With the majority of extruders currently in use, dualmode temperature control is employed. Electrical power is supplied to the resistance or induction heaters by means of electromagnetic contactors chiefly for the reason of simplicity and low cost. However, switching the power circuits by mechanical contacts can cause high frequency interference problems during both closing and opening of the contacts. Contact bounce, random contact opening, and sharp interruption of inductive circuits can lead to high induced voltage transients, contact arcing, dielectric breakdown and associated phenomena presenting severe disadvantages. The advent of advanced process control systems as well as process computers makes more versatile equipment, which allows continuous variation of the average power to the heaters, a necessity. The solution could be solid-state AC power actuators based upon variable-pulse-length switching control of thyristors (silicon controlled rectifiers and triacs) in conjunction with sophisticated integrated circuits. The paper describes a device which passes a variable number of full cycles of the AC supply in an specified period. This period may be made quite short, about two seconds or less, so that if a heater having a fairly large thermal capacity is used, power will appear to be varied continuously. Zero-crossing techniques are employed forcing the solid-state switch to close at the instant when the voltage across it is zero, and to open at the instant when the current through it is zero. The variation of average power to the heaters is linearly dependent on the actuator input signal which is 4 to 20 mA DC supplied by the output of continuousaction process controllers. A simplified version of the device, intended for use in conjunction with process computers or other digital equipment, operates in the on-off mode controlled by logic input signals. In both cases the input of the actuator is isolated from AC power.