Is it possible to make a resistor?

[fabbie]

Is it possible to make a resistor? I need to find a 0.1 ohm resisitor but finding that in my country(malaysia) is a bit hard.

 

[samcheetah]

well, yes you can certainly make resistors yourself. take a small piece of paper and a pencil. now mark a few lines on the paper. there you have it!!!! your own home made resistor. now just connect the two ends of the resistor to an ohm meter to see what value it has. if you want to have different resistance values try shading the whole piece of paper. and also try shading it with different pencils ( B, H and HB pencils ) the amount of the carbon deposited on the paper will determine the resistance.

but i dont think you will be able to make a 0.1 ohm resistor yourself. and even if you did it will have an unknown tolerance and reliability. and you certainly wont be able to make 5W resistors with the method i told above. so its better if you find some 0.1 ohm resistors

 

[John Sorensen]

You could certainly make a 0.1 ohm resistor. A wire with the proper length will have that resistance. The hard part will be determining when you've got it right, I suppose you could pass a known current through it and measure the voltage drop. Don't forget to wind it for low reactance.

j.

 

[fabbie]

If i wind a wire, wouldnt it become an inductor? But since DC doesnt have any frequency, i dont suppose i have to worry about reactance

 

[Nigel Goodwin]

If i wind a wire, wouldnt it become an inductor? But since DC doesnt have any frequency, i dont suppose i have to worry about reactance

Even a straight wire has inductance!, a 0.1 ohm resistor won't take much wire, so won't have much inductance even if you wind it. You should use resistance wire rather than normal copper wire, this could well reduce the length to a cm or two.

 

[fabbie]

What can be considered as resistance wire? :roll: I have never dealt with anything other than copper wire

 

[Nigel Goodwin]

What can be considered as resistance wire? :roll: I have never dealt with anything other than copper wire

It's made of different metals, iron and constantin are common ones. An easy way to get some is from an electric fire element.

 

[John Sorensen]

"According to the Radio Engineers Handbook, it shows nine possible ways
(including Ayrton-Perry) to wind a resistor with "that minimize reactive
effects". Under the section,"Non-reactive Wire-wound Resistors", it
says,"To keep the inductance low, each turn should enclose the minimum
possible area, and the wire should have as many ohms per foot of length
as possible so that the length required to obtain the desired resistance
will be small". Of course we all know this as we'd be winding an
inductor, the more turns, the more inductance. Then it goes on, "In
addition, it is desirable that adjacent turns carry current in opposite
directions so that the residual inductance of an individual turn is
neutralized by the effect of the adjacent turns. A low capacitive
reactance associated with a resistor is obtained by arranging the
winding in such a way that adjacent turns of wire have a low potential
difference between them and as far apart as possible".

There are tables that will give you the ohms per length of wire. That would be a good start.

j.

 

[JohnBrown]

Just make sure you turn the electric fire off before attempting to remove some of the wire.

 

[fabbie]

Mind explaning to me what is the difference between a normal resistor and a resistor with HIGH power rating

I tried buying a 0.1ohm resistor but all i can find is a 5W 0.1ohm resistor. I hope this would be suitable for current sensing applications

 

[Nigel Goodwin]

Mind explaning to me what is the difference between a normal resistor and a resistor with HIGH power rating

Higher rated ones are larger, with wirewound resistors for anything except fairly small ones - it's a question of how much heat they can dissipate.

I tried buying a 0.1ohm resistor but all i can find is a 5W 0.1ohm resistor. I hope this would be suitable for current sensing applications

That's fine, it's the sort of resistor you should be looking at for current sensing.

 

[stevez]

Nichrome wire is often used for heating elements and has a relatively high resistance per unit length. I was fortunate enough to find a 500 ft spool of 28 ga nichrome wire for less than a dollar (US) at a flea market. Note that if the temperature changes significantly the resistance will change as well.

 

[stevez]

I forgot to mention that discarded computer power supplies can be a good source of low value power resistors.

 

[fabbie]

hey,

Normally comercial copper wire sold at electronics shop arent pure copper, right? I was planning at first to make a 0.1ohm resistor using copper wire found at my college. However, measuring the resistance across a single straight copper wire was already 0.6 ohm :shock: . thats when i gave up making a resistor through the use of copper wire(most probably it has impurities in it)

 

[stevez]

Variations in chemical compostion, density, resistivity, dimensions, etc, etc are quite normal for most every material. It would be considered normal to find some impurities in copper wire. In using any material you have to account for these variations if they might affect the final product. In this case you might construct a resistor in a way that allows some fine tuning or adjustment of the value. Note that in measuring low ohmic values sources of significant error can be the measuring instrument, the leads, alligator clips, etc.

 

[Nigel Goodwin]

Note that in measuring low ohmic values sources of significant error can be the measuring instrument, the leads, alligator clips, etc.

Yes, I would suggest using two meters, one on amps in series with the wire, and one on volts across the wire. Pass a reasonable current through the wire (1A is a nice easy figure to use!) and measure the voltage drop across a section of the wire. Assuming you're using 1A current, slide the voltmeter probes apart on the wire until it reads 0.1V - the distance between the probes is then 0.1 ohm. If you're not using 1A current, adjust the voltage reading accordingly.

Copper wire, impurities or not, shouldn't read 0.6 ohms over a short length - I suggest that is probably a measurement error due to the low value.