JPS6475970A - Detecting device of current - Google Patents

Abstract

PURPOSE:To prevent the occurrence of false detection due to a lightning surge current, by providing a discriminating means to discriminate the amplitude of a voltage which is reverse in polarity to a voltage in the direction of charg ing a capacitor and is generated at both ends of a first resistor, so as to detect a primary current of a current transformer. CONSTITUTION:When a negative lightning surge current flows through a primary winding n1 of a current transformer 6, as a primary current I1 to be detected, only a voltage V1 of 0.7V clamped by a diode D1 is generated at both ends of a resistor R1, and therefore the breakdown of a circuit due to the lightning surge current can be prevented. Although a charge current flows to a capacitor C1 through a resistor R2, on the other side, it is little charged due to a high resistance value of the resistor R2, and a voltage at the opposite ends thereof is very low. Accordingly, the capacitor C1 does not give a discharge current strong enough to offset a counterelectromotive voltage generated in a secondary winding n2 of the current transformer 6, and consequently the counterelectromotive voltage is generated. However, a discriminator 9 is a device to detect a current of negative polarity, and therefore this counterelectromo tive voltage causes no problem does not need to be suppressed particularly.