How to test a capacitor

How to test a capacitor Discharging Testing Procedures

Summary: Learn how to test capacitors and even make a capacitor discharging tool.

Capacitors are conductors wrapped in insulators. They look like a short wide battery.

A capacitor is an electronic device that can store energy in the electric field between a pair of conductors (called "plates"). The process of storing energy in the capacitor is known as "charging", and involves electric charges of equal magnitude, but opposite polarity, building up on each plate

Capacitors are energy storing devices that require a charge, and then release all its energy at once.

A simple example is a digital camera flash, which is powered by a small capacitor. The capacitor is charged, and when the flash button is pressed, then the capacitor releases its energy and is routed to the flash bulb. A typical capacitor is shown below.

Capacitor

Before I go into testing capacitors, I want to stress some safety. Capacitors will discharge into whatever both leads touch at the same time. This means, if you brush up against both leads, then it discharges into your body. It will hurt at the very least, large capacitors could kill! If you are not confident, don't touch it, call an expert.

The capacitor's capacitance (C) is a measure of the amount of charge (Q) stored on each plate for a given voltage (V) which appears between the plates:

$C = {Q \over V}$

A capacitor has a capacitance of one farad when one coulomb of charge is stored due to one volt applied potential difference across the plates. The farad is a very large unit; values of capacitors are usually dealt in microfarads (湩), Nan farads (nF), or Pico farads (pF).

Before even hooking up a multimeter, there are a few things you should look for on the capacitor to see if it is bad. One is a bulging electrolyte (ceramic outer material). Another thing to look for is corrosion around the terminals. If either of these is spotted, then your capacitor is leaky and must be replaced.

Leaky capacitor

Discharging

First things first, you should always discharge the capacitor before testing. To do this you need to find a resistor within 5 to 50 ohms larger than the capacitance of the capacitor.

For low capacitance capacitors- It is best to de-solder the capacitor from the board first to avoid damaging other components. All you have to do is short both capacitor leads together with a screwdriver. Make sure you don't touch the metal shank while doing this though, you will get shocked.

For high capacitance capacitors- Solder one end of the appropriate size resistor (for your application) to a well insulated clip lead about 3 feet long.

Solder the other end of the resistor to a well insulated contact point such as a 4 inch length of bare #14 copper wire mounted on the end of a 1 1/2 foot piece of PVC rod which will act as an extension and handle.

Secure everything to the insulating rod with electrical tape. The discharge tool will keep you clear of the danger zone. Now touch the bare copper wire to both capacitor leads, and you will begin discharging safely into the resistor.

Discharge it for a full 30 seconds at least to make sure you get all the juice out. Of course, if it is a high farad or big capacitor, then hold it on longer just to be sure.

Testing- Testing capacitors can be tricky at best. The quick and easy way for the average home electrician is to hook up your multimeter to the discharged leads of the capacitor. You will have to find the polarity of the capacitor, and then hook up the corresponding meter leads.

Unfortunately with most meters, unless it's very new or expensive, you will only be checking if the capacitor is shorted or not. Also, in most cases, you will need to take at least one lead off the circuit card. Once your leads are hooked up as stated above, your readings should be: Any capacitor that measures a few ohms or less is bad. Most should test infinite even on the highest resistance range.

For electrolytes in the uF range or larger, you should be able to see the cap charge when you use a high ohms scale with the proper polarity, the resistance will increase until it goes to infinity. If the capacitor is shorted, then it will never charge. If it is open, the resistance will be infinite immediately and won't change. If the polarity of the meter leads are reversed, it will not charge properly either, which is why you must determine the polarity of your meter and mark it, they are not all the same.

If after all these tests, you are still confused, or are in doubt of your testing, then I would suggest de-soldering the remaining attached capacitor lead, and just go ahead and change it. They are not that expensive in most cases and are easy to do. Be sure to check the polarity and match it against the circuit board before mounting it though.

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