How to work out what resistor you will need:
If you do not have a resistor in series ( in front of) your muscle it is likely to burn up.
The equation to work out what resistor you will need in series with your wire is :
resistor (Ohms) = (power supply(Volts) /current required to make the muscle move(Amps) ) – total resistance of the muscle (Ohms)
First of all find the diameter of the wire your using:
Measure the length of your muscle wire you will be using in one wire:
Use the datasheet listed above to look up the Resistance (Ohms/Inch) and the Approximate* Current
at Room Temperature (mA) of your muscle wire.
Note down what voltage you will be putting through the muscle wire. (this should be done using a transistor and from a separate power supply NOT from your computer through the arduino )
Now that we know the voltage the length of our muscle wire, how much resistance it has per inch (Resistance (Ohms/Inch)) and how much current it wants (Approximate* Current) . we can use ohms law to calculate what resistor we’ll need.
For example we’re making a muscle out of a piece of 0.008″ Dynalloy Flexinol Wire, our muscle will be made of 2 inches of Flexinol Wire and we are using a 9 volt power supply. Using the dtasheet we find the 0.008″ wire has a resistance of 0.8 Ohms per inch and wants 610 mA of power in order to move.
So we have the following.
Voltage supply (V) = 9v ;
Resistance of our muscle in Ohms = 0.8 * 2 (inches) = 1.6Ohms
note, you should also double check the actual resistance of your muscle by measuring it with a multimeter as there are many factors that can change it’s resistance.
Now using ohms law we find that if we did not use a resistor in series with our muscle the muscle would be getting 9 (volts) / 1.6 (Ohms) giving the muscle 5.625 Amps of current, this is far to much current as the muscle should be getting 610mA or .61 Amps.
To find out what resistor we will need we use the equation 9 (V) / 1.6(Ohms) + x(Ohms) = .61(Amp) where the value of x is our resistor in Ohms, so if we rearrange the equation we find that x = (9 (V) /.61 (A) ) – 1.6 so X = 13.15 so we will need a resistor around 13.15 Ohms.
Because a lot of current will be running through this resistor you should use a high wattage resistor these are usually physically bigger.
Where to buy:
You can buy muscle wire from many places on the internet, but the cheapest I’ve found it for is here.
You need this to work out what resistor you will need in series with the muscle.
demo’s of different ways of using muscle wire.
Wikipedia on Shape memory alloy
nothing to do with muscle wire but a must see for kinetic sculpture.