Discussion in 'Electronic Basics' started by Gunnar G, Jun 7, 2005.

1. ### Gunnar GGuest

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?

2. ### John PopelishGuest

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.

3. ### Gunnar GGuest

Probably. It depends on how hot that element can get and still remain
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?

4. ### Roger JohanssonGuest

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?

5. ### John PopelishGuest

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

According to:
http://www.goodfellow.com/csp/active/STATIC/E/Constantan.HTML
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.

6. ### Gunnar GGuest

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!