Voltage and Current Measurement Circuits of Digital Multimeters
A digital multimeter can be used to measure various circuits. Today, the editor will list two types in detail: voltage and current measurement circuits. Let's take a look together. The following is for reference only:
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%.
DC voltage measurement circuit
The following points need to be explained:
① The input resistance of each voltage level is 10M Ω, considering the input resistance RIN=102 Ω=10000M Ω (typical value) of ICL7106 single-chip 31/2 bit A/D converter. When designing a multi range digital voltmeter, the input resistance RIN=0.001rIN=10M Ω of the instrument is generally selected to make RI>RIN, so the shunt effect of RIN on the signal can be completely ignored.
② In order to save components in digital multimeters, they can also be borrowed when measuring AC voltage
Precision resistor voltage divider for DC voltage range. In addition, the above-mentioned voltage divider resistor also serves as a standard circuit for resistor blocking. Considering that there are only 5 levels of DC voltage and 6 levels of resistors (200 Ω, 20k Ω, 200k Ω, 2M Ω, 20M Ω), R5 is actually composed of 100 Ω and 900 Ω resistors connected in series, where 100 Ω can be used as the standard resistor for the 200 Ω resistor level (the standard resistance value for the resistor level should be equal to half of the resistor range).
③ Due to the high resistance value of R1, in order to facilitate the calibration of resistance values, two paired resistors are often connected in series to form 9M Ω. Some digital multimeters also use a high resistance constant resistor and a low resistance adjustable resistor connected in series to form R1.
④ In order to filter out high-frequency noise at the input end, some digital multimeters also connect a small capacitor of several tens of picofarads in parallel to the DC voltage divider.
