Measurement of direct current with a digital multimeter
Digital multimeters have become mainstream, with a trend towards replacing analog instruments. Compared with analog instruments, digital instruments have high sensitivity, accuracy, clear display, strong overload capacity, are easy to carry, and are simpler to use. Taking the VC9802 digital multimeter as an example, this article briefly introduces its usage and precautions.
(1) Before using method a, carefully read the relevant user manual and familiarize yourself with the functions of the power switch, range switch, socket, and special socket. b Place the power switch in the ON position. Measurement of AC/DC voltage: Set the range switch to the appropriate range of DCV (DC) or ACV (AC) as needed, insert the red probe into the V/Ω hole, insert the black probe into the COM hole, and connect the probe in parallel with the measured circuit. The reading will be displayed. Measurement of AC/DC current: Set the range switch to the appropriate range for DCA (DC) or ACA (AC), insert the red probe into the mA hole (<200mA) or 10A hole (>200mA), insert the black probe into the COM hole, and connect the multimeter in series with the tested circuit. When measuring direct flow, the digital multimeter can automatically display polarity. Measurement of resistance: Set the range switch to the appropriate range of Ω, insert the red probe into the V/Ω hole, and insert the black probe into the COM hole. If the measured resistance value exceeds the maximum value of the selected range, the multimeter will display "1", and a higher range should be selected. When measuring resistance, the red probe is positive and the black probe is negative, which is exactly the opposite of a pointer multimeter. Therefore, when measuring polarized components such as transistors and electrolytic capacitors, attention must be paid to the polarity of the probes.
(2) If it is not possible to estimate the magnitude of the measured voltage or current in advance, first switch to the highest range and measure once, then gradually reduce the range to the appropriate position depending on the situation. After the measurement is completed, the range switch should be turned to the highest voltage level and the power should be turned off. When the range is full, the instrument only displays the number "1" at the highest position, and all other positions disappear. At this time, a higher range should be selected. When measuring voltage, a digital multimeter should be connected in parallel with the circuit being tested. When measuring current, it should be connected in series with the circuit being measured, and there is no need to consider positive or negative polarity when measuring direct current. When the AC voltage gear is misused to measure DC voltage, or when the DC voltage gear is misused to measure AC voltage, the display screen will show "000" or the numbers in the lower digits will jump. It is prohibited to change the range when measuring high voltage (above 220V) or high current (above 0.5A) to prevent arcing and burning of switch contacts. When "", "BATT" or "LOW BAT" is displayed, it indicates that the battery voltage is lower than the operating voltage.
