Determining the Polarity of Electrolytic Capacitors with a Multimeter
Depending on the capacitance value of the electrolytic capacitor, the R×10, R×100 or R×1K resistance ranges of a multimeter are normally selected for testing. Connect the red and black test leads to the negative terminal of the capacitor respectively (the capacitor must be discharged before each test). The quality of the capacitor can be judged by the deflection of the meter needle.
If the needle swings rapidly to the right and then slowly returns left to its original position, the capacitor is generally in good condition. If the needle swings right but does not return, the capacitor has been broken down. If the needle swings right and then stops at a certain position during its return, the capacitor suffers from leakage. If the needle does not swing at all, the electrolyte inside the capacitor has dried up and the capacitor has lost its capacitance.
For some leaky capacitors, it is difficult to accurately judge their quality using the above method. When the rated withstand voltage of the capacitor is higher than the internal battery voltage of the multimeter, the R×10K range can be used for reverse charging of the capacitor based on the characteristic that electrolytic capacitors have low leakage current during forward charging and high leakage current during reverse charging. Observe whether the position where the needle settles is stable (i.e. whether the reverse leakage current is constant). This method offers higher accuracy for evaluating capacitor quality.
Connect the black test lead to the negative terminal and the red test lead to the positive terminal of the capacitor. If the needle swings rapidly right and then gradually stops at a certain position, the capacitor is good. Capacitors whose needle position is unstable or drifts slowly to the right after settling have leakage and cannot be used further. The needle generally stabilizes within the 50–200K scale range.
The positive and negative polarities are marked on the housing of electrolytic capacitors. Polarity must be observed during use; otherwise, there is a risk of explosion. If the polarity marking of an electrolytic capacitor is unidentifiable, an analog multimeter can be used to determine its polarity.
Since electrolytic capacitors have positive and negative terminals, resistance measurement with a multimeter has a specific feature: when the positive terminal is connected to the black test lead (positive supply) and the negative terminal to the red test lead (negative supply), the electrolytic capacitor has a small leakage current (high leakage resistance). Conversely, the leakage current increases (leakage resistance decreases).
During measurement, assume one terminal is the positive (+) pole and connect it to the black test lead of the multimeter, and connect the other terminal to the red test lead. Record the scale reading where the needle stops (readings on the left side correspond to higher resistance). Then discharge the capacitor (short the two leads together), reverse the two test leads, and repeat the measurement. Between the two measurements, the test with the needle settling further to the left (higher resistance) indicates that the terminal connected to the black test lead is the positive pole of the electrolytic capacitor.
