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Old 20th January 2014, 03:39 PM   #31
PLB is offline PLB  United Kingdom
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Hi Ian,

One more thing, I sprayed the 0R05 with freezer spray and its resistance dropped slightly, which reduced the voltage across it. The THD went down too, but returned to its original THD level when I readjusted the output voltage of the amplifier for the same voltage across the resistor.
So the non-linearity of the resistor seems to be sensitive to the voltage across it, not the current through it! I say this because the 0R05 resistor is in series with the 8R0 resistor, which makes the 8R0 resistor appear as a constant current source to the 0R05 resistor, since it is 80 times greater.
I welcome any comments on this.

Regards

Peter
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Old 20th January 2014, 04:01 PM   #32
SY is offline SY  United States
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Thermocouple effects are DC unless the thermal mass is extremely low. Probably not the issue here.
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Old 20th January 2014, 05:27 PM   #33
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Peter,

Thank you for this. I think you've answered your own question empirically. Your measurements wouldindicate the effect appears to be the result of the dissimilar metal weld. One would need to look at the nature of the crystal strucuture through the weld interface to obtain more insight.

As a chemical engineer dissimilar metal welds are a topic of concern due to the mechanical properties and galvanic corrosion potential, there are reams of literature on the topic, none of which however, for obvious reasons, consider the transmission of low level electrical signals.

Your observations would likely make a good research topic for a masters degree.
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Old 20th January 2014, 07:10 PM   #34
DF96 is offline DF96  England
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Quote:
Originally Posted by PLB
7) These resistors were sent to me by a company in China, so I would think it likely that they were made in China.
Perhaps Chinese ohms are as reliable as Chinese watts?

A poor weld might force the current to have to tunnel between the various conductors? That would be non-linear. The surprising thing is that this manufacturer seems capable of reliably making poor welds! Very useful for someone investigating poor welds.
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Old 21st January 2014, 11:37 PM   #35
PLB is offline PLB  United Kingdom
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Hi SY,

Yes, thatís what I concluded. If there were a possibility of a thermocouple junction at the end caps, there would be a thermocouple junction at each end, and they would be wired in series back-to-back with the 0R05 resistance wire in-between. Their junction potentials would therefore be nulled, yes?

Hi OldMike2,

Yes, there are at least 4 different metals involved here, so it doesnít surprise me that someone like yourself with your background should mention galvanic corrosion. I would imagine that this effect would take some time to mature to a point where it would degrade the electrical connection. Having said that, I have had these resistors for at least 7 years and I have no idea how old they were when I got them. Feel free to comment further.

Hi DF96,

Well, if it is something like galvanic corrosion that has caused the problem, they might have left the factory where they were made with good welds.

Regards

Peter
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