Multimeter - how much do you know about the principle of current measurement
The basic principle of digital multimeter to measure current is to use Ohm's law: I=U/R. The digital multimeter has multiple current gears, which correspond to multiple sampling resistors. When measuring, connect the multimeter in series to the circuit under test, select the corresponding gear, and the flowing current will generate a voltage on the sampling resistor. The voltage value is sent to the A/D analog-to-digital conversion chip, which is converted from analog to digital, and then counted by an electronic counter, and finally the value is displayed on the screen. There is a series sampling resistor inside the multimeter. When the multimeter is connected in series to the circuit to be tested, a current will flow through the sampling resistor, and a voltage difference will be formed at both ends of the resistor when the current flows through. The voltage detected by the ADC is converted into a value, and then the voltage value is converted into a current value through Ohm’s law. The LCD screen is displayed.
The above is just the basic principle. For the specific implementation, we should also consider the size of the current to be measured, divide it into different gears, and take into account the overcurrent protection. The specific practical circuit is as follows:
The practical circuit is divided into 200uA, 2mA, 20mA, 200mA, 10A and other gears. The sampling resistors connected in series with different gears are different. The principle is that the sampling resistance value of the small current gear is large, and the sampling resistance value of the high current gear is small. The size of the sampling resistor will have a certain impact on the current of the circuit to be tested. In actual use, it is necessary to estimate the size of the current and select a suitable gear to reduce the measurement error. Considering that the user may connect the wrong gear and burn the sampling resistor due to overcurrent, diodes D1 and D2 are connected in parallel with the sampling resistor in the design. When the current of the sampling resistor is too large, the voltage rises. When the voltage high voltage diode conducts the voltage , the diode conducts and shunts the current of the sampling resistor to prevent the sampling resistor from being burned when the current is too large. In addition, in order to improve the anti-interference ability, a 470K resistor and a 0.22uF capacitor are added to the voltage detection terminal to form a low-pass filter to filter out the interference source.
The current measurement method of the AC gear is similar to the method of the DC gear, except that the low-pass filter is replaced with an AC-DC converter, and the others remain unchanged.
