Activating Nitinol Wire

Activating Nitinol Wire

As mentioned, nitinol wire may be heated by simply passing an electrical current through the wire. The resistance heats the wire and it contracts. The volume of the wire doesn't change during contraction (see figure 3). As the wire decreases in length, its diameter increases by a proportional amount, keeping the volume of the wire constant. The activation temperature of the wire is 70 C or 158F.

Direct Electrical Heating
Nitinol wire can be activated using a low voltage DC (6-12volts) power supply. A simple circuit can be constructed using a battery, switch and a small length of nitinol wire (see figure 4). When activating a wire using DC current it is important not to overheat the wire, or its properties will degrade. DC current doesn't heat the wire evenly. A better method is to heat the wire using pulse width modulation.