Voltage sensitivity on the multimeter
Sensitivity is a technical indicator that represents the degree to which an instrument responds to weak energy.
Due to the fact that the energy driving the instrument measurement mechanism to deflect is taken from the current in the circuit under test, if the instrument pointer deflects significantly and uses less energy, its sensitivity will also be higher.
The sensitivity of a multimeter can be divided into three indicators: DC voltage sensitivity, AC voltage sensitivity, and meter head sensitivity. The DC voltage sensitivity is the main indicator. AC voltage sensitivity is generally lower than DC voltage sensitivity due to meter circuit design factors. They are labeled on the dial in ohms per volt (Ω/V).
The sensitivity of the meter head indicates the full scale current value of the meter head, including the internal resistance and linearity of the meter head. It is the basis for calculating the meter circuit and also determines the voltage sensitivity of the entire multimeter; The internal resistance of the meter head refers to the sum of the resistance values of the moving coil of the meter needle and the upper and lower groups of hairspring; Linearity refers to the degree of consistency between the current intensity passing through the meter head and the deflection amplitude of the meter needle, which serves as the basis for drawing the dial scale. Here, the focus is on the DC voltage sensitivity of the multimeter.
When measuring, the Voltmeter is connected in parallel with the two points to be measured. Because of the existence of the internal resistance of the Voltmeter, it is equivalent to that a resistance is connected in parallel between the two points to reduce the total impedance between the two points to be measured; In addition, its shunt effect on the circuit causes the measured voltage value to be lower than the actual value. Therefore, when measuring voltage, it is required that the multimeter has a large internal resistance (i.e. sensitivity Ω/V should be high) to reduce this error.
For example, if the DC voltage range of the MF30 multimeter is 0-1-5-25-100-500V and the dial is marked with 20000 Ω/V, the internal resistance of the 1V range is 20k Ω xl=20k Ω; The internal resistance within the 5V range is 20k Ω x 5=100k Ω, and so on.
Tips for selecting the sensitivity of a multimeter:
1) If two multimeters have the same range but different voltage sensitivities, the one with higher voltage sensitivity will have smaller measurement errors when measuring the same high internal resistance power supply voltage separately.
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 in measuring high internal resistance power supply voltage, it is sometimes preferable to choose a higher voltage range to increase the internal resistance of the multimeter. Of course, the range should not be too high to avoid increasing reading error due to the small deflection angle of the pointer when measuring low voltage. For low internal resistance power supply voltages (such as 220V AC power supply), a multimeter with lower voltage sensitivity can be selected 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 range of the multimeter is more than 100 times greater than the internal resistance of the tested power supply, there is no need to consider the shunt effect of the multimeter on the tested power supply.
