Resistance Wire?

[bioborg]

Anyone know a quick way to distinquish between a common buss wire and a resistance wire ?

 

[williB]

i may be wrong but i take a stab at an answer..
resistance wire isnt copper ,it is a special wire used as a heating element.
while common bus wire is copper..

 

[stevez]

I'd think you could measure the resistance of what you have and compare it to the resistance you'd expect for a copper wire of the same size. That ought to tell the story.

 

[bioborg]

We use it in a circuit, it's a piece 20awg about 1"long. Not even with my Fluke ohm meter that reads .0001(millionth of an ohm) could I see a reading that I could rely upon to distinguish between the two. The resistance wire is so sensitive to the circuit(NOT MY DESIGN), that it must be @ least 1/16"from the board to function acceptably.
Anyway I am doing some research and will post any findings I have.
Thanks :wink:

 

[zevon8]

The only way ( outside knowing the actual resistance per milimeter ) to measure this type of alloy is by using a 4 connection Kelvin measurement.

What you need to do is connect a known current source to the wire, at a known length, and measure the current through it, while at the same time using an acurate meter measure the voltage drop produced at the known measurement.

this is how current shunts work, and why they have 4 terminals. The 2 large ones are for the main current path, the 2 smaller ones for the voltage measurement.

I have used various alloy ( nickle, copper etc, ) resistance wire in current measuring circuits, and tyhis is how we would calibrate the length needed. We would scribe the wire at a known measure and check the drop. then the wire was inserted in the circuit with small wire taps soldered to the scribe lines.

Here is a pretty good link describing the technique:

 

[williB]

i take it that somone mixed the two wire types into the same box..?

 

[panic mode]

there are different types of resistance wire. they all have some common things such that resistance is not linear and hence it is not easy to measure. it will also depend on temperature etc.
common practice for measuring low resistance value is to use 'big' current.
while the 'big' current doesn't have to be in order of 1000A, anything higher than your meter will be more precise. if you ever wondered, current used by your multimeter is probably much less than 1mA even for low resistance circuits.
try this: use LM317 for example (or 7805 if you want to) to make
current source of say 1A or at least 100mA (this can easily be measured with your meter).
then let this current flow through unknown resistor of low value.
use meter to measure voltage drop accross resistor. use calculator...

R=V/I

if the current was set to 1A, voltage drop reading will represent resistance of the wire. For example 12mV would mean 12mOhm or 0.012 Ohm.

you can use any number (even something odd like 247.9mA) but the decimal values such as 10mA, 100mA or 1A make it easy to read without need for calculator. Note that you should check your regulator rating.
Don't think you can get 10A out of 317L.
the L version of regulator is good only for 100mA so the resistance between Adj and Vout should not be less than 12 Ohm.
Say you use 22 Ohm. You shoudl be getting current of about 54.5mA so if you read voltage drop on that piece of wire (say it's 13.9mV) when this current is flowing through it, put those numbers in the formula above and you will get something like 0.25Ohm...
Now you can go to Fluke and complain

 

[panic mode]

here is quick and dirty version. note that you can use any voltage (5 or 12 or 16 or 24V or whatever) as long as you don't exceed ratings of the chip and regulator has enough headroom to operate (give it at least 4V):

 

[bioborg]

Thankyou!!!!

This has been a very informative feedback result. Thank-you everyone, I think I can tackle the problem if the need should arise.