Sensitivity and selection skills of multimeter
Regarding the sensitivity and selection skills of the multimeter, the sensitivity is a technical index indicating the degree to which the instrument responds to weak energy. The sensitivity of the multimeter can be divided into three indicators: DC voltage sensitivity, AC voltage sensitivity and gauge sensitivity.
1. The sensitivity and selection skills of the multimeter
Sensitivity is a technical indicator that indicates the degree to which the meter responds to weak energy.
Since the energy used to drive the deflection of the measuring mechanism of the meter is taken from the current in the circuit under test, if the pointer of the meter deflects relatively large and uses less energy, its sensitivity will be higher.
The sensitivity of the multimeter can be divided into three indicators: DC voltage sensitivity, AC voltage sensitivity and meter sensitivity. Among them, DC voltage sensitivity is the main indicator. AC voltage sensitivity is generally lower than DC voltage sensitivity due to circuit design factors. They are marked on the dial in ohms per volt (Ω/V), so that we can see them at a glance. The sensitivity of the meter head indicates the full-scale current value of the meter head, and also includes two indicators of meter head internal resistance and linearity, which are the basis for calculating the meter circuit and also determine the voltage sensitivity of the entire multimeter; the meter head internal resistance refers to the meter needle The sum of the resistance values of the moving coil and the upper and lower groups of hairsprings; linearity refers to the degree of consistency between the current intensity passing through the meter head and the deflection range of the hands, which is used as the basis for drawing the dial scale. Here we focus on the DC voltage sensitivity of the multimeter.
When the voltmeter is used for measurement, it is connected in parallel with the two points to be measured. Due to the existence of the internal resistance of the voltmeter, it is equivalent to connecting a resistor in parallel between the two points to be measured, so that the total impedance between the two points to be measured is reduced; and Coupled with its shunting effect on the circuit, the measured voltage value is lower than the actual value. Therefore, when making voltage measurements, the multimeter is required to have a larger internal resistance (that is, the sensitivity Ω/V number should be high) to reduce this error.
For example, the ranges of the MF30 multimeter DC voltage range are 0-1-5-25-100-500V, and the dial is marked with 20000Ω/V, then the internal resistance of the 1V range is 20kΩxl=20kΩ; the internal resistance of the 5V range is 20kΩx5=100kΩ , and so on.
The sensitivity of the multimeter can be divided into two indicators: meter sensitivity and voltage sensitivity (including DC voltage sensitivity and AC voltage sensitivity).
The full-scale value Ig (full-scale current) of the meter used by the multimeter is called the meter sensitivity. Ig is generally 9.2-200μA. The smaller the Ig, the higher the sensitivity of the meter. The full-scale value of the high-sensitivity meter is generally less than 10μA, the full-scale value of the medium-sensitivity meter is usually 30-100μA, and more than 100μA is a low-sensitivity meter.
Selection skills of multimeter sensitivity:
1) If two multimeters have the same range but different voltage sensitivities, when they are used to measure the same high internal resistance power supply voltage, the meter with higher voltage sensitivity will have a smaller measurement error.
2) For the same multimeter, the higher the voltage range, the greater the internal resistance, and the smaller the measurement error caused.
In order to reduce the error of measuring the high internal resistance power supply voltage, sometimes it is better to choose a higher voltage range to increase the internal resistance of the multimeter. Of course, the range should not be selected too high, so as not to increase the reading error due to the small deflection angle of the pointer when measuring low voltage. For power supply voltages with low internal resistance (for example, 220V AC power supply), a multimeter with low voltage sensitivity can be used for measurement. In other words, a high-sensitivity multimeter is suitable for electronic measurements, while a low-sensitivity multimeter is suitable for electrical measurements.
3) When the internal resistance of the voltage block of the multimeter is more than 100 times greater than that of the power supply under test, there is no need to consider the shunting effect of the multimeter on the power supply under test.
