In article , Tim Williams says...

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.

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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In article , Tim Williams says...

The best I've got so far is 7.2 x 10^-7 ohm-meter,

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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Tim Williams wrote:
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.

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.

In article , jim rozen says...

In article , Tim Williams says...

The best I've got so far is 7.2 x 10^-7 ohm-meter,

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?

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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please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
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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 one or two of them if you'd like. Email me
if interested.

On Mon, 17 Jan 2005 17:29:16 -0600, "Tim Williams"
wrote:

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

On Tue, 18 Jan 2005 14:22:54 -0600, Don Foreman
wrote:

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 one or two of them if you'd like. Email me
if interested.

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.