0.1 ohm Resistors from Scratch Again

That takes me right back to Junior high school.

I suppose I could retaliate by spreading your given name across the newsgroups.

-Chuck

Virtually all materials have a resistivity which is temperature dependent. Most are positive relationship (resistance goes up with temp) but a very few are negative.

On Fri, 23 Sep 2005 08:32:16 -0400, Chuck Harris Gave us:

Nope. Real e-mail addresses are far more annoying.

Okay ... but we are talking about copper here. What other materials do isn't relavent.

Mark

Maybe I should have said, copper, like MOST materials, has a resistivity which is temperature dependent. Better?

Well, it appeared that you were specifically addressing this question:

Saying that copper's resistance is temperature dependent still does not answer the question, since the known temperature dependence (for copper) is in the opposite direction to what the op measured. That is what the question is all about: why is the measurement contrary to what is known about copper?

But perhaps I'm mistaken and you weren't trying to address the above question at all, just interjecting some general knowledge for our benefit. :-)

Mark

Ah, I had indeed missed that. I didn't pay attention to the sense of the change. Interesting!

A coil on a resistor body will heat up more than a few inches of wire in open air. Air cooling is less when the wire is on a surface. In addition, probably more significant, is wire turns on a resistor body being heated by heat from adjacent turns of the coiled winding.

- Don Klipstein ( snipped-for-privacy@misty.com)

In article , Chuck Harris wrote in part:

That's a more difficult part, given temperature coefficient of resistance of copper! How to measure cold and when in use? Maybe make a resistor measuring only a little low ehen cold, and see what it does when in use, and improve from that!

- Don ( snipped-for-privacy@misty.com)

One thing to keep in mind:

Wirewound power resistors are typically made with nichrome wire, which has an unusually low temperature coefficient of resistance about half that of copper and most other metals.

- Don ( snipped-for-privacy@misty.com)

This is opposite of what should be the truth. Copper wire has resistance increasing with temperature, roughly proportionately with degrees K.

Looks like you have some measurement error.

- Don ( snipped-for-privacy@misty.com)

I read in alt.binaries.schematics.electronic that Don Klipstein wrote (in ) about '0.1 ohm Resistors from Scratch Again', on Mon, 26 Sep 2005:

Well, they MAY be, but there are many alloys with much lower TCRs. Constantan is one, and Eureka is similar. Manganin has very nearly zero TCR and is used for high-precision resistors.

Perhaps the heating/cooling changes the stress on a contact somewhere (from thermal expansion), and that might be what is causing the resistance change. The differences involved, 0.005 ohms, is not very much.

Mark

Really, I don't know what this is all about, every bit of the "experiment" can be predicted by anybody, so WTF is the OP doing? He should just take some Konstantan wire, I have some at hand. It says 2.5 Ohm/m, has 0.5mm dia. and looks yellowish/greenish. It is solderable, but should be welded to be useable at higher temperatures, otherwise the total dissipation would be the same as the resistor. My 0.1R resistors have to have much more dissipation capability than 1/4W to be useable.

Is that mined in Constantanople?

No, but it's used for making istanbullets.

"Constant resistance since 1453".

Best regards, Spehro Pefhany

I read in alt.binaries.schematics.electronic that Spehro Pefhany wrote (in ) about '0.1 ohm Resistors from Scratch Again', on Tue, 27 Sep 2005:

Well, it's 2032 here now, so that's less than 6 hours. What's the LONG-term stability like?