I decided
that I would finally test a 10cm length of 36 AWG nichrome wire
with a current limited supply of varying voltages. Since my windmill generates under 1 Amp of electricity I decided to use an LM317 integrated circuit to fix the current at a max of 220mA. I put an LM317 in a bread board and attached its output to a 10cm length of 36 AWG nichrome wire (from the hair curler heating element). Below is the circuit diagram for my LM317 set to limit current to 220mA:
Using the LM317 as a current limiter
Need to connect the 3 terminal chip as below:
pic
To calculate the resistance that you need to limit the current to what you want:
formula
In an effort to maximize the
effectiveness of the heating element I decided to test different
thicknesses (or gauges - higher the gauge thinner the wire) of Nichrome wire to see which would work best (produce
the most heat with the power produced by the windmill - just under 2 watts). With this in
mind I ordered 40 and 38 AWG Nichrome wire from Jacobs Online
. They mainly sell Nichrome wire for making foam cutters and
igniters for model rockets (the larger scale rockets). They are
fairly inexpensive since you can order lengths as short as 10 feet –
for $6 I got 10 feet of each of the 2 sizes I mentioned before (if I
had wanted 100 feet of each I think it would have only cost me around
$12 – better deal but I just don't want all the excess around).
Some basic notes on Nichrome wire:
- higher gauge (thinner) = higher resistance per unit length and heats up at lower power (less current)
- coiled wire produces higher temps than straight wire (with the same voltage and current)
More in depth info on nichrome wire take a look at the Wikipedia Page for Nichrome Wire
Resistivity of 1 foot straight Nichrome wire
at room temperature:
AWG Ohms
40 70.2
38 42.2
36 27.0
Need some type of temperature resistant
insulation between the wire and the aluminum casing to prevent
shorting out the current to the wire (do not want the wire in direct
contact with the aluminum since it also conducts electricity).
Insulation: Mica sheet, fiber glass fabric (welding supply, car body
work with Bondo)
Ni-chrome Wire Calculator
Gives you some ideas of what you need
even though the lowest temperature on the calculator is 400 degrees
Fahrenheit.
If average wind speed is 15mph –
which produces 220mA at about 9volts. That gives us our average
power available. Ohms law states V=IR rearranging we get R=V/I:
R = 9v/0.220Amps
R = 40.9 Ohms
So, with the windmill running with a
15mph wind producing 9 volts with 220mA (which is just under 2 wattS –
P=9v*0.22A) if we use a length of wire with 40.9 ohms resistance it
should use up all the power. Yes, I know its not perfect reasoning
since resistance increases with increasing temperature in the wire
but I'm just trying to get an idea of what I need before I begin
trying things.
