Capacitors are conductors wrapped in insulators, and are commonly used to store energy in an electrical field.
A capacitor is an electronic device that can store energy in the electrical 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.
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, it will discharges into your body. Some capacitors will hurt when discharged into your body, and very large capacitors could kill! If you are not confident working with capacitors, do not touch them.
The 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. The common equation in capacitance is seen below.
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, so values of capacitors are usually read in microfarads (µF), nano farads (nF), or pico farads (pF).
Capacitor Visual Inspection
Before testing with a multimeter, there are a few things you should look for on the capacitor to see if it is faulty. The first is a bulging electrolyte, or ceramic outer material leaking out. 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.
Discharging a Capacitor
You should always discharge the capacitor before testing. To do this you need to find a high value resistor that can safely handle the output of the capacitor.
Low voltage capacitors- It is best to desolder the capacitor from the board first to avoid damaging other components. Hold a high resistance resistor with a pair of insulated needle nose pliers. Now touch the two metal leads from the resistor to the two capacitor lead points on the circuit card. This will slowly drain out the stored energy from the capacitor into the resistor.
High voltage capacitors- My recommendation on high voltage capacitors is to call an experienced electrician for replacement or testing. High voltage capacitors are dangerous and should not be touched by anyone other than a trained professional.
Test a Capacitor
ESR meter test- For the most accurate testing of electrolytic capacitors, use an equivalent series resistance (ESR) meter pictured below. The meter sends in small pulses of voltage which are essentially too small to charge the capacitor under test. The ESR meter is also convenient because it allows the capacitor to remain wired into the circuit. The ideal capacitor has an ESR reading of zero. As the ESR level increases, the capacitor becomes more likely to leak, fail, and damage other components in the circuit.
To test a capacitor for resistance using an ESR meter, follow the steps below.
Step 1- Remove power from the circuit.
Be certain the circuit is not connected to a power source and is dead.
Step 2- Discharge the capacitor.
Discharge the capacitor using a resistor with a resistance 5 to 50 times the rated voltage of the capacitor. Hold the resistor with insulated needle nose pliers and touch the resistor leads to the capacitor leads. Hold for several seconds to drain the capacitor into the resistor.
Step 3- Test the capacitor with an ESR meter.
Connect the ESR meter probes to the capacitor using the correct polarity. The negative side of tube type capacitors are clearly marked with a " - " symbol (See photo below). Connect the black probe to the negative lead and the red probe to the positive lead.
Negative side marked
Step 5- Review the test results.
Look at the readout on the ESR meter, which could be analog, LED readout, or digital. The reading is the equivalent series resistance which should be close to zero for a good capacitor. The higher the number, the more defective the capacitor. Remember, zero is ideal, but capacitors with age may climb higher than that.
Alternate Capacitor Test Method
This method uses a common multimeter to test a capacitor. This test method is less accurate than the ESR test and requires at least one side of the capacitor to be removed from the circuit.
Step 1- Connect the meter to the capacitor.
Connect your meter to the discharged leads of the capacitor. Be certain to connect the red probe to the positive capacitor lead, and the black probe to the negative lead.
Step 2- Watch the capacitor charge.
For capacitors in the microfarad uF range or larger, you should be able to see the capacitor charge when connected to the meter. This is because the meter will charge the capacitor.
Step 3- Review the test results.
When the meter is set in the high resistance range, the resistance should start out low and increase until it goes to open. If the capacitor is shorted, then it will never charge and resistance will not increase. If the capacitor is open, the resistance will be infinite immediately and will not change.
If after testing the capacitor you are still in doubt of the results, then I would suggest to replace the capacitor. Be sure to check the polarity of the capacitor, and match it against the circuit board before soldering.