Resistors made from wire - Page 1
Forum section: electronics / beginners

author: M0BSW
author_rank: Frequent Contributor
timestamp: May 10, 2013, 12:03:57 pm
content: 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

author: Bored@Work
author_rank: Super Contributor
timestamp: May 10, 2013, 12:09:34 pm
content: http://en.wikipedia.org/wiki/Resistance_wire

author: mikes
author_rank: Regular Contributor
timestamp: May 10, 2013, 01:14:27 pm
content: 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.

author: M0BSW
author_rank: Frequent Contributor
timestamp: May 11, 2013, 04:49:38 am
content: Thank you for the replies to my pondering's, I'm just curious how and why certain things are done.

author: TriodeTiger
author_rank: Regular Contributor
timestamp: May 11, 2013, 07:11:41 am
content: Quote from: M0BSWThe useful unit here isresistivity(greek letter rho), or ohm-metres. Copper has aresistivity of 1.68e-8andnichrome of 1.10e-6at 20 degrees C.Code: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 formulaA = pi r2we find it has a cross-sectional area of1.075e-7m2Lets say your target is1 ohmof resistance. To make it simple, we can get ohms per metre length of wire,R/L = rho/A:Code:1.68e-8 / 1.075e-7 m^2 = ~ 0.156 ohms per metre1.10e-6 / 1.075e-7 m^2 = ~10.232 ohms per metreWith these you can easily find the required length:1 ohm / 0.156 = ~6.31mof this copper wire to get 1 ohm.1 ohm / 10.232 = ~0.097m,or 9.7cm of this nichrome wire to get 1 ohm.QuoteI 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.

author: saturation
author_rank: Super Contributor
timestamp: May 11, 2013, 11:28:22 am
content: In addition, see technical data from below, who make precision resistors wirewound:http://www.precisionresistor.com/Benefits-of-WWs.html

author: ptricks
author_rank: Frequent Contributor
timestamp: May 11, 2013, 01:52:45 pm
content: 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.