A switching circuit includes a bridge rectifier to receive voltage inputs and an optical isolator to receive a logic input signal and generate an output signal based on the logic input signal. The high speed switching circuit also includes a field effect transistor (FET) with a source connected to a negative output of the bridge rectifier, a drain connected to a positive output of the bridge rectifier through a load, and a gate driven by the output signal of the optical isolator. First and second resistors connect the voltage inputs to the gate of the FET through first and second diodes. The first and second resistors and the first and second diodes limit current flowing to the gate of the FET. A Zener diode connected to the gate of the FET limits voltage to the gate of the FET below a maximum voltage rating of the FET.

Exemplary embodiments or implementations are disclosed of methods, apparatus, and systems for operating motors in variable speed situations. In an exemplary implementation, a method of controlling a variable-speed motor includes defining a control duration as a predetermined number of cycles of a multiple-phase power supply. Each speed in a range of speeds is defined by a corresponding number of the cycles of the control duration. Power is provided to the motor from the power supply at a selected one of the speeds, by enabling input from the power supply for the number of the cycles of the control duration corresponding to the selected speed.

A switching circuit includes a bridge rectifier to receive voltage inputs across a load and an optical isolator to receive a logic input signal and generate an output signal based on the logic input signal. The high speed switching circuit also includes a field effect transistor (FET) with a source connected to a negative output of the bridge rectifier, a drain connected to a positive output of the bridge rectifier through a second load, and a gate driven by the output signal of the optical isolator. First and second resistors connect the voltage inputs to the gate of the FET through first and second diodes. The first and second resistors and the first and second diodes limit current flowing to the gate of the FET. A Zener diode connected to the gate of the FET limits voltage to the gate of the FET below a maximum voltage rating of the FET.

An apparatus for controlling one or more switched high power loads (or heating elements). The apparatus including: one or more switched high power loads (or heating elements), each high power load being powered from a common alternating current power source, and wherein each load is independently switched using a switching signal for zero crossing switching to achieve a desired average power output; the switching signal is generated that comprises a repeated switching sequence; the switching sequence indicates a respective selecting activation for each of the switched high power load over a sequence of half or full cycles.

A DC/AC conversion circuit and a control method therefor, and a modulation method for a cycloconverter are provided. The circuit includes: an inverter unit, which is configured for converting a direct current into an alternating current; at least one cycloconverter, which includes a plurality of sets of switching elements, the plurality of sets of switching elements are connected to an output port of the inverter unit respectively and are configured for performing AC-to-AC conversion, and an output port of the at least one cycloconverter is configured to connect to a power grid and provide an alternating current output; and a controller, which is connected to the inverter unit and the at least one cycloconverter respectively and is configured for controlling, when a grid voltage is within a threshold range, corresponding two sets of switching elements among the plurality of sets of switching elements to complementarily turn on.