In the figure we have a constant current source for Shape Memory Alloy in wires or Alloy with Shape Memory (Nitinol). The value of R is 1k ohms. The transistor must be equipped with a heatsink. The current setting is made in 10 ohm potentiometer or trimpot. The circuit is from the book Mechatronics Sourcebook by Newton C. Braga, published in the United States.
