# a question about power

• posted

Probably. It depends on how hot that element can get and still remain a stable resistance, and how easily heat is conducted from it to the environment around it. It takes an object about as large as your little finger to get rid of 6 watts and remain cool enough to not blister you. So it may take two strands in parallel (and twice as long, each) wound around something about that size, and covered in a layer of epoxy, to keep the wire temperature reasonable at that wattage. Bigger is better.

• posted

Hi. I'm a total clueless newbie and I've just bought my first soldering tool and I'm about to construct a resistor from a small thread with a resistance of

30 Ohm per meter. The question is: how much power can this thread take? It's going to be a 10 Ohm resistor and the DC current will be like 0.8 A and it gives a power of 6.4 W. I don't want things to start burning, is there a risk that 6.4 W too much power for this small thread?
• posted

It takes some area to dissipate some power to heat in the air.

If you heat up a big objekt, like a car, it can warm up the air around it, and 5Watt would easily be moved over from the car to the air around it. The car would just have to be slightly warmer than the air.

Think about a small object, like the thin wire inside a bicycle lamp. If you heat that wire with 5Watt of power it will have problems getting rid of that heat. It can do it, but it needs to be very much hotter than the air to do it.

That is why size means so much for the ability to move heat, to dissipate heat, without getting very high temperatures.

Wood and epoxy are usable up to a certain temperature, if your wires don't get too hot it can work.

What are you really trying to do?

```--
Roger J.```
• posted

It says "konstantan" on the package. I think I've heard that that is a material that changes its resistance very little with temperature.

The calculation with my little finger and 6 watts, what do you base that on?

The plan is to wire the strands around a piece of wood. The epoxy, is that to fix the wires so they don't get in contact with each other, or does it have any other possivitve effects when it comes to heat disapation? I have some of that termal paste you use under your heatsink in the computer, might that be of any use?

Sorry for the bad english.

• posted

It is true that over some operating temperature range, that alloy changes resistance little.

According to:

the maximum operating temperature is 500 C.

The normal size of resistors conservatively rated for similar power dissipation. They are normally coated with ceramic or silicone material which has a higher temperature rating than epoxy.

The coating not only provides electrical insulation, it helps the heat to spread from the thin wire to the nearby surface, to allow it to get out into the air with a lower temperature rise. By the way, epoxy is limited to something like 100 C and wood will shrink at less than that. I suggest you use something mineral, ceramic or glass. If you can scrounge up some much higher resistance 10 watt resistor, that would make an ideal body for your custom resistor.

Not if you intend to use epoxy, also. Those materials are usually based on silicone grease, which prevents epoxy from bonding. But the thermally conductive solids they contain make a good additive to epoxy. Some examples would be zinc oxide, alumina and powdered aluminum. But this enhancement is of little importance because the main thermal resistance is not that between the wire and the surface it is wound on, but between that surface and the surrounding air. Very good thermal contact with the resistive wire is only important when brief, large pulses of power must be handled.

I hadn't noticed. If anything I say is not clear, please ask for me to say it another way to help your understanding.

• posted

One more thing:

*-Constantan . ..

Yeah--better than we see from some native speakers.

• posted

Good question. The project changed dramatically last night when I (please don't laugh, I thought I knew this stuff, but obviously I didn't) realized that an adapter marked 7,5V 1A does not send out a constant voltage of 7,5 AND a constant current of 7,5V. It's rather a 7,5V voltage and a maximum of 1A.

My earlier questions are therefor irrelevant.

To answer your question, I was trying to make a resistor that would be warmed up with 6W. With last nights revelation, its no longer something I'm going to do. Silly me. But at least I learnt something!

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