| 摘要 |
PURPOSE:To prevent wrong operation, by coupling a level detection circuit to the contact of switch element, which generates the minimum of reference currents, when changing the matching value of a matching circuit. CONSTITUTION:The input of the protective relay is rectified by a rectifier circuit REC so that a negative signal current I1 is applied to an inverting terminal of an operational amplifier OA. A positive reference current I2 is applied from fixed contacts a, b, c of the switch SW to the inverting terminal of the operational amplifier OA. Both the currents I1, I2 are compared with each other to produce a trip signal. A diode D is connected between a moving contact d of the switch SW and the fixed contact a which supplies the reference current corresponding to the minimum matching value. Thus, the reference current I2 is applied to the operational amplifier OA even if the moving contact d remains disconnected from the fixed contacts a-c at the time of shifting of the switch SW. This eliminates the wrong operation due to the alteration of the matching value. |
