1. Field of the Invention
The present invention relates to a current measuring method and apparatus therefor and more particularly to a technique of measuring a DC current or AC current flowing in a conductor in a non-contact manner by use of a magnetic sensor.
2. Description of the Prior Art
Methods heretofore known conventionally for measuring current flow include one in which a part of a current path is separated and an ammeter is connected across the separated current path so as to measure the value of the current.
There is another method in which noting the fact that the flow of current in a conductor produces magnetic flux around the outer periphery of the conductor and that this magnetic flux is proportional to the current flowing in the conductor, the value of the magnetic flux is measured to indirectly measure the value of the current flowing in the conductor.
FIG. 1 is a schematic diagram of one of the prior art current measuring apparatus according to the latter method. In the Figure, numeral 11 designates a magnetic yoke, 1 a conductor in which a current to be measured flows, 12 a magnetic sensor for measuring the magnetic flux in the magnetic yoke 11, 13 an amplifier, 14 a measuring device for receiving the output of the amplifier 13 to indicate a detected value, and 15 DC power sources for driving the amplifier 13.
In the current measuring apparatus constructed as described above, when a current I.sub.m flows in the conductor 1, magnetic flux is produced around the outer periphery of the conductor 1 so that the magnetic flux is converged by the magnetic yoke and detected by the magnetic sensor 12 disposed in the gap of the magnetic yoke thereby converting it to an electric signal. The signal is amplified by the amplifier 13 whose output in turn operates the measuring device 14 to indicate the value of the current flowing in the conductor 1.
However, the conventional current measuring apparatus of the above type involves the following various problems and it is desired to enhance and improve the various efficiencies with a view to overcoming these problems:
(1) Due to the low conversion efficiency of the magnetic sensor for converting to an electric signal the magnetic flux produced in proportion to the value of a current flowing in the conductor, it is impossible to measure small currents and the minimum value of measurable currents by the commercially available products is not less than 100 mA. PA1 (2) Where the current to be measured is an AC current, particularly a high-frequency current, a measuring error is caused due for example to the reduced effective transmissibility and the eddy current loss caused in the magnetic yoke. PA1 (3) While a Hall element is generally used as the magnetic sensor, the Hall element is a semiconductor and therefore it is disadvantageous in that the temperature characteristics are not good, that a complicated circuit is required for ensuring a constant current source drive and so on.