Current Probe Introduction_Century Wisdom (Suzhou) Testing Technology Co., Ltd

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Current Probe Introduction

Release time:

2023-05-04 15:19

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　　1. Preface

This article mainly introduces the purpose, structure, characteristics, calibration method and test application of current probe. Let everyone have a more comprehensive understanding of the current probe related technical knowledge, can be used in EMC testing and calibration laboratory.

　　2. Current Probe Overview

　　2.1 Uses

The current probe is usually used as a sensing component for disturbance measurement, which converts the disturbance current into a measurement receiver.

to detect the voltage. The asymmetrical disturbance current on the cable can be measured with a specially made card-type current converter, and there is no need to make direct conductive contact with the disturbance source wire or change its circuit. The practicality of this method is self-evident; for complex wire systems, electronic circuits, etc., the measurement can be carried out without affecting normal operation or normal configuration. The current probe is constructed so that it can easily catch the wire under test as a primary coil of one turn, and the secondary coil is included in the current probe.

Although the main measurement frequency range of the current probe is 30Hz to 100MHz, the current probe can be manufactured for measurement in the frequency range of 30Hz to 1000MHz. When measuring the standing wave current of the regular power system above 100MHz, the current probe should be placed at the position of the maximum current.

The current probe is designed to have a flat frequency response in the pass band. For frequency ranges below this flat passband, the current probe can still make accurate measurements, but its sensitivity is reduced due to the reduction of the transfer impedance. For frequency ranges above the flat passband, the measurement will no longer be accurate due to the resonance generated by the current probe.

When a shielding structure is attached, the current probe can measure asymmetric (common mode) currents or symmetric (differential mode) currents.

Fig. 1 Real diagram of current probe (F-57)

　　2.2 structure

The current probe is usually in the shape of a ring, and the wire to be measured is placed in the center of the ring. According to the existing requirements and manufacturer's specifications, the inner diameter of the ring of the current probe is 2mm ~ 30cm. The secondary coil is placed inside the ring body to perform the function of a clip-type current clamp. The toroidal core and coil are shielded to prevent electrostatic coupling. The purpose of the gap on the shield shell is to avoid the formation of a short circuit turn on the transducer.

A typical current probe for disturbance measurement has 7 to 8 secondary turns. This is an optimum turns ratio, enabling the widest flat frequency range and insertion impedance of 1Ω or less. In the frequency range below 100kHz, a silicon steel sheet core is used. In the frequency range of 100kHz to 400MHz, a ferrite core is used; in the frequency range of 200 MHz to 1000 MHz, a hollow core is used and a balanced-unbalanced 50Ω output converter is provided. 2 shows the structure of a typical current probe.

Figure 2 Typical current probe structure

　　2.3 characteristics

Insertion impedance: ≤ 1Ω.

Transfer impedance: When the current probe is terminated with a 50Ω load, it is 0.1Ω ~ 5Ω in the flat linear range; when it is lower than the flat linear range, it is 0.001 Ω ~ 0.1 Ω.

　　Note: The reciprocal of the transfer impedance, I .e., the transfer admittance [dB(S)], can also be used. When expressed in decibels, the reading of the measuring receiver is added to the admittance. To calibrate the transfer impedance or transfer admittance, a dedicated current probe fixture may be used.

Additional parallel capacitance: The capacitance between the current probe housing and the wire under test should be less than 25pF.

Frequency Response: Calibrate the transfer impedance of the probe within the specified frequency range. The typical frequency range of a single probe is: 100kHz ~ 100MHz;100MHz ~ 300MHz;200MHz ~ 1000MHz.

Impulse response: TBD.

Magnetic saturation: The maximum DC or AC supply current in the primary conductor shall be specified with an error not exceeding 1dB.

Transfer impedance tolerance: to be determined.

Influence of external magnetic field: When the current carrying wire is moved from the inside of the probe aperture to the vicinity of the outside of the probe, the reading of the indicator should be reduced by at least 40dB.

Influence of electric field: It should not be sensitive to electric field below 10 V/m.

Influence of position: when using the probe, the wire of any size shall be placed at any position within the caliber, and the change of the measured value shall be less than 1dB when it is not more than 30 MHz; when it is in the frequency range of 30 MHz ~ 1000MHz, the change of the measured value shall be less than 2.5dB.

Aperture of current probe: at least 15mm.

　　3. Current probe calibration method

　　3.1 metrological characteristics

3.1.1 Insertion loss

Measurement frequency range: 5Hz ~ 1000MHz.

3.1.2 Transfer impedance

Measurement frequency range: 5Hz ~ 1000MHz.

　　3.2 calibration conditions

3.2.1 Environmental conditions

a) Ambient temperature:(20±10)℃;

B) Relative humidity: not more than 80%;

c) Atmospheric pressure: 86kPa ~ 106kPa;

d) Power supply: voltage (220±22)V, frequency (50±1)Hz;

e) No corrosive, inflammable and explosive gas around;

f) There shall be no mechanical vibration or electromagnetic interference affecting the normal operation of the calibration system.

3.2.2 Measurement standard

Use the etalon for the network analyzer (see Figure 3) and the current probe fixture ((see Figure 4):

Figure 3 Network Analyzer

Fig. 4 Current Probe Fixture

　　3.3 calibration items and methods

3.3.1 Insertion loss and transfer impedance

3.3.2 Calibration method

Fig. 5 Connection diagram of current probe transfer impedance calibration

The measurement mode of the network analyzer is set to S21, the starting frequency and ending frequency of the network analyzer are set according to the bandwidth of the current probe, the resolution bandwidth is generally 1KHz, and the frequency sweep mode is changed to logarithmic frequency sweep. The insertion loss (voltage transmission coefficient) A of the current clamp can be calculated according to the following formula:

　　A= S21' S21

S21': The indicated value (dB) of the network analyzer is approximately zero when the current probe is not added and only the test fixture (a 10dB isolation attenuator is added at both ends of the fixture) and the coaxial cable are normalized;

S21: After the current probe is clamped into the test fixture, the receiving end of the network meter is connected to the current probe, the output end is connected to one end of the fixture, and the other end of the fixture is added with a 50Ω terminal. The indicated value (dB) of the network analyzer.

The transfer impedance Z of the current probe can be calculated by the following formula:

Z = A 34

The coefficient 34 is relative to the load impedance of 50 ohms. Therefore, it can be seen that after measuring the voltage transmission coefficient of the current probe, the transfer impedance of the current probe is calculated, and the transfer admittance can also be obtained:

　　S=A-34

　　4. Test Application

The current probe is the sensing component of disturbance measurement. It converts disturbance current into voltage V that can be detected by the measuring receiver. The test EUT current I = receiver voltage V current probe transfers admittance S. Mainly used in EMC laboratory conducted emission project testing. The following figure shows the application of current probe in auto parts conduction emission-current probe method project.

Fig. 6 Layout of Conducted Emission-Current Probe Method

This article introduces that we should have a general understanding of the relevant knowledge of the current probe, but there is also an injection current probe, which is different from the current probe. We should pay attention to the distinction. The injection current probe is mainly used in anti-interference testing. The next article will focus on explaining it for you.

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