Better Electrical Control

Better Electrical Control

On the previous page, we activated nitinol wire using a basic DC current. While this works, it is easy to overheat and potentially damage the nitinol wire. This month we will construct a pulse width modulation (PWM) circuit. Activating nitinol wire using pulse width modulation has distinct advantages. The oscillating on-off of the power allows for more even heating of the wire (reduces hot spots). The duty cycle of the square wave can be varied to generate a degree of proportional control over the contraction. These factors allow us to activate the nitinol wire with better control for longer periods of time without causing heat damage to the crystalline structure of the nitinol alloy.

The PWM circuit is made from a 555 timer, (see figure 6). By using two transistors Q1 and Q2 and potentiometer with the 555 timer we can create an output with a relative constant frequency with a variable duty cycle.

When the output from the timer (pin 3) goes high, both transistors Q1 and Q2 turned on. The current through Q1, R3 and portion of the Potentiometer (R4) designated RA charges the timing capacitor C1. When the voltage on C1 reaches 2/3 Vcc, the output on the 555 timer (pin 3) goes low.

At this point both transistors Q1 and Q2 turn off. Capacitor C1 begins to discharge through the portion of the potentiometer (R4) designated as RB and R5 via pin 7 (Discharge pin) of the 555 timer. When the voltage on C1 drops to 1/3 Vcc the output switches high and the timing cycle repeats.

The duty cycle of the square wave output can be changed by varying the resistance of the potentiometer.

The output of the 555 timer (pin 3) connects to a MOSFET transistor that switches the current on and off to the nitinol wire. If the current from the PWM circuit is too powerful to control the nitinol wire proportionally, place an 8-ohm (2 watt or greater) resistor in series with the nitinol wire to reduce power.