Test The Capacitors

Wednesday, April 11th, 2018 - Troubleshooting

Capacitors that have been used for several times can be damaged therefore, we have to test the capacitors. Capacitors are used for hundreds of different purposes, such as filtering, voltage regulating, bypassing, power phase correction, source of power, and frequency controlling. They come in various sizes, shapes, types, and values. Basically, a capacitor is a device that has the ability to store an electric charge. Capacitors consist of two conducting plates, separated by an insulating dielectric material. Types of capacitors include mica, paper, ceramic, plastic, aluminum, and tantalum. The value of a capacitor is expressed by the unit farad (F), but most capacitors are rated in microfarads (abbreviated μF), which represent the amount of electric charge that the capacitor can store.

Techniques For Test The Capacitors

There are several techniques that can be used to test the capacitors:

  1. Resistance measurement (ohmmeter)
  2. Capacitance measurement (capacitor checker)
  3. Spark test
  4. Bridging
  5. Substitution

Test The Capacitors With Multimeter

A capacitor below 0.25 μF should not show a reading on the ohmmeter since it is too small to advance the meter movement. A near-zero reading indicates a short-circuited capacitor. All capacitors above 0.25 μF should register on the ohmmeter.

When you check a capacitor, place the ohmmeter on a high scale such as 10,000 Ω and place the leads of the ohmmeter across the leads of the capacitor. First, make sure you have discharged the capacitor by short-circuiting the leads with a piece of wire or a screwdriver. When the leads of the meter have been placed across the capacitor, the needle should deflect upward and then slowly drop back down to near zero. Failure to deflect the needle indicates an open capacitor, and failure of the needle to drop down indicates a shorted capacitor (see the following picture).

Test The Capacitors With MultimeterTest The Capacitors With Multimeter

Test The Capacitors With Spark Test

Another method used to check larger capacitors is the spark test method. Momentarily, connect the capacitor across a voltage source. Generally, just 1 second (s) is enough to charge the capacitor. Never leave the voltage applied for too long, or else damage or injury may result. Also, make sure the voltage you have applied does not exceed the voltage rating specified on the capacitor (see the following picture). After the capacitor has been charged, short its terminals together, using a screwdriver or a similar device with an isolating handle so you will not get shocked. A good capacitor will show a spark. Absence of a spark means it is defective.

Test The Capacitors With Spark TestTest The Capacitors With Spark Test

A capacitor checker is a useful device for testing the performance of a capacitor. Besides, checking its specific capacitance rating, it can also test for other characteristics of a capacitor, such as leakage and opens. Some capacitors can be checked while in the circuit; however, it is generally necessary to test them while they are out of the circuit.

Bridging Method For Test Capacitors

The bridging method is also a good way to check the performance of a capacitor. Usually, filter capacitors that are suspected of being open will be bridged before being replaced. This is a simple method in which the suspected defective capacitor is bridged (or jumped) with a known good capacitor within +10 percent of the rated value. A noticeable difference in the quality of performance of the product or device (such as a radio or TV) under test should result upon bridging the capacitor.

For example, defective filter capacitors often produce a noticeable hum in a radio. By bridging a defective capacitor with a good one, the proper circuit operation will be restored and the hum will disappear.

Subtitution Technique For Test The Capacitors

The substitution technique, like the bridging method, determines the quality of the capacitor by means of another capacitor. When using the substitution method, you simply replace the suspected defective capacitor with a known good capacitor of similar value and rating. The performance of the product or device indicates the effect of the new capacitor. Remember, never exceed the voltage rating of a capacitor. A capacitor rated at 100 V should only be replaced with one at 100 V or more; otherwise, it may be destroyed. Substitution boxes are handy aids to assist the troubleshooting. These boxes contain the most common-valued capacitors, eliminating the need to obtain individual capacitors. These substitution boxes can easily be made or purchased, and they offer a quick, convenient, and accessible means of obtaining a substitute for test the capacitors.

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