How to Measure Capacitor Health with a Multimeter

To measure the health of a capacitor, it is best to use an analog multimeter as it provides a more intuitive reading. Here, we will use an analog multimeter to illustrate how to measure a capacitor’s health.

First, to measure the capacitance, set the multimeter to the resistance range Rx100 for capacitors larger than 100μF, Rx10 for capacitors larger than several thousand microfarads, and RxK for hundreds of microfarads. Measure by touching the probes to either end of the capacitor; the needle will exhibit the following movements:

a. The needle swings quickly to the right (the larger the capacitance, the greater the swing angle), then slowly returns to its original position. This indicates that the capacitor is good.

b. The needle swings quickly to the right, reaches full scale pointing to zero, then stops moving. This indicates that the capacitor has internally shorted or broken down. If the needle does not move at all, the capacitor is also bad.

c. If the needle swings to the right and then slowly returns to the center, this indicates that the capacitor is leaking internally, which is also bad.

d. If you do not have a multimeter, you can use a battery and a small bulb to determine its condition. Connect the capacitor in series with the battery and bulb; if the bulb lights up, the capacitor is bad; if it does not light, it is good.

e. For capacitors in the range of several thousand microfarads, if the needle barely moves, it is good; if the needle reaches full scale, it is bad.

1. Most digital multimeters have a capacitance range (some have a capacitance testing socket on the multimeter, while others use probes).

2. If there is no capacitance range, you can use the diode setting (if the probes are connected to the capacitor terminals and you hear a beep, then it stops, the display usually shows 1. If you reverse the probes and see the same phenomenon, the capacitor is good. If the probes are connected to the capacitor terminals and the meter continuously beeps, the capacitor has broken down).

Note: During the time the meter beeps before displaying 1, the displayed number is continuously increasing.

Method 2 only checks if the capacitor is good or bad, but cannot measure its capacitance.

1. Direct Measurement with the Capacitance Range Some digital multimeters have a capacitance measurement function, with ranges of 2000pF, 20nF, 200nF, 2μF, and 20μF. When measuring, insert the leads of the discharged capacitor directly into the Cx socket on the multimeter, select the appropriate range, and read the displayed data. The 000p range is suitable for measuring capacitors less than 2000pF; the 20n range is suitable for measuring capacitors between 2000pF and 20nF; the 200n range is suitable for measuring capacitors between 20nF and 200nF; the 2μ range is suitable for measuring capacitors between 200nF and 2μF; the 20μ range is suitable for measuring capacitors between 2μF and 20μF. It has been verified that some models of digital multimeters (e.g., DT890B+) have significant errors when measuring small capacitors below 50pF, with little reference value for measuring below 20pF. In such cases, a series method can be used to measure small capacitance values. The method is to first find a capacitor of about 220pF, measure its actual capacitance C1 with a digital multimeter, and then measure the total capacitance C2 by connecting the small capacitor in parallel. The difference (C1-C2) will give the capacitance of the small capacitor. This method is very accurate for measuring small capacitors between 1 and 20pF.

2. Measuring with the Resistance Range It has been proven that a digital multimeter can also observe the charging process of a capacitor, which reflects the change in charging voltage in discrete digital values. If the measurement rate of the digital multimeter is n times per second, then during the observation of the capacitor’s charging process, you can see n independent and sequentially increasing readings per second. Based on this display characteristic, you can check the capacitor’s health and estimate its capacitance value. Below is a method for using the resistance range of a digital multimeter to test capacitors, which is very practical for instruments that do not have a capacitance range. This method is suitable for measuring large capacitors ranging from 0.1μF to several thousand microfarads. Set the digital multimeter to an appropriate resistance range, connect the red and black probes to the two terminals of the capacitor Cx, and the displayed value will start from “000” and gradually increase until it shows an overflow symbol “1”. If it always shows “000”, it indicates that the capacitor is internally shorted; if it always shows overflow, it may indicate that the capacitor is internally open, or the selected resistance range is not suitable. When checking electrolytic capacitors, it is important to connect the red probe (positive) to the positive terminal of the capacitor and the black probe to the negative terminal.

3. Measuring with the Voltage Range Measuring with the voltage range is actually an indirect measurement method, which is the most precise method. Set the multimeter to the DC current range, connect the probes to the capacitor, charge the capacitor, and then use a formula to calculate the capacitance. The various methods of measuring capacitance with a multimeter utilize the principle of capacitor charging; as the charge increases, the current flowing through causes changes in the readings on the multimeter. The multimeter is a precision instrument, but there are some precautions to take during use, such as ensuring the correct connection of the red and black probes and not misplacing the voltage/current range to avoid damage to the instrument.

Before measuring, make sure to disconnect the power and discharge the capacitor. The discharging method is to find a metal object like a screwdriver, hold the insulated handle, and touch the exposed metal part to the two leads of the capacitor being tested. For measuring capacitors using a digital multimeter, find the capacitance range, then insert the discharged capacitor leads into the capacitance measurement socket. Wait for the readings on the LCD screen to stabilize; the displayed value will be the capacitance of the capacitor being tested. If checking for leakage, you can use the resistance range on an analog multimeter. For small capacitors, set the multimeter to RX1K or RX100. When the probes are connected to the capacitor leads, the needle will deflect clockwise, and as the capacitor charges, the current flow will stop, and the needle will return counterclockwise towards infinity. The greater the angle of deflection, the larger the capacitance. During the deflection process, the needle should swing uniformly and then return to infinity, indicating that the capacitor is not leaking. If at any point the needle slows down or does not return, it indicates leakage at some point in the capacitor. If it finally shows infinity, it indicates no leakage. However, this is only a preliminary assessment; for precise values, a capacitance meter and leakage testing instrument or an oscilloscope should be used to observe characteristics, which is generally not available to the average person. Additionally, capacitors have voltage ratings; the voltage rating for electrolytic capacitors is usually marked on them, while some ceramic capacitors may not be labeled, so caution is advised when selecting.