This section provides overview, applications, and principles of current probes. Also, please take a look at the list of 11 current probe manufacturers and their company rankings.
A current probe is an instrument that detects the flowing current by inserting a wire between the probes and converting it into a voltage signal.
Usually used in conjunction with an oscilloscope, the measured value is displayed on a graph. Different probes have different bandwidths, so check the bandwidth before use. They are convenient because measurements can be made by simply inserting them without the need to cut and connect the wires of the circuit every time, but many models are relatively expensive.
In addition, it is important to adjust the measurement conditions in order to reduce errors. Magnetization of the current probe can be a factor in error.
Current probes are used for current measurement in industrial and electronic equipment because they allow nondestructive inspection of circuits. Lineup of current probes are available for high current, low current, and high-speed current, depending on the magnitude and nature of the current. There are models that can measure both DC and AC power sources and models that can measure only AC power sources.
Specifically, they are used together with oscilloscopes for various measurements such as the current measurement of inverter equipment, load current of motors, observation of substations, measurement of switching power supplies, and evaluation of LED drive circuits, and they all have a wide range of applications.
The current probes utilizes the magnetic flux generated by the current flowing through the power cable, so measurements can be made without disconnecting the circuit.
First, current probes are inserted into the power cable. The probe captures the magnetic flux created by the current flowing in the cable and converts it into a voltage. The sensing part of the probe, which captures the magnetic flux, is a transformer. A coil pre-wound on the core of the transformer serves as the secondary winding and flows into the probe as a current. The current flow is converted into a voltage by the load resistor in the probe and displayed on the oscilloscope connected to the probe.
If the current is so small that the oscilloscope waveform is buried in noise, the amplitude can be increased in proportion to the number of windings by winding the cable between the current probes multiple times.
However, attaching the current probes to the circuit creates an impedance, albeit a small one. This is called insertion impedance, which has characteristics depending on the respective current probes and frequency, and is specified in the instruction manual, etc.
There are two items that should be adjusted prior to using current probes. These are DC offset cancellation and probe skew adjustments.
AC/DC current probes are very easy to use as they can measure from DC to 120MHz AC. However, since they use Hall elements for current detection and their output is amplified by a DC amplifier and connected to the input terminal of an oscilloscope, a DC offset is inevitable. Therefore, it is necessary to cancel it for accurate measurement.The procedure is as follows.
First, a degaussing process is performed to eliminate the residual magnetism of the core at the tip of the probe.
Then, using the offset voltage adjustment function, adjust the oscilloscope display so that it reads 0A.
After this adjustment, the current probes are attached to the circuit under test. However, if the measurement is continued for a long time, the DC offset will gradually fluctuate, and the 0A position will change, so the above procedure must be repeated from time to time.
When observing current and voltage waveforms simultaneously using current and voltage probes, such as power measurement in a power circuit, it is necessary to adjust the phase of the signal waveforms, or so-called skew adjustment, because the delay time of signals reaching the oscilloscope body is different for each probe.
Adjustment devices such as power measurement deskew fixtures are available, so please use these to adjust the phase between probes.
The following is a list of special precautions to be taken when measuring with current probes.
The AC current probes uses the transformer principle to detect the current flowing in the circuit under test, but the waveform is small at low-frequency currents. In particular, low-speed pulse signals cause waveform distortion due to sag. Therefore, when measuring low-frequency signals, including DC, AC/DC probes should be selected.
The magnitude of current that can be handled by current probes depends on its frequency, and the higher the frequency, the lower the current that can be measured. This is because the higher the frequency, the more heat is generated in the core and transformer. Therefore, it is necessary to select the model of the probe to be used based on the frequency of the current to be measured.
Attaching current probes to the circuit under test means inserting a small impedance into the circuit under test. The effect of this impedance on the circuit is so small that it can usually be ignored. However, if the line through which the current flows is wound around the core multiple times in order to measure a small current, the aforementioned impedance becomes twice as large as the number of times it is wound around and thus has a greater potential to affect the circuit under test.
*Including some distributors, etc.
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