| 摘要 |
PURPOSE:To improve the reliability with a simple device by detecting the consecutive operating time of a process load from the frequency of a commercial AC power supplied thereto. CONSTITUTION:The lighting of a halogen lamp LA1 being an original exposing lamp of a copying machine is detected by a current flowing to the lamp LA1 by means of a current transformer CT, and a pulse corresponding to the commercial frequency of the current, for example, 50Hz is obtained by a lighting detecting section. The pulse is counted by a frequency divider Q953 so as to set accurately the time, and when the halogen lamp LA1 keeps lighting over the set time, a transistor Q954 drives a relay K951 so as to extinguish the lamp LA1. The frequency divider Q953 is reset at the extinguishment so as not to produce a pulse at a coarse set time depending on a capacitor C951 and a resistor R954, and the protection circuit is formed with simple constitution without an accurate oscillator. |
