Clamp Meters Working Principle

 Current transformer clamp meters are equipped with rigid jaws made of ferrite iron. Coils of copper wire individually wrap the jaws. Together, they form a magnetic core during measurements.

Their essential operation is like that of a transformer. It works with one primary turn or winding, which is the conductor being measured in nearly all cases. The coils around the jaws assist as a secondary winding of the current transformer.

Current flowing through the conductor generates an alternating magnetic field that rotates around it. This field is concentrated by the clamp's iron core, inducing a current flow in the meter's secondary windings. The amount of magnetic field passing through the conductor (or any surface) is called magnetic flux, denoted by the Greek letter phi, Φm.

The signal is proportional to the ratio of the turns. A much smaller current is delivered to the meter's input due to the ratio of the number of secondary windings (those wrapped around the jaws of the clamp) vs. the number of primary windings wrapped around the core.

If, for example, the secondary has 1000 windings, then the secondary current is 1/1000 of the current flowing in the primary. Thus 1 amp of current in the conductor being measured would produce 0.001 amps, or one milliamp, of current at the meter's input. With this technique, much larger currents can be easily measured by increasing the secondary turns.

Internally, the conductor's current flow can be measured either as a current—some older clamp meter accessories plug into the current jacks of a digital multimeter or converted to a voltage. Most clamp meters now have mV output.

Current transformer clamp meters only respond to ac waveforms.

Clamp Meters using Hall Effect for AC & DC Measurements

Hall Effect clamp meter can measure both ac and dc current up to the kilohertz (1000 Hz) range.

Hall Effect clamp meters use rigid iron jaws to concentrate the magnetic field that encircles the conductor being measured like current transformer types.

Unlike current transformer clamp meter, the jaws are not wrapped by copper wires. Instead, the magnetic field made by the conductor is focused across one or more gaps in the core after the jaws are clamped around the conductor.

Notice the point where the jaw tips of a Hall Effect clamp meter meet.

A gap exists where a Hall Effect clamp meter meets the jaw tips, creating an air pocket that the magnetic field (aka magnetic flux) must jump. This gap limits the magnetic flux so that the core cannot immerse.

In contrast, the jaws of an ac-only current transformer clamp are flush when closed. When opened, the tips of the jaws show bare metal core faces.

In that gap, covered by thin plastic molding, is a semiconductor known as a Hall Effect sensor. This transducer varies its output voltage when responding to magnetic fields; in this case, the conductor's magnetic field or a wire being measured. Its purpose is to measure magnetic flux directly. The sensor's output voltage is then amplified and scaled to represent the current flowing through the conductor that lies inside the clamp's jaws.

As current flows through a conductor being measured, the iron core made by the jaws of a Hall Effect clamp meter allows the magnetic field to pass through—more easily, in fact, than air efficiently.

When the magnetic field (flux) comes to that small air gap in the jaw tips, the field has to jump that gap. Because the gap is small, the field remains concentrated across the gap, and the Hall Effect sensor—which sits in the gap—produces a voltage proportional to the magnetic flux in the gap that the clamp translates into a current reading.

In Hall Effect devices, dc magnetic fields are also concentrated through the core, like a permanent magnet sticking to the iron. Because of the earth's dc magnetic field and the possibility of other magnetic fields near the measurement site, these clamps require the reading to be "zeroed" before measuring to eliminate offsets.

American physicist Edwin Hall (1855-1938) is credited for identifying the Hall Effect in 1879.