Resistors made from wire

[M0BSW]

Hello everyone, I have a 2 part  question, when you see resistors in a  resistance box ,made from wire, is that wire just ordinary copper wire & how is the length or turns calculated to give the required resistance, because I  imagine they would be precise 

[Bored@Work]

http://en.wikipedia.org/wiki/Resistance_wire

[mikes]

Precision wire resistors as found in a decade box would (should) typically be made of Manganin, since it changes resistance very little for a change in temperature. The above link is to "resistance wire," and references Nichrome, which has a tempco about 10x Manganin. For the same reason, copper isn't used (about 270x).

That may be different for non-precision power resistors, which are used for different purposes.

[M0BSW]

Thank you for the replies to my pondering's, I'm just curious how and why certain things are done.

[TriodeTiger]

& how is the length or turns calculated to give the required resistance

The useful unit here is resistivity  (greek letter rho), or ohm-metres. Copper has a resistivity of 1.68e-8 and nichrome of 1.10e-6 at 20 degrees C.

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R = rho * (length L in metres) / (cross-section A in metres^2)
This figure however is for a uniform cross-sectional area, and so you will require the wire's in question.

Using a wire with a 0.185mm radius, and the formula A = pi r² we find it has a cross-sectional area of 1.075e-7m²

Lets say your target is 1 ohm of resistance. To make it simple, we can get ohms per metre length of wire, R/L = rho/A:

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1.68e-8 / 1.075e-7 m^2 = ~ 0.156 ohms per metre
1.10e-6 / 1.075e-7 m^2 = ~10.232 ohms per metre
With these you can easily find the required length:

1 ohm /  0.156 = ~6.31m of this copper wire to get 1 ohm.
1 ohm / 10.232 = ~0.097m, or 9.7cm of this nichrome wire to get 1 ohm.

or turns

I suppose you can translate this in to turns, by wrapping it around an object once and measuring the length to multiply if this is easier.

Above posts mentioned temperature coefficients (tempcos, they are often called) of which are pretty important in precise resistors, and the linked table in my resistivity figures has a few.

Tiger.

[saturation]

In addition, see technical data  from below, who make precision resistors wirewound:

http://www.precisionresistor.com/Benefits-of-WWs.html

[ptricks]

try building your own parts , just for fun.  The wire in a toaster works well for resistors. You can make capacitors from aluminum foil and plastic wrap. In fact, years ago there was a need for a very high voltage capacitor, it used sheets of aluminum separated by sheets of glass. Batteries are fun to make as well, you can make an easy battery from copper pennies by sanding off one side, they are zinc inside and then spacing them out with paper dipped in salt water.