3 Ways to Measure Electrical Current with Sensors

Measuring current using current probes is a great way to measure the flow of electrical charge without interrupting the circuit. The probe usually contains a direct/alternating current sensor—typically a Hall-effect sensor—and measures the current flowing through the conductor by passing the conductor through the ring structure.

The ring structure contains a hall-effect sensor that detects the strength of the magnetic field, which is proportional to the current flowing through the conductor. It’s an effective way of measuring both DC and AC currents. However, certain precautions are associated with connecting a current probe.

For example, only one wire should go through the current probe. Passing two or more wires could cause the magnetic fields to interfere with one another and offset the measurement by quite a substantial amount. Also, the direction of the probe should match the direction of the current (in the case of DC measurements); otherwise, the sign of the current value will be negative.

We have already established that measuring current in a circuit implies interrupting the circuit, which also means interrupting circuit operation. However, voltage is far easier to measure than current because the measuring instruments are connected in parallel, which doesn’t imply breaking up the circuit.

Ohm’s law states that the electric current through a conductor between two points is directly proportional to the voltage across said points, provided that all other physical conditions remain constant—including electrical resistance.

By introducing a known, low-value resistance into the circuit (in series) and measuring the voltage drop across it, the current can be easily calculated indirectly using Ohm’s law, which states that the current equals voltage divided by resistance.

This measurement approach is commonly used with a shunt resistor—a very precise low-value resistor—in high-current applications. The low resistance ensures that the voltage drop across the resistor is minimal, thus severely limiting the resistor’s impact on the circuit’s function.

By measuring the small voltage drop across the shunt resistor and knowing the shunt’s exact resistance value, technicians can accurately measure the value of the current flowing through the circuit.

However, these measurements have some limitations. Using a shunt resistor to measure current requires that you implement an appropriate resistor. Inadequate resistors will certainly produce a measurement error.

To do this, consider the minimum voltage at which the load can work and divide that value by the load’s current consumption. The resulting value, and/or lower, is your shunt resistance. After implementing the resistor into the circuit, you can then derive the current value based on the measured voltage.

KEYENCE’s NR-X Series multi-input DAQ can measure current directly because of a built-in circuit, taking the worry out of choosing the correct resistance value., Otherwise, a resistor in combination with NR-HV04, NR-FV04, and NR-TH08 measurement units, can derive a current value by measuring the voltage drop across a known resistance alue. This is useful when measuring other signal types with the same input module.

Current transformers are exceptionally useful in measuring current in high-current AC applications. Transformers are rather simple machines with exceptionally high efficiency of up to 99%. The conductor of interest passes through the CT and acts as the primary winding that generates a magnetic field in the CT’s core.

This magnetic field then induces a current in the secondary winding, which usually has multiple wire turns around the CT’s core. The induced current is proportionally lower than the primary current, based on the turns ratio of CT—the number of primary turns against the number of secondary turns of the wire.

The circuit has to be closed for the current to flow, so a burden resistor is added to the circuit to act as a load. By measuring the voltage across the voltage across the resistor and using Ohm’s law, we can derive the current measurement within the circuit of the secondary winding and multiply it with the turns ratio to get the current value in the primary winding.