CAPACITANCE

what is capacitor?

 A capacitor is an  electrical/electronics component on most circuit boards that has positive and negative pin and  can perform many functions, when it’s  placed in a circuit with an active current, electrons from the negative side build up on the closest plate. The negative flows to the positive—that is why the negative is the active lead, although many capacitors are not polarized. Once the plate can no longer hold them, they are forced past the dielectric and onto the other plate, thus displacing the electrons back into the circuit. This is called the discharge. Electrical components are very sensitive to voltage swings, and as such a power spike can kill those expensive parts. Capacitors condition DC voltage to other components and thus provide a steady power supply. AC current is rectified by diodes, so instead of AC, there are pulses of DC from zero volts to peak. When a capacitor from the power line is connected to ground and the DC will not pass, but as the pulse fills up the cap, it reduces the current flow and the effective voltage. While the feed voltage goes down to zero, the capacitor begins to leak out its contents, this will smooth the output voltage and current. Therefore a capacitor is placed inline to a component, allowing for absorption of spikes and supplementing valleys, this, in turn, keeps a constant power supply to the component. Now lets look at how to measure capacitor.

Measuring Capacitors

Measuring the value of a giving capacitor has can be accomplished in the  following ways :

markings on the capacitor itself.

The above capacitor has a capacitance of 6800μF (micro farad) and  it has a voltage rating of 63V. The arrangement of the leads all show that it is a radial capacitor. Both leads exit on one side versus an axial arrangement where one lead exits from either side of the capacitors body. Also, the ash side of the capacitor with negative symbol  indicates the negative pin.

 voltage test:

 

capacitors store a potential difference of charges across their plate, the charge are voltages. A capacitor has an anode which has a positive voltage and a cathode which has a negative voltage. One way to check if a capacitor is working is to charge it up with a DC voltage and then read the voltage across the anode and cathode. In this case polarity is very important. If this capacitor has a positive and negative lead, it is a polarized capacitors (electrolytic capacitors). Positive voltage will go to the anode, and negative goes to the cathode of the capacitor. Remember to check the markings on the capacitor to be tested. Then apply a voltage, which should be less than the voltage the capacitor is rated for, for a few seconds.

After the charge is finished, disconnect the battery from the capacitor. Use the multimeter and read the voltage on the capacitor leads. The voltage should read starting from the battery volt rate and  will discharge rapidly to 0V because the capacitor is discharging through the multimeter. If the capacitor will not retain that voltage, it is defective and should be replace

Bulging tops

bulging top capacitor

Some capacitors do not require any test to determine failure. If a visual inspection of the capacitors reveal any sign of bulging tops, those need to be replaced. This is the most common failure in power supplies. When replacing a capacitor, it is of utmost importance to replace it with a capacitor of the same, or higher value. Never subsidize with a capacitor of lesser value.

If the capacitor that is going to be replaced or checked, does not have any markings on it, a schematic will be necessary. The image below from  shows a few symbols for capacitors that are used on a schematic.

Knowing if the capacitor needs replacement:

for testin a capacitor on a circuit board

an ESR meter will be the best approach in checking a capacitor that is still being installed in a circuit. If the capacitor is removed from the circuit then a multimeter set as an ohm meter can be used, but only to perform an all-or-nothing test. This test will only show if the capacitor is completely dead, or not. It will not determine if the capacitor is in good or poor condition. To determine if a capacitor is functioning at the right value (capacitance), a capacitor tester will be necessary. Of course, this also holds true to determine the value of an unknown capacitor.

Always discharge a capacitor before testing it, it will be a shocking surprise if this does not get done. Very small capacitors can be discharged by bridging both leads with a screw driver. A better way of doing it would be by discharging the capacitor through a load. In this case alligator cables and a resistor will accomplish this. Here is a  showing how to construct a discharge tools.

To test the capacitor with a multimeter, set the meter to read in the high ohms range, somewhere above 10k and 1m ohms. Touch the meter leads to the corresponding leads on the capacitor, red to positive and black to negative. The meter should start at zero and then moving slowly toward infinity. This means that the capacitor is in working condition. If the meter stays at zero, the capacitor is not charging through the battery of the meter, meaning it is not working.

This will also work with SMD caps. Same test with the needle of the multimeter moving slowly in the same direction.

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