Heating Circuit

Steve Newport schrieb:

Hello,

to calculate the resulting temperature is very difficult.

But I suspect that four 1.5 AA cells are far to weak for the heating you want to do.

Give us more details what you want to do.

Bye

Actually, the other day when using a digital camera I got mild warmth for about an hour. It uses 4 2200 mA-hour NiMH AA batteries. So something for mild heat for limited time might be possible. (I am not suggesting you use a digital camera for heat!) But more details are certainly necessary to figure out how to do this and how reasonable it will be. --Phil

I am not sure if this will be any help at all, but have you ever used those battery socks? They keep your feet pretty warm on a couple of D cells....maybe some reverse engineering is in order here.....have a great holiday...Ross

You've given far too little information to answer completely.

Certainly 4xAA batteries can generate quite a large amount of heat in a small volume, take for example the inside of a flashlight bulb.

You want to start by estimating the number of watts you want to dissipate and the volume (or area) you want to heat.

If one typical AA alkaline battery has 2800mAh at 1.5V then 4 in parallel will give you 11.2 Ah at 1.5V ir 4 in series will give you 2800mAh at 6V. Either way you can get about (P=IV) 16.8W for about an hour out of it.

Say you wanted to make a 10W hand warmer and the area of my palm is about 10 square inches. You would need to dissipate 1W/square inch. (10W might be a bit hot in your hand if not spread out properly)

We'll use 2.8A and 6V for 1 hour as the source ratings. For 10W and 6V you want the current to be 1.6A (P=IV). This should last 1.75 hours. The total wire needs to be R=P/I^2 or 10/2.56= 3.9 ohms. Double check with V=IR

6=1.6*3.9 (actually 6.24V since I rounded somewhere).

Now say you made a zig zag pattern of 5 wires 1 inch apart. Each inch of wire would need to dissipate roughly 1W (ignoring insulators in the area). You end up with 30 inches of wire total. and each inch should be 130 mOhm/inch.

The maximum temp is calculated under the worse case of a fully charged battery with no current regulation. It is complicated because the physical configuration of the heater in the application greatly effects heat conduction away from the source. You need to know the heat conductivity of the media and the temperature change tolerated etc.

For a cheap source of resistance wire, try the spiral binding of a notebook. It is usually Ni-Cr and very suitable for many heater uses.

Chris

Hi both,

Thanks for the replies, not too sure what other details I can, or need, to give.

I had assumed that the first step would be to find a suitable resistance wire that would suit the batteries. Again, I assumed that too thick a wire would not provide heat but would just short the batteries. Again, I was thinking of something like a 0.25mm thick wire.

Then I thought it would be a case of calculating how long it would take for the batteries to discharge throuh the wire and how high the resulting temerature would be.

The element size I realise must be a central factor; so I am estimating that the wire length (before it meets normal copper) will be approx 27cm.

Should I view this from the perspective that the wire I select should be on the basis of the thinnest I can use without the batteries being able to "burn" the element?; for example I have seen 0.05mm diameter wire but I don't know how to calculate the temperature this would reach or the length of time the batteries would take to discharge.

Sorry, have I confused everybody now?

Actually, the other day when using a digital camera I got mild warmth

If all you had to choose from was copper then thickness (or wire gague) would be the main concern. With other metal alloys available, the wire size might be quite large. The key factor is the resistance per unit length or Ohms per foot. Even at single strand thicknesses, you would need at least several feet of wire and the risk of fusing it open is high.

If cost were not a concern, you would decide upon the length and thickness as a matter of mechanical design considerations first . Then after calculating the resistance per inch you can go out and select a material that can be bent, formed, stamped or molded to your desired shape.

If cost is a concern, try ripping apart an old electric blanket. Experiment with other types of wire not intended for electrical use (packing or bailing wire). Look for something not steel or copper. Nickel or Chrome alloys conduct poorly and should work well. You can get a heating element from the hardware store and dismantle it. Once you find a material close enough, you can look at ways to pack it into the area you want to heat. overlapping coils is a good way to distribute the heat evenly and increase the heat density (watts per unit area).

Bare wire is safe in a 6V application but for higher voltages, or any AV voltage source that may contact a person directly, you would want an insulated heating element. Now, some companies offer resistance strips made on flex-PCB, these offer more flexibility.

There are some other ways to do this too. You can power a electro heat exchanger or you can power a chemical reaction. Both of these may be more efficient then the heat dissipation of a resistor.

If you are trying to keep your feet warm, it may be easier to but hunters socks.

Sincerely,

Donald L. Phillips, Jr., P.E. Worthington Engineering, Inc.

145 Greenglade Avenue Worthington, OH 43085-2264

snipped-for-privacy@worthingtonNSengineering.com (remove NS to use the address)

614.937.0463 voice 208.975.1011 fax

No way. You're forgetting heat. Try grabbing hold of a short piece of bare #24 and connecting it across a fresh AAA battery. On second thought, don't touch the wire, hold it with pliers, and watch what happens. Heck - if you put bare #12 across a 6 volt auto battery (if you can find one any more) you'll melt the wire. Been there, done that.

There's a company out there making cigarette lighters that use one or two AA batteries to heat an element enough to like your smoke. It worked pretty well, even with the tiny voltage being applied. I don't recall who made it, but you may be able to find one on the .net. Best of all, it was only a few bucks.

A quick search on Ebay found this item:

Just a note, I don't know this seller, never delt with him, and can't vouch for him. However, this is the lighter I've used in the past.

Anyway, two of these could fit the bill for four batteries.

Good luck,

Pagan