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Resistor

Tom Hall

May 25, 2018
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I'm building a welder from an alternator and need a 1 1/2 ohm 8 amp resistor. Can anyone show me how to make one using copper wire?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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1 1/2 ohm 8 amp resistor

That's not a normal rating for a resistor.

The resistor will be dissipating 96W, so it's a 1.5Ω 100W resistor.

To make one out of copper wire, you'd have to find some thin copper wire that is capable of carrying more than 8A and cutting a length long enough to have a resistance of 1.5 ohms.

If you looked at a table like this, you could determine that you need 36.75 feet of 26 gauge wire. 26 gauge wire can carry about 20A, so that's reasonably safe.

However, you'll need to keep all that wire insulated, and it's not going to fit in a small space.

A neater option would be to get three resistors like this, attach them to a heatsink and connect them in series.

For additional safety margin, I'd probably get 4 of these and connect them in series/parallel.

You can get higher wattage resistors, but they're generally disproportionately expensive.
 
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WHONOES

May 20, 2017
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Don't like the copper wire solution. The 26AWG wire suggested is way too thin and would get hot when passing 8A especially if wound into a coil. If you were determined to use copper wire I would advise at least a 1mm cross section which equates to about 17AWG. Don't forget that as the wire warms up its resistance will change.
The resistance of the flood lamp will change wildly between cold and hot.
In the past, I have used iron wire, the sort of stuff you might use for fencing, to make small value resistors.
To determine what length of wire is needed, pass a known current (I use 1A) through a known length of wire and measure the voltage drop across it. It is then a simple matter of ohms law.
 

WHONOES

May 20, 2017
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ok well how about nichrome wire?
Nichrome works but, it would need to be on a heatsink of some sort or else its resistance will increase with temperature.
The best solution would be for a thin film high wattage resistor on a heatsink.
 

Chemelec

Jul 12, 2016
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Nichrome Wire remains Fairly constant in resistance with heating.

I have some 16 AWG Nichrome wire (0.268 Ohms per Foot) I could mail to him for Free.
(And It Does NOT get Red Hot at 8 Amps.)
 
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Chemelec

Jul 12, 2016
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If you looked at a table like this, you could determine that you need 36.75 feet of 26 gauge wire. 26 gauge wire can carry about 20A, so that's reasonably safe.

A 26 AWG wire is only good Maybe 5 Amps in Free Air.
Definitely NOT 20 Amps.
 

Tom Hall

May 25, 2018
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Nichrome Wire remains Fairly constant in resistance with heating.

I have some 16 AWG Nichrome wire (0.268 Ohms per Foot) I could mail to him for Free.
(And It Does NOT get Red Hot at 8 Amps.)
That would be great! How do I get my address to you? ( I'm new at this)
[Email Removed]
 
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Chemelec

Jul 12, 2016
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That would be great! How do I get my address to you? ( I'm new at this)

I am in Canada, so it just cost me $2.00 Postage.
This wire is from a Used Heating Element,
I will cut you a 1.5 Ohm Length from it.

Send me an Email with your Name and Address.
chemelec at Hotmail.com16 AWG Nichrome.JPG
 
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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A 26 AWG wire is only good Maybe 5 Amps in Free Air.
Definitely NOT 20 Amps.

The figures I used for copper wire were really designed to show that it was an unwieldy solution.

I didn't suggest nichrome wire because that is also unwieldy.

Why not use appropriately rated resistors so you don't have to concern yourself with keeping the whole thing insulated?
 

Chemelec

Jul 12, 2016
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100w 1.5r aluminium mount case aus$6.50 ebay....
Considering it is Wire Wound and Looking at the Size of that resistor, I'll bet it gets Really Hot at 100 watts.
Must be a Small Size Wire inside.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I have a "decade dummy load" comprising resistors from 0.01Ω to 10kΩ at various power ratings from 50W to 200W. This gets used for testing power supplies.

My 1Ω value is comprised of 4 50W 1Ω resistors. All the resistors (they are aluminium clad ones similar to the ones I linked to) are mounted on a single large heatsink.

With 10A flowing through the 4 series/parallel resistors, the resistors remain cool enough that you can comfortably touch them. If you look at their specs, unlike semiconductors, they can safely maintain extremely high case temperatures, but for the stability of their resistance it's best not to subject then to that.
 
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