How to Test Outside-Foil Capacitors

Use a multimeter to test the charge capability of an outside-foil capacitor.

Outside-foil capacitors are created using two strips of metal foil wrapped around a long strip of insulator. The foil-wrapped insulator is wound into a coil with leads attached to each of the metal strips. The whole coil is covered to prevent moisture buildup. Foil capacitors power a variety of electrical devices including guitar amplifiers, receivers, home audio electronics and appliances. Test outside-foil capacitors with a multimeter set to measure resistance. Always discharge the capacitor before testing to ensure accurate results.

Things You'll Need

• Heavy-gauge insulated-copper wire
• Wire strippers
• Multimeter

Instructions

1. Strip several inches of insulation from the heavy-gauge wire. Use the wire strippers to cut a ring in the insulation down to the wire. Pull the insulation from the wire to expose bare-copper wire.

2. Remove power to the device in which the capacitor is located.

3. Bend the wire to allow both leads of the outside-foil capacitor to be touched simultaneously. Grasp the wire by the insulation and press the bare wire against the outside-foil capacitors two leads. Do not touch the wire against anything other than the leads during the discharge process. Hold the wire against the leads for 30 seconds to discharge any stored electrical energy.

4. Set the multimeter tool to measure resistance. Resistance is measured in ohms. Ohms may be displayed on the multimeter as the Greek letter, "Omega," which is formed like an upside down horseshoe.

5. Touch the two multimeter leads to each of the capacitor leads,

6. Monitor the measurement output on the multimeter to determine the functional state of the outside-foil capacitor. Functional foil capacitors initially display zero and consistently climb toward infinity. Open capacitors fail to register any measurement. Electrically shorted capacitors only register a zero resistance and do not climb. Leaking capacitors register zero, climb and then stop short of infinity due to electricity leaking from the capacitor.

7. Replace open, shorted and leaking capacitors.

Tips & Warnings

• Discharge the capacitor before each test. The multimeter's battery charges the capacitor when the multimeter leads are connected to the capacitor leads.

• Do not touch the capacitor's leads without first discharging to avoid electric shock.

References

• Aiken Amps: Outside Foil Capacitors
• OhmCheck: How to Test a Capacitor
• Tech-FAQ: Capacitor