What are Current Probes?

A current probe or a current clamp is an current transformer used to measure the current flowing through a wire or a conductor without having to connect the conductor to a device. In EMC testing, the current probe senses the current flowing through the conductor or cable of interest and converts it into a proportional voltage that can represent the amplitude of the current. This measurement can be viewed with the help of instruments like a spectrum analyzer or EMI test receiver. In EMC emissions testing, current probes are used to measure the conducted noise current (RF current) flowing through a conductor for compliance to standards such as MIL-STD-461, DO-160, CISPR, etc.

Types of current probes:

There are two types of current probes, both use different sensor techniques to measure the unknown currents that flow through a conductor. The techniques are the Hall Effect sensor technique and the current transformer technique.

Type 1: Hall Effect sensor current probe:
 

Figure 1: Hall Effect Current probe

The Hall Effect sensor method is shown in figure 1. The Hall-Effect current sensor consists of a magnetic core that surrounds the conductor, with a Hall sensor is mounted in the air gap of the magnetic core. The unknown AC flowing through the conductor magnetizes the magnetic core. The Hall sensor senses the magnetic field generated by the target current (I) and converts it into voltage. Usually, the Hall-Effect current sensor needs an amplifier to get the required output (because of the low amplitude output of the Hall sensor). The output connector of the current probe is connected to the spectrum analyzer or EMI test receiver to display the output voltage. The Hall Effect current probe is ideal for measuring DC and low-frequency AC.

Type 2: Current transformer technique current probe:

Figure 2: Current transformer technique current probe

The current transformer type of the current probe is shown in Figure 2. This current probe is an RF current transformer and can measure AC current only. In this probe, the conductor acts as a primary winding and the clamp surrounding the conductor acts as a magnetic core, and the coil wound on the magnetic core acts as a secondary winding. A shunt resistor (typical value 50 ohm) is connected across the terminals of the secondary winding.

In EMC testing, when the current probe is clamped over the conductor in which the RF current needs to be measured, the unknown RF current flowing through the conductor magnetizes the magnetic core. This magnetic field induces an emf across the secondary winding. Since the secondary winding is shunted with a resistor, it results in the current flowing through the secondary coil. This secondary current induces a new magnetic field, which opposes the main magnetic field. The current flowing through the secondary coil creates a voltage drop (ES) across the shunt resistor. This voltage drop magnitude is proportional to the RF current that is to be measured. Now, connect the output connector of the current probe to the spectrum analyzer or EMI test receiver to measure the output voltage (ES).

The RF current (IP) in microamps flowing through the conductor under test is evaluated from the reading of the current probe output in microvolts (ES) divided by the current probe transfer impedance (ZT).

IP = ES/Z

Or, in dB IP(dBμμA) = ES(dBμμV) – ZT(dB)

The datasheet of a current probe shows the graph that contains the typical values of transfer impedance (ZT) of the current probe throughout the frequency range.

There are also some current probes that utilize both Hall Effect and current transformer techniques to measure the AC and DC. These are called Hybrid Current Probes.

Key Specifications of Current Probes:

• Frequency range: Represents the useable frequency range of the current probe. Usually, the range frequency may vary from kHz to MHz range.
• Maximum primary current (DC-400Hz): Represents the maximum primary current that can be accepted by the current probe. The frequency range is usually DC-400 Hz.
• Maximum primary current (RF): Represents the maximum RF primary current that can be accepted by the current probe at the radio frequency range. 
• Transfer impedance dBΩ: Represents the Transfer impedance of current probe in dBΩ
• Transfer impedance ZTΩ (nominal): Represents the Transfer impedance of current probe in Ω
• Maximum peak pulse current: Represents the maximum peak pulse current. This rating should not exceed, if exceeds means the core will saturate. It is represented in A.
• Maximum current (continuous) RMS: Represents the maximum continuous current that can be accepted by the current probe. It is represented in A.
• Connector type: Represents the connector type used in the current probe. Usually, it is in N-type.
• Load impedance: Represents the load impedance connected across the output. It is represented in ohm.
• Diameter of measurable conductors: Represents the diameter limit of a measurable conductor in mm.
• Aperture: Represents the aperture size of the RF current probe in mm.
• Internal diameter: Represents the inner diameter of the RF current probe in mm.
• External diameter: Represents the outer diameter of the RF current probe in mm.
• Maximum core temperature: Represents the core temperature limit of the current probe for safe operation. It is represented in degrees Celsius.
• Operating temperature range: Represents the operating temperature of the current probe for safe operation. It is represented in degrees Celsius.
• Storage temperature range: Represents the storage temperature of the current probe. It is represented in degrees Celsius.

Current Probes are used in a wide range of applications including electromagnetic compatibility (EMC) testing and to troubleshoot circuits. In the realm of electromagnetic interference (EMI) and electromagnetic compatibility (EMC) testing, RF current probes are essential tools for detecting and measuring EMI to ensure devices meet regulatory standards.