How does a multimeter measure AC and DC current?

Multimeters are commonly known as all-purpose meters or multimeters. They are generally used to measure electrical parameters such as current, voltage and resistance of the object being measured. Some mid-to-high-end brands of multimeters also have the functions of measuring capacitance, frequency, transistors, duty cycle, temperature, non-contact measurement (NCV), etc. At present, most multimeters on the market are digital multimeters. Digital multimeters have the characteristics of high sensitivity, high accuracy, clear display, strong overload capacity, and easy to carry. As a simple and practical measuring instrument, multimeters are widely used in various fields of power electronics.

The essence of the pointer multimeter is a magnetoelectric DC ammeter, which has the disadvantages of low input impedance and inconvenient reading. The digital multimeter has the advantages of high input impedance, convenient reading, multiple functions and high intelligence. Its core is a DC voltage millivolt meter. When the measured voltage enters the multimeter through the meter pen, the voltage is attenuated by the voltage divider resistor in the meter, and then calculated by the calculation unit, and the specific number of the measured voltage is displayed on the LCD. The measured current and voltage are converted into corresponding voltages through current/voltage converters and resistance/voltage converters, and then the measured current and resistance values ​​are obtained. In daily work, it is very simple and convenient to measure voltage and resistance with a multimeter, but most friends may not know how to use a multimeter to directly measure AC and DC currents. Here we will give a brief introduction on how to correctly use a multimeter to measure AC and DC:

1. Demonstration equipment:

1. Equipment under test: electric fan (measure AC current), circuit board (measure DC current)

2. Measuring instrument: DELIXI Electric DE78AS digital multimeter

3. Temporary power supply: any socket, AA battery box

As the tested equipment, we chose a common electric fan and a DC circuit board in our daily life. As the multimeter, we chose DELIXI Electric's DE78AS digital multimeter, which has a large wide-angle screen, a fully transparent high-definition display, and true effective value measurement. In order to facilitate the measurement, we chose the alligator clips as accessories of the multimeter to assist the operation.

2. AC current measurement:

1. Set the multimeter to the AC current 20A range (to ensure the safety of personnel and measuring instruments, for manual range measuring instruments, the maximum range must be used for the first measurement, and the appropriate measurement range can be selected after the measurement results are obtained), turn on the multimeter backlight, insert the red test lead into the 20A jack, and the black test lead into the COM port

2. Connect one end of the alligator clip to the fan plug.

1. Connect the red probe of the multimeter to the other plug of the fan plug.

2. First, turn off the power switch of the power strip, plug the other end of the alligator clip into the socket, and insert the black test lead into the other socket of the socket.

3. Then press the power switch of the socket to connect the entire circuit, and the electric fan starts to work. At this time, the display value of the multimeter is 0.09A, which proves that the current of the electric fan is roughly around 90mA when it is working.

Note: All operations must pay attention to electrical safety, strictly follow the electrician's wiring requirements, and make sure the circuit is correctly connected before powering on the test

6. In order to measure the working current of the electric fan more accurately, we disconnect the circuit, insert the red probe of the multimeter into the uAmA jack, and turn the multimeter handle to 600mA.

7. Connect the red test lead to the fan plug. After the circuit is connected, the multimeter displays a value of 96.7mA, which means the normal working current of the fan is around 96.7mA.

3. DC current measurement (wiring guidelines follow the above safety requirements)

1. First, turn the multimeter knob to 600mA DC (since the device under test we chose is a digital display circuit board, I can be sure that its operating current will not exceed 600mA, but if the current of the device under test cannot be determined, it is recommended to start measuring from the maximum range), insert the red test lead into the uAmA jack of the multimeter, and insert the black test lead into the COM port of the multimeter.

2. Since DC current has positive and negative poles, we must choose the correct polarity when measuring DC current. The red probe of the multimeter is connected to the negative pole of the battery inside the multimeter, and the black probe is connected to the positive pole inside the multimeter. Therefore, when we connect an external DC circuit, the red probe needs to be connected to the positive pole of the measurement circuit, and the black probe to the negative pole of the measurement circuit. The correct connection method for the following circuit is to connect the positive pole of the battery to the red probe of the multimeter, the negative pole of the battery to the positive pole of the circuit board being measured, and the negative pole of the circuit board to the black probe of the multimeter. After the circuit is correctly connected, the working current of the circuit board is 6.57mA.