The electrolytic capacitor (cap) is a vital component in most electrical circuits. Acting as a sort of mini battery, caps store up a charge from a power supply that is then distributed on demand when called on by other circuit components. Often when a single cap goes bad, the whole circuit will stop working. Checking the capacitors in a circuit is a good place to start when troubleshooting. This can be accomplished easily with a digital multimeter that is capable of reading high-range voltages. For other tests, an ohmmeter or device that can measure capacitance may be required.
- Skill level:
Things you need
- Digital multimeter
- Functioning capacitor
- Loose wire
- High ohms, high wattage resistor
Use a small piece of loose wire to jump the two terminals of the capacitor and discharge any stored voltage. Hold the capacitor by its insulated body and grasp the wire with a pair of pliers, touching an end of the wire each to one of the capacitor's leads. While the energy stored in most small capacitors is no more than enough to cause a mild shock, touching the leads or bare wire should be avoided while discharging.
For larger capacitors measuring around and above 400uF or those that are part of a power supply chain and that carry a high voltage, use a high ohms, high wattage resistor--such as a 2K Ohm/25 Watt--to discharge the cap. Hold the resistor in place for several seconds to ensure that the capacitor is fully discharged.
Connect the negative (black) probe to the black socket on the your digital multimeter. Connect the red (positive) probe to the red socket labelled for reading resistance. Turn the knob on the meter to a high ohms range setting, between 10K and 1M ohms.
Touch the red probe to the cap's positive lead, and the black probe to the cap's negative lead. A capacitor is a directional component, so it is important to properly identify the positive and negative leads. Typically these are marked with an arrow signifying the direction of current flow through an electrolytic cap, with the tip of the arrow pointing towards the negative lead.
Observe the meter display. If the capacitor is in good working condition, the display will read zero, then move towards infinity and stop there. If the meter reads zero and there is no change, the cap is not holding a charge and therefore is bad.
Perform the same operation on a known working capacitor of a similar rating to qualify your results.
Tips and warnings
- If the meter comes to rest at a point below infinity after rising from zero, there may be a voltage leakage in the cap. To confirm this, compare with the results of a similar capacitor that is known to work.
- Always fully discharge the capacitor before testing, as failure to do so could result in damage to your capacitor or multimeter.
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