how to test a capacitor with an ohmmeter

Daniel Parker

3 min read

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20-01-2025

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Capacitors are essential components in many electronic circuits, storing electrical energy. Over time, capacitors can fail, leading to malfunctioning devices. Fortunately, you can often test a capacitor's functionality using a simple ohmmeter. This article will guide you through the process. Knowing how to test a capacitor with an ohmmeter is a valuable skill for any electronics enthusiast or technician.

Understanding Capacitors and Their Failures

Before diving into the testing process, let's briefly review what capacitors do and how they fail. A capacitor stores electrical energy in an electric field between two conductive plates separated by an insulator (dielectric). Common capacitor failures include:

• Short Circuit: The dielectric breaks down, creating a direct path between the plates. This results in a low resistance reading on an ohmmeter.
• Open Circuit: The dielectric becomes completely non-conductive. This leads to an infinite resistance reading (or a lack of reading).
• Leakage: The dielectric's insulating properties degrade, allowing a small current to leak between the plates. This shows up as a lower than expected resistance reading that may change over time.

Tools and Safety Precautions

Testing a capacitor with an ohmmeter requires minimal tools:

• Ohmmeter: A multimeter with an ohmmeter function is essential. Ensure your multimeter is properly calibrated.
• Capacitor under test: Obviously! Make sure you identify the capacitor's voltage rating to avoid damaging the device.
• Screwdriver (if necessary): Some capacitors may be soldered into circuits; you may need a screwdriver to access them.

Safety First! Always disconnect the capacitor from any circuit before testing. Even seemingly discharged capacitors can retain a dangerous charge. Be especially careful with larger capacitors, which can store a significant amount of energy.

How to Test a Capacitor with an Ohmmeter: Step-by-Step Guide

Here's a detailed procedure on how to test a capacitor using your ohmmeter:

1. Set the Multimeter: Turn on your multimeter and select the lowest resistance range (usually 200 ohms). Many multimeters have a diode testing setting which can also be used.

2. Discharge the Capacitor: This is crucial for safety. If you suspect a capacitor is charged, briefly short the leads together using a screwdriver with insulated handle.

3. Connect the Ohmmeter: Connect the ohmmeter leads to the capacitor's terminals.

4. Observe the Reading:

   • Initially, the ohmmeter needle should deflect briefly toward zero, indicating a low resistance. This is because the capacitor is initially charging through the ohmmeter. The needle should then slowly move toward infinity, as the capacitor charges. This is a sign of a good capacitor.

   • If the needle remains at zero, the capacitor is likely shorted. This indicates a faulty capacitor.

   • If the needle immediately goes to infinity (no deflection), the capacitor is likely open. This is another indicator of a faulty capacitor.

   • If the needle moves slowly toward infinity, but not all the way, the capacitor might have some leakage. This could still be functional, but its performance is likely degraded.

5. Reverse the Leads: Repeat steps 3 and 4, reversing the ohmmeter leads. The behavior should be similar. Significant differences in readings could suggest a problem.

6. Interpret the Results: Based on your observations, determine whether the capacitor is good, shorted, open, or has leakage. Consult the specifications for the capacitor if you are unsure whether leakage is acceptable.

Troubleshooting Tips

• Low Readings that Don't Change: A capacitor with significant leakage will show a low resistance that doesn't change much over time.
• Very High Readings or No Readings: This suggests an open capacitor.
• Electrolytic Capacitors: Pay special attention to the polarity. Connecting an electrolytic capacitor backward can damage the capacitor and your multimeter.

When to Replace a Capacitor

If your tests reveal a shorted, open, or severely leaky capacitor, it's time for replacement. A faulty capacitor can disrupt your circuits, leading to erratic behavior or complete failure of the device. It's always recommended to replace a faulty capacitor with one of the same specifications (voltage, capacitance).

Conclusion

Testing a capacitor with an ohmmeter is a straightforward yet crucial skill for anyone working with electronics. By carefully following these steps, you can effectively diagnose capacitor health and maintain your electronic projects. Remember safety and always discharge the capacitor before testing! Understanding how to test a capacitor with an ohmmeter can save you time and money in the long run.