The PR3-E S/SP/SPM range of thyristor stacks provide full, seamless control, of three phase, resistive loads using a two thirds control technique. Control of the power level is either by a dc signal or by manual control using a 5kΩ potentiometer. The PR3-E burst firing control stacks use fast pulse, zero volts switching technology, to minimise flicker and eliminate RFI problems. The controllers also incorporate an automatically resetting temperature trip, integral semiconductor fuses, and an integral volt free trip/enable input for wiring of an external thermal trip.