# 12vdc resistance heater using copper wire (low temp)

#### RogueRose

Joined Oct 10, 2014
375
I'm trying to figure out what temps I can get and how wire gauge and length effect temp. I need a low temp heating element anything from 25 - 100 C.

I calculated that
26g - 25ft = 1 ohm
24g - 39ft = 1 ohm

The thing is that I don't know how to calculate what wire to use to try this and if I can add a pot or resistors to change the temp.

I have 17, 24, 26, 28 & 30g copper wire that I can try with this.

Can anyone point me in the direction to figure this out or give me some insight into this?

I just calculated the wire parameters and it says that a single 24g wire would draw 3.18 amps. If I apply 12v then I should get about 38.16 watts - or about 1 watt per foot. What I am not sure of is how resistance comes into play here.

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#### LesJones

Joined Jan 8, 2017
4,191
The biggest factor is the heat loss from the wire. If the wire is in a straight line with a good air flow across it it will require more power to reach the required temperature than if it was wound into a tight coil and surounded by thermal insulation.

Les.

#### Alec_t

Joined Sep 17, 2013
14,332
Thermal insulation is the key factor in determining the temperature. In theory, a perfectly thermally insulated 1 milliWatt heater wire would reach its melting point! You will need to experiment to see how much current will get your wire to its target temperature. A thermostat can control the current.

#### crutschow

Joined Mar 14, 2008
34,464
What I am not sure of is how resistance comes into play here.
Power dissipated in the wire is V²/R and the current is V/R, so you select a wire size and wire length to get the desired power dissipation and current for the supply voltage you have.
The main limitation is, you have to stay below the maximum current a given wire gauge can carry, as shown in the wire table.

#### RogueRose

Joined Oct 10, 2014
375
Thanks for all the responses! I'm planning on using solid 24g CAT5e twisted pair - still twisted. It's going to be spun in a tight coil, each row touching the next and a solid aluminum plate sitting on top of it.

I was planning on using a computer PSU but if the problem is that I don't want max current, then that will be an issue. What happens if maximum current is applied?

I also have a 32v power supply that supplies 3.18a - which, coincidentally, is the same max amperage of the wire.

I could make the wire longer or add an extra smaller wire running parallel, so maybe add a 26g wire in addition, to allow for more current carrying capacity.?

Joined Jan 15, 2015
7,523
Using 24 gauge wire typical AWG 24 has a max current rating of about 3.5 amps. That is single conductor copper wire. The resistance is about 88 Ohms per 1,000 meters (3,280 feet). If you use a twisted pair, insulated, you can half the resistance numbers. The diameter in inches is about 0.020" for AWG 24 single conductor copper (less any insulation).

How much heat are you looking to get? How thick is the aluminum plate and do you plan on any type of temperature control? Using a copper coil resistance heater may not be the best way to go depending on exactly, in some detail, on what you want to do? Twelve volt cartridge heaters may be a better way to go about this. There are other solutions like Flexible Silicone Rubber Fiberglass Insulated Heaters for lower temperature applications.The links are merely examples of some of what is available over the counter.

Ron

#### GopherT

Joined Nov 23, 2012
8,009
Thanks for all the responses! I'm planning on using solid 24g CAT5e twisted pair - still twisted. It's going to be spun in a tight coil, each row touching the next and a solid aluminum plate sitting on top of it.

I was planning on using a computer PSU but if the problem is that I don't want max current, then that will be an issue. What happens if maximum current is applied?

I also have a 32v power supply that supplies 3.18a - which, coincidentally, is the same max amperage of the wire.

I could make the wire longer or add an extra smaller wire running parallel, so maybe add a 26g wire in addition, to allow for more current carrying capacity.?
You have 32v and 3amps. You can get 100 watts.
To limit your 32V amps, you'll need about 10 ohms of resistance.

If you want to use 24 gauge wire, you'll need about 110 meters of 24 gauge wire.

If you are using CAT5e with 8 strands (4 pairs), you can use a 14 meter section of CAT5e, connect each pair on one end (4-connections), then connect appropriately on the other end to make all 8 wires connect in series - then supply current to that end. (Measure resistance first)

The problem is that keeping this from overheating can be a challenge. Don't coil too tightly or 100W in a very small area can get to very high temps quickly - note how hot a 100 w bulb gets. If you keep it extended, you'll be able to transfer the 100watts into the room but it will be a challenge to push the heat into a small area.

Good luck -make sure you look at the temp rating on the insulation. 75°C is common.

Done.

#### WBahn

Joined Mar 31, 2012
30,076

You don't just grab some wire and figure out how many watts of heat you can produce.

The first thing you need to do is figure out how many watts of heat you NEED to accomplish your objective. THEN you start looking for ways to get THAT amount of heat.

#### GopherT

Joined Nov 23, 2012
8,009

You don't just grab some wire and figure out how many watts of heat you can produce.

The first thing you need to do is figure out how many watts of heat you NEED to accomplish your objective. THEN you start looking for ways to get THAT amount of heat.
How would you go about calculating how many watts of heat I NEED to cook my Christmas turkey?

#### RogueRose

Joined Oct 10, 2014
375
Well, I was using this project as a starting point for some low wattage projects. heat transfer will be pretty good as it will be wrapped around an aluminum pipe, glass bottle or aluminum or copper plate and sometimes having fiber glass cloth as the outer insulating later helping keep heat in and hold the wire in place. high temp tape will help hold wire in place. There may be instances where the wires are submerged in a liquid at temps between 100-160F

I'd like to make some setups that can put out 25 - 150 watts. If one setup is possible, that is great, or if I have to have a few sets of wires then that is fine as well.

I have a wide range of power supplies from standard computer PSU's, 24vdc, 32vdc. Some only do a couple to few amps, others are capable of 20-50 amps. I can get a psu that will do whatever I need, that isn't so much the problem.

What I need to figure out is what wires will work best for this. Using 113 meters of wire is not practical in this instance. If I'm using Cat5 cable, I seperate the strands into the twisted pairs and sometimes untwist the pairs for single strands.

#### GopherT

Joined Nov 23, 2012
8,009
Well, I was using this project as a starting point for some low wattage projects. heat transfer will be pretty good as it will be wrapped around an aluminum pipe, glass bottle or aluminum or copper plate and sometimes having fiber glass cloth as the outer insulating later helping keep heat in and hold the wire in place. high temp tape will help hold wire in place. There may be instances where the wires are submerged in a liquid at temps between 100-160F

I'd like to make some setups that can put out 25 - 150 watts. If one setup is possible, that is great, or if I have to have a few sets of wires then that is fine as well.

I have a wide range of power supplies from standard computer PSU's, 24vdc, 32vdc. Some only do a couple to few amps, others are capable of 20-50 amps. I can get a psu that will do whatever I need, that isn't so much the problem.

What I need to figure out is what wires will work best for this. Using 113 meters of wire is not practical in this instance. If I'm using Cat5 cable, I seperate the strands into the twisted pairs and sometimes untwist the pairs for single strands.
I would suggest leaving the CAT5e in their insulating and use only 14 meters.

Or, use 10 of those 100 ohm sand-bar resistors in parallel (10 ohms overall). Use the 10W size so overall you'll have 100W worth. Much more compact.

Joined Jan 15, 2015
7,523
Or, use 10 of those 100 ohm sand-bar resistors in parallel (10 ohms overall). Use the 10W size so overall you'll have 100W worth. Much more compact.
Those or the aluminum ones. That would be my choice.

Ron

#### GopherT

Joined Nov 23, 2012
8,009
Those or the aluminum ones. That would be my choice.

Ron
They get expensive but they are great at transferring heat. We used them on the build plate for RepRap 3D printer.

We used four of these in parallel on a 1/4" aluminum plate. 12V ATX power supply and 8 ohms of resistance.

Joined Jan 15, 2015
7,523
They get expensive but they are great at transferring heat. We used them on the build plate for RepRap 3D printer.

We used four of these in parallel on a 1/4" aluminum plate. 12V ATX power supply and 8 ohms of resistance.
Years ago I built an ATX PSU load bank for testing ATX PSUs and used dozens of them mounted on aluminum plated (1/4") with cooling fans. Still have remains around here somewhere. They did make heat running of ATX power supplies.

I also have a bag of 25 Watt 30 Volt cartridge heaters, round 0.370" diameter and about 2.5" long.

Ron

#### RogueRose

Joined Oct 10, 2014
375
I have an old PSU from an HP printer or mointor that is 32v 3.17a and when I connect it to the wire, the voltage drops to zero. Since the current of the PSU and the max current of the wire are the same, I thought this would work for at least a couple minutes to see how quickly things heat up. But it seems to auto shut-off immediately upon connecting it. The resistance in the wire is 1 ohm (at least that is what the calculation show, my meter isn't reliable as it seems to jump around quite a bit.

I'm using 39 ft of 24g wire - single strand. I can also add an additional 40ft of 26g wire without too much problem to this setup if that would help things - running the wire in parallel is what I mean. I can vary the voltage and can supply as much current as the wires can carry or I need a way to limit the current, which is where I always get confused as to how to do this, maybe add additional resistors in line or in series with the wire?

I have 12vdc PSU's that are 1, 1.5, 2, 2.5 and 3a. I have 16-19vdc PSU's that vary from 2.5A up to 5a. I have 24vdc supplies at 1, 2 and 3a, and a 32vdc 3.17a PSU.. Then I have the PC PSU with 3.3, 5 & 12vdc outputs with high amperage.

From what I understand from above, is that I don't want to exceed the rated current of the wire. I thought that a wire would only carry so much current

#### GopherT

Joined Nov 23, 2012
8,009
I have an old PSU from an HP printer or mointor that is 32v 3.17a and when I connect it to the wire, the voltage drops to zero. Since the current of the PSU and the max current of the wire are the same, I thought this would work for at least a couple minutes to see how quickly things heat up. But it seems to auto shut-off immediately upon connecting it. The resistance in the wire is 1 ohm (at least that is what the calculation show, my meter isn't reliable as it seems to jump around quite a bit.

I'm using 39 ft of 24g wire - single strand. I can also add an additional 40ft of 26g wire without too much problem to this setup if that would help things - running the wire in parallel is what I mean. I can vary the voltage and can supply as much current as the wires can carry or I need a way to limit the current, which is where I always get confused as to how to do this, maybe add additional resistors in line or in series with the wire?

I have 12vdc PSU's that are 1, 1.5, 2, 2.5 and 3a. I have 16-19vdc PSU's that vary from 2.5A up to 5a. I have 24vdc supplies at 1, 2 and 3a, and a 32vdc 3.17a PSU.. Then I have the PC PSU with 3.3, 5 & 12vdc outputs with high amperage.

From what I understand from above, is that I don't want to exceed the rated current of the wire. I thought that a wire would only carry so much current
The current of the wire?

That is a maximum number. You need to use Ohms law like I showed in post 7.

A short piece of wire will have low resistance and pop the power supplies circuit breaker.

Measure the resistance and then use 32v / resistance = 3 amps. You want resistance to be about 10 ohms or more. Plain copper is an excellent conductor so you'll need a long piece of thin copper to get to 10 ohms.

#### RogueRose

Joined Oct 10, 2014
375
The current of the wire?

That is a maximum number. You need to use Ohms law like I showed in post 7.

A short piece of wire will have low resistance and pop the power supplies circuit breaker.

Measure the resistance and then use 32v / resistance = 3 amps. You want resistance to be about 10 ohms or more. Plain copper is an excellent conductor so you'll need a long piece of thin copper to get to 10 ohms.
Ok, so if the charts are correct, then the wire I have is 1 ohm (My meter shows resistance of 1.1 - 1.5 ohm..). I have a good assortment of resistors and can come up with a combination that is ~ 9 ohm, but wouldn't most of the heat be in the resistor instead of the wire?

#### WBahn

Joined Mar 31, 2012
30,076
How would you go about calculating how many watts of heat I NEED to cook my Christmas turkey?
Well, either you're trying to troll me again, or the approach you would apparently take if you had to solve that problem would be to calculate how much heat would be produced in an extension cord shorted across your car battery.

Since I don't think even you would take such a backward approach, I'll conclude that you are just doing your usual trolling.

Please do everyone a favor and just stop.

#### RogueRose

Joined Oct 10, 2014
375
Well, either you're trying to troll me again, or the approach you would apparently take if you had to solve that problem would be to calculate how much heat would be produced in an extension cord shorted across your car battery.

Since I don't think even you would take such a backward approach, I'll conclude that you are just doing your usual trolling.

Please do everyone a favor and just stop.

No, not trolling. Didn't know I did it before and if you think I did, I'm sorry, but I didn't do it purposely, so I think you must have misunderstood my previous intentions.

What I'm doing is trying to build a small heater with what I have on hand. I'd like it to heat slowly if possible. It is going to be wrapped around a pipe and needs to heat what is inside it. The temp inside needs to be from 100 - 160F depending upon pressure inside the vessel (it varies between liquids and how much vacuum can be applied at a constant rate - which is more difficult than it sounds with evaporating liquids).

The pipe or glass bottle has a flat bottom and a coil of wire will be tightly wound in a spiral and then up the side of the pipe or vessel.
In another application, the wire will be inside the liquid and allow for better contact with the solution during evaporation.

I have a lot of low ohm resistors from 3, 5, 10, 15 & 20 watt resistors and .22, .47, .56, .62, .68, 1, 1.5, 2.2, 4.7, 5, 6, 10, 15, 30, I could easily get 9-10 ohms from these at whatever wattage is needed and use these at the base of the pipe or bottle (or mount them to an aluminum plate).

#### WBahn

Joined Mar 31, 2012
30,076
No, not trolling. Didn't know I did it before and if you think I did, I'm sorry, but I didn't do it purposely, so I think you must have misunderstood my previous intentions.
Don't worry -- YOU are not the troll.

What I'm doing is trying to build a small heater with what I have on hand. I'd like it to heat slowly if possible. It is going to be wrapped around a pipe and needs to heat what is inside it. The temp inside needs to be from 100 - 160F depending upon pressure inside the vessel (it varies between liquids and how much vacuum can be applied at a constant rate - which is more difficult than it sounds with evaporating liquids).

The pipe or glass bottle has a flat bottom and a coil of wire will be tightly wound in a spiral and then up the side of the pipe or vessel.
In another application, the wire will be inside the liquid and allow for better contact with the solution during evaporation.

I have a lot of low ohm resistors from 3, 5, 10, 15 & 20 watt resistors and .22, .47, .56, .62, .68, 1, 1.5, 2.2, 4.7, 5, 6, 10, 15, 30, I could easily get 9-10 ohms from these at whatever wattage is needed and use these at the base of the pipe or bottle (or mount them to an aluminum plate).
I stand by my suggestion that you are approaching this backwards. Now, if you WANT to essentially try a random heater without any idea whether it is way too little or way too much and see how well it does or does not work, and then design a second not-quite-so random heater leveraging what you learned from the first one, this is a viable technique and will eventually get you something useable and possibly without too many attempts -- the learning curve can be pretty steep if you approach it methodically.

But I would recommend getting at least some very rough requirement numbers first. What is the solution that is being evaporated and how much of there is it in the bottle (let's leave the pipe for later)? Trying to heat a bottle that holds an ounce and a bottle that holds a gallon are very, very different things. Do you know the specific heat of it? If not, you can probably measure that fairly easily (to a good enough approximation). If nothing else, start with the assumption that you are evaporating plain ole water and then build in a decent capability margin.

A lot will depend on how well you can insulate the system, and that will depend a lot on how much control you have over these details.

Since you are evaporating things, another big factor is how much heat the evaporated vapor takes away with it. Do you have any idea of this? If not, it might be time for another crude experiment to get a rough idea.