Here's kind of a wierd question. I would like to try to heat wire (any kind) to ~40 - 50 degrees for a sustained time with batteries. I would love to get 10+ hours out of C or D batteries. My question is, is there an easy way to determine how thick the wire should be, and how many batteries I would need to do this? The temperature does not need to be exact, just within 10 - 15 degrees.
The problem has very little to do with the wires and everything to do with how much insulation you wrap around them. That insulation will have to be pretty good, to keep even a short length of wire (or other resistor) above 50C in a much cooler environment for 10+ hours. For instance, a millimeter length of bare wire (submerged in 25C water) would take many times the power to sustain a 50C surface temperature than a kilometer of fine wire (wound on a small spool that is enclosed in close fitting Styrofoam box a few centimeters thick surrounded by 25C air) would need.
So lets get started with what you actually need to have happen (not how you envision doing it), and we can work towards a means to that end and see if that means is practical with power available from some number of C or D batteries.
This isn't a weird question at all. The whole point of resistance heaters is precisely that.
But you didn't mention what length of wire you wanted, whether it would be in free-standing air or another medium, whether the air or other medium was still or moving and at what speed, and whether the wire would be actually doing any work or heating something which might change the temperature. Kind of difficult to give you any advice given those constraints.
Another difficulty you might have is load life for your batteries, which might be the final determining factor. A couple of D alkaline batteries can light a 1/4 to 1/2 amp bulb for 8 or 10 hours. That will limit you to 3/4 to 1-1/2 watts, which isn't much power. However, your temp requirement might not be that strict if you're talking about 20 degrees C ambient in still air.
What you might want to do is scrounge some 22 to 220 ohm, 10 watt power wirewound resistors and gently tap off the ceramic overcoating so as not to break the resistance wire. If you then break the ceramic core, you can unspool it and have several inches to a foot or so of high quality resistance wire. Connect alligator clips to your battery wires, then starting with the 22 ohm wire, clamp the clips to the wire, let it stabilize, and measure temp. For the 22 ohm resistor, your minimum length would be around 1/4 of the total length of wire, or about 6 ohms. More than that will use the battery up too fast. Do the length calculations for the longer lengths of wire so minimum resistance doesn't go below 6 ohms or so. Then just find what you're comfortable with.
As I said, if your project requires that the wire actually heat something, or if you're working with something besides free still air, you might have to get more complicated. Feel free to post back with more info.
Cheers Chris
Forgot to mention that the above is based on *two* D sized alkaline cells. Sorry.
Cheers Chris
What I would like to do (if humanly possible), is make something like a heated driveway, only smaller, using wire and batteries. I was thinking of putting wire between two sheets of plastic and connecting them to a battery pack. The wire would have to be warm on the outside, even at very cold temperatures. Am I wasting my time, or could this be done?
I forgot... maybe the most important part... I would like this to melt snow, and further more, continue to melt snow as it falls on the plastic. That's probably an important detail to leave out!
Okay. This is a pretty demanding requirement, since ice absorbs so much energy per gram to melt. What area would you like to have this melting action take place? A pea sized area may be practical.
Apologies for sounding blunt. But I think you would have more success with eating a bannana and going out there with a stiff sweeping brush. Alot more environmentally friendly too.
A chemical solution could also work. Very salty water has a lower freezing temperature than water/snow alone.
You are looking at an area of maybe 1 sq mm in that case.
Huh. Thanks for the blunt replies. That is what I was looking for!
If you could let up in on the planned use we might be able to come up with good, workable suggestions.
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Hmmm. they've got 6 watts per foot stuff here:
I don't know how long you can get 6 watts out of a few D cells, but they do sell battery-powered socks and gloves. ;-)
Cheers! Rich
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