How to ohm out a capacitor

What does it mean to 'ohm out' a capacitor?

Testing a capacitor by measuring its resistance with a multimeter is commonly referred to as 'ohming it out.' This process helps determine if the capacitor is functioning correctly or if it has failed due to common issues like short circuits or open circuits. While not a definitive test for all capacitor failures, especially for intermittent issues or degradation, it's a quick and accessible method for basic diagnostics.

How to Ohm Out a Capacitor Using a Multimeter

Before you begin, ensure your multimeter is functioning correctly and has a fresh battery. Capacitors can store a charge, so it's advisable to discharge them before testing, especially larger ones. You can do this by briefly touching a resistor across the terminals.

Step 1: Select the Resistance Setting

Turn your multimeter dial to the resistance (Ω) setting. For most standard capacitors, the 20kΩ or 200kΩ range is usually appropriate. If you have an auto-ranging multimeter, it will select the best range automatically.

Step 2: Identify Capacitor Terminals

Locate the two leads or terminals of the capacitor. For polarized capacitors (like electrolytic or tantalum capacitors), pay close attention to polarity. The positive (+) terminal is usually marked with a '+' sign or a longer lead, while the negative (-) terminal is marked with a '-' sign or a shorter lead. Non-polarized capacitors (like ceramic or film capacitors) can be connected in either direction.

Step 3: Discharge the Capacitor (If Necessary)

Larger capacitors, especially those found in power supplies or high-voltage circuits, can hold a significant charge. To safely discharge them, use an insulated screwdriver or a resistor (e.g., 1kΩ to 10kΩ) and briefly touch it across the capacitor's terminals. This prevents potential damage to your multimeter or yourself. Smaller capacitors typically hold negligible charge.

Step 4: Connect Multimeter Probes

Touch the multimeter's red probe to one terminal of the capacitor and the black probe to the other. For polarized capacitors, connect the red probe to the positive terminal and the black probe to the negative terminal. If you are unsure about polarity or it's a non-polarized capacitor, you can reverse the probes after the first reading.

Step 5: Observe the Resistance Reading

Watch the multimeter's display. Here's what you should expect to see:

• Good Capacitor: The resistance reading will start very low (close to 0 ohms) and then gradually increase. As the multimeter's battery charges the capacitor, the resistance will climb steadily towards infinity (or the maximum value the multimeter can display, often shown as 'OL' or '1'). The speed at which the resistance increases depends on the capacitor's value and the multimeter's output voltage. Larger capacitors will take longer to charge.
• Shorted Capacitor: If the resistance reading immediately shows a very low value (close to 0 ohms) and stays there, the capacitor is likely shorted and needs to be replaced.
• Open Capacitor: If the resistance reading immediately shows a very high value (infinity or 'OL') and never changes, the capacitor is likely open (its internal circuit is broken) and needs replacement.
• Leaky Capacitor: A capacitor that charges but then settles at a resistance value that is not infinite might be leaky. This is more common in older or lower-quality capacitors.

Step 6: Reverse Probes (For Polarized Capacitors or Confirmation)

If testing a polarized capacitor, after observing the initial charging behavior, reverse the probes. The reading should immediately jump to infinity ('OL') and stay there. This confirms the capacitor's polarity and that it's not shorted in reverse.

Interpreting the Results

The 'ohming out' test is most effective for detecting catastrophic failures like shorts and opens. It's less reliable for subtle issues such as capacitance drift, increased Equivalent Series Resistance (ESR), or dielectric breakdown under load. For a more comprehensive test, especially for critical applications, a capacitance meter or an ESR meter is recommended.

Important Considerations

• Capacitor Value: The time it takes for the resistance to climb varies significantly with capacitance. A small ceramic capacitor (e.g., 0.1µF) might charge almost instantly, showing infinity quickly. A large electrolytic capacitor (e.g., 4700µF) could take several seconds or even minutes for the reading to climb fully.
• Multimeter Limitations: The voltage provided by a multimeter's battery is usually low (e.g., 1.5V to 9V). This might not be sufficient to reveal certain types of failures that only occur at higher voltages.
• Safety: Always be mindful of stored charge in capacitors, especially in power supply units. Discharging them before testing is crucial for safety.

In summary, ohming out a capacitor is a useful troubleshooting step that can quickly identify if a capacitor is completely dead (shorted or open). However, for precise measurements or to diagnose more complex failures, specialized equipment is necessary.