Automatic range selector for volt-ammeter instrument

Abstract

An automatic ranging clamp-on ammeter includes an openable normally closed magnetic core for encircling an A.C. current carrying cable has a measuring winding and a sensing winding. A current meter, a diode rectifier and a pair of variable first and second resistors are connected in series across the measuring winding. A relay solenoid is shunted by a Zener diode and is connected in series with a variable resistor across the sensing winding. A sensitivity reducing shunt variable resistor is connected through the relay normally open contacts betwen the junction of the first and second resistors and the meter opposite terminal. An audible signal generator may be connected across the shunt resistor. When the measured current exceeds a predetermined value the relay closes and inserts the shunt resistor across the meter to reduce the measuring sensitivity and increase its range and energizes the audible signal generator. Alternatively the current from the sensing winding is rectified and fed through a first resistor to a meter, the series connected first resistor and meter being shunted by a series connected unidirectional switch and second resistor whose gate is connected to the rectified current by way of a capacitor and resistor so that the switch is closed when the rectified voltage exceeds a set value to increase the meter range.