Hello Im currently in the process of building a efficient small DC powered heater and would like suggestions. It needs to be the least amount of watts vs. maximum heat output. The closest thing I found so far are the 10 and 8 ohm ceramic resisters wrapped with aluminum. When they are hooked in series and parallel and shorted out with a dc power source they get hot. The problem is shorting out uses lots of power. especially when hooked to a battery. Ive managed to get the watt usage down to around 200 watts with an Iota 500 watt ac/dc inverter but the other problem is when you blow air on them they cool off. Thats probably the biggest factor. Thats probably why you would want a vacuum sealed element and perhaps a better thermal conductor. If you have a DC motor that uses around 300 watts and hook the resisters in parallel to reduce current to the motor works to but the resisters get hot but the motor doesn't have much power. I got the frame built with a duel fan PCB type brushless motors from an old Xbox360 which is highly efficient at 12 volts like 5 or 10 watts with high air output. The frame came out really nice, made with good quality metal. I'm probably going to use the 18650 lithium ion batteries to run it. So I basically need to how to build a circuit that can cancel out the short to the battery and any advice on types of efficient DC heating elements. I would like to get the wattage down under 100 watts with a high amount of heat output. I think its ridicules that your average ceramic heater uses around 1500 watts when a 900 degree soldiering iron uses 40 watts
closed as unclear what you're asking by PeterJ, Ricardo, JRE, Dwayne Reid, Daniel Grillo Aug 5 '15 at 14:44
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Let's say you have a normal room in a normal house with some basic insulation. Let's assume that the temperature difference between the inside of the room and the outside of the room at the current time is diffusing 500 watts worth of heat to the outside of the room through the walls, floor, door, windows, etc. (There are far more complicated calculations that take into account the thermal conductivity of these surfaces and the actual temperature differential but let's just ignore that for now and assume we're losing 500W of power.)
So to heat the room we need to have a heater that generates more heat over time than the room is losing over time.
Most resistive heaters will be very close to 100% efficiency in converting electricity to heat. Most efficiency losses in electrical systems are caused by waste heat being produced by components in the circuit, and reducing that waste heat will increase the efficiency of the circuit. Heaters don't really have the same problem, producing waste heat is precisely what they aim to do.
Going past 100% efficiency is physically impossible. That would mean your heater is producing energy out of nothing. What you need to do instead is better insulate the space you are attempting to heat to reduce the heat lost to the outside space and thereby reduce the power required to heat the room.
The other part of your question is about current limiting. If your circuit is nothing but a resistor then you can simply use Ohm's law and tune your input voltage to get your desired power output. If you cannot change your input voltage then you'll need to consider other options such as current limiter circuitry, which you can find hundreds of questions about with a quick search.