A multimeter is a magnetic-electric instrument that can measure various electrical parameters such as AC voltage, DC voltage, and resistance. By changing the knob switch, it can measure multiple electrical parameters, making it an important instrument for tractor electrical repairs. Below, we will introduce the structural features and technical usage points of pointer multimeters using the 500 model as an example.
The 500 model pointer multimeter is a highly sensitive, multi-range portable rectifying instrument. This meter has a total of 24 measurement ranges and can complete measurements of AC and DC voltage, DC current, resistance, and audio levels, as well as estimate the performance of capacitors and identify various types of diodes, transistors, and their polarities.
(1) Meter Head and Dial. The meter head is an important component of the multimeter that determines its sensitivity. The dial consists of various scale lines and symbols that serve as explanations. Only by correctly understanding the reading methods of various scale lines and the meanings represented by various symbols can one skillfully and accurately use the multimeter. There are 4 types of scale markings on the scale, from top to bottom: ohm scale, AC and DC 50 V and 250 V scale, dedicated AC 10 V scale, and dB scale. The DC current and DC voltage share one scale, and the markings on that scale are uniform and evenly divided, while the markings on other scales are uneven. Different numbers are marked on the same scale to avoid the need for conversion when using certain ranges. The reading method for resistance values is exactly the opposite of other reading methods, starting from the zero position on the right to infinity on the left, while other range readings start from the zero position on the left. There are three methods for reading measurement results: the first is the limit method, where the value of the measured object is within the full deflection range of the scale, allowing for direct reading, such as measuring voltage values within 250 V; the second is the range method, where the measurement result equals the reading multiplied by the range, which applies to the resistance range. For example, if the resistance range switch is set to R×100 and the reading is 8, then the measured resistance is 800 Ω; the third is the conversion method, such as for voltage ranges above 250 V and dB ranges above +22 dB.
(2) Selector Switch. Different models of multimeters have different working methods for the selector switch, which can be a combined function switch and range switch, a separate function switch and range switch, or an interactive use type. Some multimeters also have dedicated sockets that work in conjunction with the function selector switch to complete certain specialized measurements. The 500 model multimeter belongs to the interactive use type. When using it, one must first familiarize themselves with the positions of the function options on the two selector switches. Based on the type of the measured object, select the corresponding measurement item, and then choose the appropriate range according to the size of the measured value, allowing for measurement. For example, when measuring a 9 V stacked battery’s voltage with the 500 model multimeter, first select the function switch by turning the right knob to the “V” position; then select the range by turning the left knob to the DC voltage 10 V range. After confirming that the red probe is inserted into “+” and the black probe into “-”, measurement can proceed, and the test result can be read from the scale. In addition to selecting the range using the above method, the measurement range can also be converted by changing the probe plug position. For example, when measuring 2500 V AC or DC voltage, keep the black probe in “+” and insert the red probe into the 2500 V socket to measure.
(3) Probe Sockets. The representations of probe sockets in different multimeters vary. Some directly use “+” and “-”, while others use “+” and “*”, or “+” and “COM”. The 500 model multimeter has 4 probe sockets corresponding to “*”, “+”, “dB” (some are for 5 A), and “2500 V” positions. When measuring, the red probe should be inserted into “+”, and the black probe should be inserted into the common terminal “*”. When using the AC, DC “2500 V”, and audio level testing ranges, the red probe should be inserted into the “2500 V” and “dB” sockets, respectively.
(1) Measuring DC Voltage. Turn the right selector switch to the DC voltage range and the left knob to the corresponding range of the DC voltage to be measured. The two probes should be connected across the two ends of the circuit. If the value of the voltage to be measured is unknown, one should follow the principle of starting from a higher range to a lower range, adjusting downwards until the pointer is within the effective deflection range. If the internal resistance of the meter is not considered to affect the measurement result, a smaller range can be selected to maximize the pointer deflection, thus providing the most accurate reading with the least error. If the internal resistance of the meter is considered, a higher range should be selected to increase the meter’s internal resistance, reducing the impact of the meter’s internal resistance on the measurement result. During measurement, if the polarity of the voltage is unknown, one probe can be connected first, and the other probe can be quickly touched on the point to be measured. If the pointer deflects left, it indicates a measurement error, and the red and black probes should be swapped. If the pointer deflects right, it indicates a correct measurement, with the red probe connected to the positive side and the black probe to the negative side, allowing for detailed measurement. Except for the 50 V and 250 V ranges, which can be read directly, other ranges require proportional conversion of the measurement results.
(2) Using the AC Voltage Range. Turn the right selector switch to the AC voltage range (shared with the DC voltage range), and the left knob to the corresponding range of the AC voltage to be measured. The selection of the range and the reading method are the same as for DC voltage. Additionally, the AC voltage range has a dedicated AC 10 V scale. Since AC voltage does not have positive and negative polarities, the probes do not have red and black distinctions. However, it should be noted that measuring AC voltage with the DC voltage range will cause the pointer to shake without deflection, and may even damage the meter. Measuring DC voltage with the AC voltage range will yield a result that is approximately double. When measuring AC voltage, if the AC signal being measured is superimposed with DC voltage, the sum of the AC and DC voltage must not exceed the limit of the range; if necessary, a coupling capacitor should be connected in series at the input, or the dB range can be used for measurement, as this socket already has a coupling capacitor in series. Therefore, when using the AC voltage range for measurement, attention should be paid to the selection of the range.
(3) Using the DC Current Range. When measuring DC current, the left selector switch should be turned to the DC current range, and the right selector switch should be turned to the corresponding range for the current to be measured. The selection of the range is the same as for measuring DC voltage. Disconnect a point in the circuit to be measured and connect the two probes in series in the circuit, ensuring that the red probe is connected to the end where the current flows in, and the black probe is connected to the end where the current flows out. During the measurement process, ensure that the contact between the two probes and the circuit remains good, and do not connect the two probes directly across the two ends of a circuit to prevent damage to the multimeter.
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