The principles and differences between multimeters and clamp meters for detecting current
The main functions and working principles of clamp meters
The most prominent feature of the clamp meter is the caliper that can be opened in the front. It can be easily inserted into the wire and measure the current in the circuit. This way, there is no need to destroy or modify the original circuit, and it can measure large currents. A multimeter also has a current measurement function, so what are the differences between it and a clamp meter for measuring current? First, let's understand the principles and differences between multimeter current detection and clamp meter detection current respectively.
How does a multimeter measure current?
When measuring current with a multimeter, you need to disconnect the circuit being measured and connect the multimeter in series to measure the current. It can be seen from the internal current detection circuit of the multimeter that the current range is actually a resistor with a very small resistance value inside the multimeter. When the current flows through this resistor, a voltage drop will occur 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 this resistor is connected in series in the loop, the current flowing through it is the current of the loop being measured.
Therefore, the current measurement circuit in the multimeter, including the current measurement circuit in many instruments, is measured by converting the current into voltage through a resistor 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 distributed, which will affect the normal operation of the measurement load. On the other hand, On the other hand, the larger the resistance value, the greater the power consumption generated on it at the same current, which will cause the resistor to heat up. Therefore, considering these two issues, the smaller the resistance value, the better.
However, the resistance value cannot be too small. If the resistance is too small, the voltage drop generated when the current flows will be smaller, which places certain requirements on the subsequent measurement circuit, because too low voltage 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 multimeter to detect current that when measuring the current, the multimeter needs to be connected in series in the circuit being measured. This is not suitable in some circuits that cannot be measured without power off. Another point is the current measurement range of the multimeter. Usually The maximum current measurement range of a multimeter is generally 10A or 20A. In order to prevent the internal current-sensing resistor from heating up, the multimeter is not allowed to measure large currents for a long time. It is not easy for ordinary multimeters to measure large currents.
How clamp meters measure current
The working principle of a clamp meter to measure current is actually basically the same as that of a multi-purpose pen to measure current. The difference is that the clamp meter does not directly detect the voltage on the shunt resistor, but uses a current transformer. Transformer is actually an application of 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, which is the number of turns inside the clamp meter, is larger. This makes the current smaller according to 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 a multimeter, a clamp meter does not need to modify the circuit when measuring current, and can measure larger currents, such as the current of inductive loads such as motors. However, since the clamp meter uses a current transformer inside, it cannot pass direct current according to the working principle of the transformer. So can a clamp meter really not measure DC current? In fact, a clamp meter can measure DC current, but it does not use a current transformer.
