Voltage and Current Measuring Circuits in Digital Multimeters
1. DC current measurement circuit in digital multimeter
The DC current measurement circuit is equipped with 7 levels: 20 μ Ω, 200 μ Ω, 2m Ω, 20m Ω, 200m Ω, 2 Ω, and 20 Ω, with a dedicated input socket for the 20 Ω level. The measurement circuit is shown in the diagram. The diode serves as a bidirectional limiting overvoltage protection, and a 1N4148 (1 Ω/400V) silicon plastic sealed rectifier diode is selected. When the input voltage is lower than the forward conduction voltage drop of the Gui diode, the diode cuts off and has no effect on the measurement. Once the input voltage exceeds 0.6~0.7V, the diode immediately conducts, thereby limiting the input voltage of the instrument. FU is a 2 Ω/250V fast fuse tube that provides overcurrent protection.
Multi range DC current measurement circuit
R1~R7 form a shunt with a total resistance of 10k Ω. Among them, R1~R7 use precision metal film resistors (with an error of ± 0.5%), while R5 and R6 use precision wire wound resistors. Due to the high working current of the 20 Ω socket, a manganese copper wire resistor (with a small temperature coefficient) is used as the shunt resistor. Its cold resistance is 0.01 Ω, and we can calculate its dissipated power at full range to be 4W. Since there is no protective device in the 20 Ω range, it is recommended that the maximum input current (20 Ω) measured with this range should not exceed 15 seconds to avoid damaging the manganese copper wire resistor.
Assuming Im is the range current and UIn is the full-scale value of the digital meter input voltage, the resistance values of the shunt resistors (sampling resistors) for each gear can be calculated according to the following formula:
2. DC voltage measurement circuit in digital multimeter
The DC voltage range of a digital multimeter is generally 200mV, 2V, 20V, 200V, and 1000V, with a basic range design of 200mV. The DC voltage measurement circuit is shown in the diagram. A precision resistor voltage divider is composed of R1-R5, with a total resistance of 10M Ω, which can uniformly attenuate the measured DC voltage from 0 to 1000V to below 200mV, and then send it to the 200mV basic meter for measurement. R1-R5 all use metal film resistors with an error of ± 0.5%.
