Design Ideas: January 19, 1995
The extremely simple circuit in Fig 1 drives a stepper motor directly from 120V ac, 60 Hz. Usually you need switched-dc voltages to drive a stepper motor. But a stepper motor will run off ac lines if you introduce a 90° phase shift between the voltages applied to the motor's two windings.The motor in Fig 1 moves 7° per step. Four steps equal one ac cycle, so the resulting motor speed is 300 rpm. Using other motors that have different step sizes results in different speeds.
The resistor's specified power dissipation and the value of the capacitor limit the current that the circuit can deliver to the motor. Because of the prices of large-value components, this technique works best with low-current motors such as the motor in Fig 1. (DI #1651)