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Power Resistor Distortion
Power Resistor Distortion
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Old 7th May 2017, 11:46 PM   #11
Conrad Hoffman is offline Conrad Hoffman  United States
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Elvee, my hat is off to you! I put a large U-shaped magnet up to the resistor body and the THD dropped to 1/3 it's previous value. Looking at the residual on the scope, the overall shape didn't change much, just the dramatic amplitude drop. My regular load resistors are immune to this. So the problem appears to be magnetic
I may be barking up the wrong tree, but at least I'm barking!
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Old 8th May 2017, 12:20 AM   #12
Max Headroom is offline Max Headroom  Australia
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Motor action between the send and return windings ?, some mechanical compliance of the fixing material ?.
Some speaker cables with compliant insulation exhibit distortion because of this.

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Old 17th September 2019, 01:13 AM   #13
davidsrsb is offline davidsrsb  Malaysia
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There is very little published on nichrome permeability. If it is magnetic, it probably has a hysteresis loop.
I doubt motor effects as the resistor would vibrate audibly
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Old 27th September 2019, 04:29 PM   #14
opcom is offline opcom  United States
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This industrial applications reference handbook concerns resistance wire of many alloys.
Please see first the table of "General Information on Metals and Alloys"

Alloy Handbook of Electrical Resistance - Wilbur B Driver Co. (incl. lavish illustrations section, 255MB PDF) Why not?

Alloy Handbook of Electrical Resistance - Wilbur B Driver Co. (sans the lavish illustrations section, 54MB PDF)

The book is old but these materials are still available albeit the trade names for some of them have changed. There may also be competitors' data books but this is what was found and what was scanned

Hope this helps!
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Old 27th September 2019, 10:05 PM   #15
Mark Tillotson is offline Mark Tillotson
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Magnetic resistance wire? Probably cheap steel or the wrong kind of stainless, no wonder it distorts measurably. Wirewound or metal film resistors should have basically unmeasurable distortion (when the right metal is used!).

Magnetic wire would have poor properties at radio frequencies too.

Nichrome wire is non-magnetic BTW - that dummy load probably had steel terminals rather than brass - same issue as steel speaker terminals.
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