Functional principle of the clamp meter and the difference between multimeters
When a multimeter measures current, it is necessary to disconnect the circuit under test and connect the multimeter in series to measure current. Through the circuit of current detection inside the multimeter, it can be seen that the current level is actually a very small resistor inside the multimeter, and when the current flows through this resistor, a voltage drop will be generated on it, because the resistance value is determined, and as long as the voltage on the resistor is measured, the current through the resistor can be calculated according to the formula, because this resistor is connected in series in the circuit, so the current that flows through it is the current of the circuit being measured.
So the current measurement circuit in the multimeter, including the current measurement circuits in many meters, is to convert the current into voltage by means of a resistor shunt to measure the current. The selection of this resistor resistance value is also required, if the resistance value is too large, the current through the resistor when the voltage drop is large, so on the one hand, it will be divided into more voltage, affecting the normal operation of the measurement load, on the other hand, the larger the resistor value, the same current, the same current, the power consumption generated on it is also the larger, which will make the resistor heat, so from the consideration of the two problems, the smaller the resistor is the better.
However, the resistance value should not be too small, the resistance is too small the voltage drop generated when the current flows through it will be smaller, which will have certain requirements for the measurement circuit behind it, because too low a voltage needs to be amplified before it can be detected by the circuit.
Disadvantages of measuring current with a multimeter
From the multimeter current detection methods and principles can be seen, the measurement of current needs to be connected in series with the multimeter in the circuit to be measured, so that in some of the circuit that can not be measured without power is not suitable, another point is the current measurement range of the multimeter, usually the maximum measurement range of the multimeter current is generally 10A or 20A, and in order to prevent the internal current detection resistance heating, the multimeter is not allowed to long time. In order to prevent the internal current detecting resistor from heating up, the multimeter is not allowed to measure large current for a long time, and it is not easy to realise the measurement of large current with the general multimeter.
Principle of Clamp Meter Current Measurement
Clamp meter current measurement principle and the principle of universal pen current measurement is actually basically the same, the difference is that the clamp meter is not directly detect the voltage on the shunt resistance, but the use of current transformers. The transformer is actually an application of the transformer, which transforms the current in a certain proportion. After the current transformer into the load, its primary is equivalent to a turn, the secondary is also the number of turns inside the clamp meter is more, so the current in accordance with a certain proportion of smaller, so the current transformer is also equivalent to a step-up transformer, the clamp meter internal circuit by detecting the voltage on the secondary side of the transformer, you can calculate the measured current.
So the clamp meter compared to the multimeter, in the measurement of current without changing the line, and can measure the larger current, such as motor and other inductive loads of current. However, since a clamp meter uses a current transformer inside, it cannot pass DC current according to the working principle of a transformer. So the clamp meter really can not measure DC current? In fact, the clamp meter can measure DC current, but it is not the use of current transformers.
Clamp meter measuring DC current principle
As DC cannot produce a change in magnetic flux, the clamp meter cannot measure DC current if it uses a current transformer. A transformer is used to measure AC current, which is called an electromagnetic transformer, while a clamp meter that measures DC current uses a different kind of sensor - a Hall sensor.
The principle of measuring DC current using Hall sensors is that a magnetic field (similar to an electromagnet) is generated when current flows through a wire, and this field is proportional to the size of the current. Clamp meter caliper will lead to the magnetic field produced by the convergence of the caliper is located in the Hall element detection, Hall element is a magnetically sensitive components, it will be converted into a magnetic field voltage signal output, this voltage signal after the circuit amplification process, you can display the load current. Nowadays, many of the clamp meter is AC and DC dual-use, internal electromagnetic transformers and Hall sensors are included to detect AC current and DC current.
