The main function and working principle of the clamp meter
The most prominent feature of the clamp meter is the caliper that can be opened at the front. It can be easily clipped into the wire to measure the current in the loop, so that there is no need to destroy or modify the original circuit, and it can measure a large current. The multimeter also has a current measurement function, so what is the difference between it and a clamp meter? First of all, let's understand the principle and difference between the multimeter and the clamp meter.
How a multimeter measures current
When the multimeter measures current, it is necessary to disconnect the circuit under test and connect the multimeter in series to measure the current. It can be seen from the current detection circuit inside the multimeter that the current gear is actually a resistor with a very small resistance value inside the multimeter. When the current flows through this resistor, a voltage drop will be generated on it, because the resistance value is determined. , as long as the voltage on the resistor is measured, the current through the resistor can be calculated according to the formula, because the resistor is connected in series in the loop, so the current flowing through it is the current of the loop under test.
Therefore, the current measurement circuit in the multimeter includes many current measurement circuits in the instrument, which are measured by converting the current into voltage by means of resistance shunt. There are also requirements for the selection of the resistance value of this resistor. If the resistance value is too large, the voltage drop generated when the current passes through the resistor will be large. On the one hand, more voltage will be divided, which will affect the normal operation of the measurement load. On the one hand, the larger the resistance value is, the greater the power consumption will be generated on it at the same current, which will cause the resistance to heat up. Therefore, considering these two issues, the smaller the resistance value, the better.
But the resistance value should not be too small. If the resistance is too small, the voltage drop generated when the current flows will be smaller, so there are certain requirements for the subsequent measurement circuit, because the voltage that is too low needs to be amplified before it can be detected by the circuit.
Disadvantages of measuring current with a multimeter
It can be seen from the method and principle of the current detection method of the multimeter that the multimeter needs to be connected in series with the circuit under test when measuring the current, which is not suitable in some circuits that cannot be powered off and measured. Another point is the current measurement range of the multimeter, usually The maximum measurement range of the current of the multimeter is generally 10A or 20A, and in order to prevent the internal current-sense resistor from heating, the multimeter is not allowed to measure large currents for a long time. For the measurement of large currents, the general multimeter is not easy to achieve.
The principle of clamp meter measuring current
The working principle of the clamp meter to measure the current is basically the same as the principle of the multi-pen to measure the current. The difference is that the clamp meter does not directly detect the voltage on the shunt resistance, but uses a current transformer. The transformer is actually an application of the transformer, which can transform the current according to a certain proportion. After the current transformer is connected to the load, its primary is equivalent to one turn, and the secondary is the number of turns inside the clamp meter, which reduces the current in a certain proportion, so the current transformer is equivalent to A step-up transformer, the circuit inside the clamp meter can calculate the measured current by detecting the voltage on the secondary side of the transformer.
Therefore, compared with the multimeter, the clamp meter does not need to change the circuit when measuring the current, and can measure the larger current, such as the current of the inductive load such as the motor. However, since the clamp meter uses a current transformer inside, according to the working principle of the transformer, it cannot pass direct current. So is the clamp meter really unable to measure DC current? In fact, the clamp meter can measure DC current, but it does not use a current transformer.
The principle of measuring DC current with clamp meter
Since DC cannot produce changes in magnetic flux, the clamp meter cannot measure DC current if it uses a current transformer. The AC current is measured by a transformer, which is called an electromagnetic transformer, and the clamp meter that measures the DC current uses another sensor-Hall sensor.
The principle of using a Hall sensor to measure DC current is that when current flows through a wire, a magnetic field (similar to an electromagnet) is generated, and this magnetic field is proportional to the magnitude of the current. After the caliper of the clamp meter collects the magnetic field generated by the wire, it is detected by the Hall element located in the caliper. The Hall element is a magnetic sensitive element, which converts the magnetic field into a voltage signal output, which is amplified by the circuit. Processing, you can display the current of the load. Many of the current clamp meters are AC and DC dual-purpose, including electromagnetic transformers and Hall sensors to detect AC current and DC current respectively.
The difference between a clamp meter and a multimeter
As mentioned above, the main function of the clamp meter is to detect the current. Compared with the multimeter, the clamp meter is more convenient to use to detect the current, and the measurement range is much larger than that of the multimeter, but there is one point , the clamp meter cannot display normally when measuring small currents (such as small currents of several hundred milliamps), and the measurement accuracy is not as good as the multimeter.
The second difference is that because the main function of a clamp meter is to detect current, it is not as good as a multimeter in other functions. Although many clamp meters now integrate many functions of multimeters, such as voltage measurement, resistance measurement, frequency measurement, temperature measurement, etc., but in general, these functions other than current measurement are not comparable to multimeters. And the accuracy of these measurement gears is generally worse than that of the multimeter.
