on microcontroller 17 operative to set the duty cycle ratio according to the magnitude of control signal v c an important feature of a hf ac chopper according to the present invention is the ability to protect the circuitry against high current as might develop if the output is shorted or as a result of inrush current inrush current develops in various loads in response to a fast increase in output voltage for example in incandescent lamp dimmer applications inrush current develops when the lamp filaments are cold until the filaments reach the elevated operating temperature this is because of the large difference in the resistance of a filament when the filament is cold and when the filament is hot under typical operating conditions the resistance of a hot filament is typically more than 10 times the resistance of a cold filament consequently when a voltage is first fed to an incandescent lamp the current is much greater than the nominal operating current this might cause interference on the power line feeding the lamp and could harm the hf ac chopper similarly a short circuit causes high currents to build up and unless controlled damages the hf ac chopper in the case of high current buildup a deadtime could be especially harmful because interruption of the high current generates very large voltage spikes that can easily cause breakdown the present invention solves the protection problem by first limiting the duty cycle ratio to lower the output voltage and if required.