FEEDING CHANGEOVER RELAY CIRCUIT

PURPOSE:To improve the reliability by providing a switch means passing an input current of a feeder line as a relay sensing current, a relay switched by a relay sensing current and a current detection control means controlling them to the relay circuit. CONSTITUTION:An input current IIN is supplied to a Zener diode 11 and a current sensing resistor 12 whose resistance is very small. When a voltage across the resistor 12 reaches a prescribed voltage, the current IIN is branched to both paths of the resistor 12 and a relay coil. Then the current IIN equivalent to a sufficient sensing current to the coil 17 flows to the resistor 12. The voltage across the resistor 12 is amplified by an amplifier 13 and when the amplified voltage exceeds a threshold voltage of a comparator 14 of a next stage, a transistor(TR) 16 is turned on. In this case, the sufficient sensing current flows to the coil 17 and a relay switch 18 is surely activated, Thus, the sensing characteristic of the relay is not required of the selection and an inexpensive relay is employed.