| 摘要 |
PURPOSE:To execute a zero level detection by a simple circuit, so that no noise is generated, by shifting an operation start level of a switching element to a zero level. CONSTITUTION:In an AC power control device for heating the glass in order to eliminate a cloud or an ice-drop, etc. adhering to the glass due to a difference between a room temperature and an open air temperature, it is necessary to detect a zero level in order to execute a zero level control of an AC waveform. Therefore, a switching element to which a DC bias voltage has been supplied in the operating direction, and a level shift means which has been set so that an operation start level of the switching element is set to a zero level, in the direction for negating a DC bias voltage are provided. For instance, a Zener diode 33c is inserted in series into a base circuit of a transistor 34, and when the Zener diode 33c is operating effectively, if its breakdown voltage is equal to a DC voltage +V supplied to the transistor 34, the transistor 34 becomes on when the level of the AC waveform has become lower by V than its original operation start level +V, and outputs a signal (b). |
