| 摘要 |
PROBLEM TO BE SOLVED: To provide power amplifiers for amplifying an output from a photoelectric voltage transformer or the like with a high gain, and for outputting it to a load such as a protecting relay on double buses, and to prevent excess currents due to deflection between those power amplifiers or the malfunction of the load when they are connected with the load in switching the buses. SOLUTION: A difference between an input voltage VI and a step-down value VO of an output voltage is amplified by an error amplifier 45, and negative feedback control is operated in this power amplifier 31(31a). In this case, the output voltage of a DC power source 34 realized by a DC/DC converter or the like is detected by a voltage detecting circuit 53, and when this is higher than a rated value, that is, the inflow of currents is detected, a switch 54 provided at the output side of the error amplifier 45 is electrically conducted to a prescribed fixed potential side, and error amplification is stopped. Therefore, the transfer of a power generated due to deflection between those power amplifiers can be reduced in a prescribed level, and excess currents or the malfunction of a load 33 can be prevented.
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