# Resistivity of stainless?

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I'm trying to find the resistivity and tempco of stainless (presumably a 304 or so, whatever this 0.02" dia. lockwire is). I need to make some 0.033 ohm power resistors cheaply and want to know how much they'll change once they burn about 30W.

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

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You'd have to have the name of the particular "stainless". Stainless, per se, is not listed in any tables I have. An approximation may be gleaned from the Handbook of Chemistry and Physics which says pure iron has a resistivity of 10 microhm-centimeters. Manganese steel is 70 whereas Siemens-Martin Steel is 18. "Nichrome" *TM is given as 100. Stainless, as such, is not listed. Suggest you check into a metals handbook.

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The tempco is going to be low. Because it's an alloy, like most restistance wire (manganin, evenohm, etc) most of the resistance is due to scattering from the 'impurity' atoms, not from phonon scattering.

I don't have the number here, but I could look it up at work in the morning.

Jim

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All I can tell 'ya is it's "Stainless Steel Safety Lock Wire", "Form No.

514", "National Standard Company", and in a 1lb. roll.

The best I've got so far is 7.2 x 10^-7 ohm-meter, which works out to about two feet for the resistance I need. The problem is, once I start pulling the 30A through it, it's going to light up like a lamp, and resistance is going to go mad. From what I've seen (which amounts to one graph for pure iron), it'll go from some small amount of resistance at 10K or so, on up to like 10 times the room temp resistance at red heat...

And that assumes stainless even has a constant tempco - plenty of alloys have zero or even negative coefficients!

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

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There is a table for different alloys that states the temperature a given wire size will reach when a certain amount of current is dragged through it. That is a sort of universal tempco table. Maybe Jim has access to it - but he will have to know the type of metal.

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Hi Tim, We used to make high power low ohm resistors from a length ( or three) of gal water pipe. Just make 2 sets of clamping blocks, bolt your wires to each, and move the blocks closer together (turned off of course) until you get the current you need. Its not pretty, or compact, but its cheap.

regards,

John

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I found 71 to 74 micro ohm-cm, which seems to be at variance with your value. This was from "Experimental Techniques in Condensed Matter Physics at Low Temperatures" by Richardson and Smith.

Maybe a units problem?

Jim

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Firstly, you almost certainly don't want to do this. If it's glowing red, it's corroding rapidly. Does it need to be small?

1/33 ohm resistors, so about a volt and 30A. I'd start with copper wire, say a meter. IIRC copper has a resistivity of 3*10^-8 ohms/meter, so maybe 0.4mm.

How robust do these things need to be? Enamelled copper wire in water can sustain quite high powers for a while.

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Maybe jim needs more coffee this early in the morning.

7.2 e-7 ohm-M = 0.72 e-6 ohm-M = 0.72 uohm-meter

0.72 uohm-M = 72 uohm-cm

So on second thought, your number sounds fine. On third thought, I should never do math without a chalkboard.

Jim

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How many resistors do you plan to make? I have some projector arclamp carbons, copper-clad, about .223 dia x 6" long. "As is" they measure 6.23 milliohms end-to-end. (Kelvin bridge measurement: millivolts with known 1000 mA flow) You could grind or etch (ferric chloride) off some copper to get .033 ohms, probably in an inch or two.

They can definitely "take the heat". I'd terminate with clamps of some sort rather than soldering wires to the copper; the solder would probably melt.

I'd be happy to send you >I'm trying to find the resistivity and tempco of stainless (presumably a 304

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Post script: I removed 1" of copper. Was going to etch it, but after taping all but one inch I thought I'd shine up the exposed copper a bit in the beadblast box. Well, that copper was gone in about 3 seconds!

The resistance is 0.055 ohms for 1 inch, dia is .218". You're still welcome to a projector carbon or two, but 3/8" or even 1/2" dia welding carbons might better suit your 30-watt .033 ohm need. From this experiment, I'm guessing a 1/2" dia welding carbon would exhibit about .010 ohm per inch of stripped carbon. 3" x 1/2" would have enough area that I don't think it'd get too terribly hot at 30 watts. The old Ungar 35-watt soldering pencils had elements of about that area.

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Stainless is specified for use up to 1600°F or so. Not that it's good in that range, of course. I personally have had this wire up to yellow, using it as nichrome, which I *don't* have on hand, in an electric furnace. Melted a pound or two of aluminum in two hours, burning 700W. I forget what the wire length was...dang!

I'm more worried about sag. I might make some porcelain insulators for support. With airflow it should be okay in normal use.

Not necessarily, but 16 might take up too much space.

Intended to spread the voltage between eight 35A 200V rectifier bridges, for my welder.

No need for offers of complete 200A bridges free in the mail, guys. I already have everything, I just need to put it together. :P

The thing with copper is that it has less bulk resistance, so you have to use thinner wire which can't dissipate as much power, so gets hotter. Stainless is about the best resistance material I have around, and it's already in convienient wire form.

Enough to handle 30W worst case scenario. My welder is rated 225A, between eight bridges (and eight resistors per side, I need a pair of resistors per bridge) that's 28A. Add short circuit conditions and we're sitting pretty good. Normal use around 100A will put all of 15A tops through each.

I'd prefer to pack this inside the welder housing, maybe adding another fan. A good breeze over the resistors will keep them dramatically cooler than still air.

Last night I constructed ten coils with 1/4" QC terminals, enough for five bridges; ample at the current levels I'm going to use mostly (1/8" rod on up to 1/4" work). I'll cast a heatsink (for the bridges) and see how DC is compared to the AC I've been using.

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

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By rapidly, I meant "won't last many hundred hours". For a welder, this may not be a problem.

Ok... Call it 28A, and 1/28th ohm. Say a 1 bridge 50C hotter than the rest. That's -2mV * 50 (per side), or 100mV. The hot bridge will take only 2.8A more (assuming that the diodes are ideal, with no resistance), so is dissipating only (about) 10% more.

For it to get 50C hotter, (assuming case temp of 50C), the temps would have to be some 500C.

Personally, I'd halve the resistance, and still be happy that there is loads of margin.

For a welder, I'd agree, this isn't appropriate. SS may be a good choice, especially as it'll be running at lower currents most of the time.

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