Testing Capacitor Leakage Current with Multimeter Voltage Range
The DC voltage range of a digital multimeter is not only used to measure voltage, but also to detect capacitors. Measuring capacitance using voltage range is actually an indirect measurement method, which can measure small capacity capacitors ranging from 220pF to 1 μ F and accurately measure the leakage current of the capacitor.
1. Measurement methods and principles. The measurement circuit is shown in Figure 1, where E is an external 1.5V dry battery. Set the digital multimeter to the DC voltage of 2V, connect the red probe to one electrode of the tested capacitor Cx, and connect the black probe to the negative terminal of the battery. The input resistance Ri for 2V gear is 10M Ω. Since the voltage across Ri is the instrument input voltage VIN, Ri actually also serves as a sampling resistor. If the leakage current of the measured capacitor is ID and the stable value displayed by the instrument is VC (unit: V), then
Here is an example to illustrate:
Example 1: The tested capacitor is a fixed capacitor with a capacitance of 1 μ F/160V. Use the DT930FD digital multimeter with a 2V/DC range (=10M Ω) to connect the circuit according to Figure 1. At first, the instrument displayed 1.521V, and then the displayed value gradually decreased. After about 1 minute, the displayed value stabilized at 0.0027V. Based on this, the leakage current of the tested capacitor was calculated: the leakage current of the tested capacitor was only 0.3nA, indicating good quality.
Example 2: The tested capacitor is a 0.022 μ F/63V polyester capacitor, and the measurement method is the same as above. Due to the small capacity of the capacitor, VIN (t) decreases rapidly during measurement. After about 4 seconds, the displayed value drops to 0.0019V. The calculated leakage current is:
The leakage current is very small, indicating good quality.
2. Precautions
(1) Before measurement, the two pins of the capacitor should be short circuited and discharged, otherwise the process of reading changes may
not be observed.
(2) During the measurement process, do not touch the capacitor electrode with both hands to avoid instrument jumping.
(3) During the measurement process, the value of VIN (t) changes exponentially, decreasing rapidly at the beginning and gradually slowing down over time. When the capacity of the tested capacitor Cx is less than a few thousand picofarads, due to the rapid decrease in VIN (t) at the beginning and the low measurement rate of the instrument, there is no time to reflect the initial voltage value. Therefore, the initial displayed value of the instrument is lower than the battery voltage E.
(4) When the measured capacitor Cx is greater than 1 μ F, in order to shorten the measurement time, resistance mode can be used for measurement. But when the capacity of the tested capacitor is less than 200pF, it is difficult to observe the charging process due to the brief change in reading.
