Introduction to How to Judge Whether a Capacitor Is Good or Bad with a Multimeter
Since the energy that drives the deflection of the instrument‑measuring mechanism is derived from the current in the measured circuit, the instrument has higher sensitivity if its pointer deflects substantially with little energy consumed.
The sensitivity of a multimeter falls into three indicators: DC voltage sensitivity, AC voltage sensitivity and meter‑movement sensitivity. Among them, DC voltage sensitivity is the primary indicator. Restricted by circuit design, AC voltage sensitivity is generally lower than DC voltage sensitivity. These values are printed on the dial in ohms‑per‑volt (Ω/V) for easy reference. Meter‑movement sensitivity refers to the full‑scale current of the meter movement. It also includes two parameters: internal resistance of the meter movement and linearity. It serves as the basis for calculating meter circuits and determines the overall voltage sensitivity of the multimeter. The internal resistance of the meter movement is the sum of resistances of the pointer moving coil and the upper and lower hairsprings. Linearity describes the consistency between the current passing through the meter movement and the pointer deflection, which is the foundation for dial‑scale graduation. This section focuses on the DC voltage sensitivity of multimeters.
When a voltmeter performs measurement, it is connected in parallel across the two measured points. Due to the internal resistance of the voltmeter, it is equivalent to connecting a resistor in parallel between the two measured points, which reduces the total impedance between them. Together with its shunt effect on the circuit, the measured voltage value becomes lower than the true value. Therefore, for voltage measurement, the multimeter shall have high internal resistance (i.e., high Ω/V sensitivity) to minimize such errors.
For example, the DC‑voltage ranges of the MF30 multimeter are 0‑1‑5‑25‑100‑500 V, marked with 20000 Ω/V on the dial. The internal resistance for the 1 V range is 20 kΩ × 1 = 20 kΩ; the internal resistance for the 5 V range is 20 kΩ × 5 = 100 kΩ, and so forth.
Multimeter sensitivity consists of two categories: meter‑movement sensitivity and voltage sensitivity (including DC voltage sensitivity and AC voltage sensitivity).
