocow said:

quote:
"There's something on the Bill Lawrence site about using an eddy current test to find shorted turns in a pickup. Maybe that's what he's asking?"

It turns out that this is correct.

The easiest way to use eddy currents to detect a shorted turn is to use an Extech LCR meter to measure the 1-KHz inductance and AC resistance of the pickup in question and also a known-good pickup of the same kind. Take the two AC resistances, subtract the corresponding DC resistance values, and look at the two difference values. A shorted turn will cause the difference (Rac-Rdc) to be much larger than normal. The inductance will also be reduced.

With an air-core coil (well away from metal), Rdc=Rac, unless there is a shorted turn. In this case, Rac will exceed Rdc, and again the inductance will be reduced. A piece of mild steel will increase inductance, but the AC resistance will still increase, so AC resistance is the most reliable test.

As for the Bill Lawrence website, I looked around, but didn't find a description of how Bill does it. Does anyone have the URL?

11/23/2005 6:29 PM

I have this cheesy Korean plain enamel wire and if you wind it too tight it shorts out internally. I had a class example of this when I didn't insulate the magnets on a tele pickup, DC resistance read correctly, inductance was right in the ballpark, even the inductance was in the ball park but the AC resistance had nearly doubled!!! Pickup sounded like it was inside a beer can trying to get out, terrible tone.....

11/23/2005 9:44 PM
Roscoe	Can you have eddy currents from different layers of windings without having the pole pieces or other metal pieces being involved? IOW can eddy currents occur between windings only?

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