Capacitors Explained. Learn how capacitors work, where we use them and why they are important.

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Remember electricity is dangerous and can be fatal you should be qualified and competent to carry out electrical work. Do not touch the terminals of a capacitor as it can cause electric shock.

What is a capacitor?

A capacitor stores electric charge. It’s a little bit like a battery except it stores energy in a different way. It can’t store as much energy, although it can charge and release its energy much faster. This is very useful and that’s why you’ll find capacitors used in almost every circuit board.

How does a capacitor work?

I want you to first think of a water pipe with water flowing through it. The water will continue to flow until we shut the valve. Then, no water can flow.

If, after the valve we let the water flow into a tank, then the tank will store some of the water but we continue to get water flowing out of the pipe. When we close the valve, water will stop pouring into the tank but we will still get a steady supply of water out until the tank empties. Once the tank is filled again, we can open and close the valve and as long as we do not completely empty the tank, we get an uninterrupted supply of water out the end of the pipe. So we can use a water tank to store water and smooth out interruptions to the supply.

In electrical circuits, the capacitor acts as the water tank and stores energy. It can release this to smooth out interruptions to the supply.

If we turned a simple circuit on an off very fast without a capacitor, then the light will flash. But if we connect a capacitor into the circuit, then the light will remain on during the interruptions, at least for a short duration, because the capacitor is now discharging and powering the circuit.

Inside a basic capacitor we have two conductive metal plates which are typically made from aluminium or aluminium as the Americans call it. These will be separated by a Dielectric insulating material such as ceramic. Dielectric means the material will polarise when in contact with an electric field. We’ll see what that means shortly.

One side of the capacitor is connected to the positive side of the circuit and the other side is connected to the negative. On the side of the capacitor you can see a stripe and symbol to indicate which side in the negative, additionally the negative leg will be shorter.

If we connect a capacitor to a battery. The voltage will push the electrons from the negative terminal over to the capacitor. The electrons will build up on one plate of the capacitor while the other plate will in turn release some electrons. The electrons can’t pass through the capacitor though because of the insulating material. Eventually the capacitor is the same voltage as the battery and no more electrons will flow.

There is now a build up of electrons on one side, this means we have stored energy and we can release it when needed. Because there are more electrons on one side compared to the other, and electrons are negatively charged, this means we have one side which is negative and one side which is positive, so there is a difference in potential or a voltage difference between the two. We can measure this with a multimeter.

Voltage is like pressure, when we measure voltage we’re measuring the difference or potential difference between two points. If you imagine a pressurised water pipe, we can see the pressure using a pressure gauge. The pressure gauge is comparing two different points also, the pressure inside the pipe compared to the atmospheric pressure outside the pipe. When the tank is empty the gauge reads zero because the pressure inside the tank is equal to the pressure outside the tank so the gauge has nothing to compare against. Both are the same pressure. The same with voltage, we’re comparing the difference between two points. If we measure across a 1.5V battery then we read a difference of 1.5V between each end, but if we measure the same end we read zero because there’s no difference, it’s the same.

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Coming back to the capacitor, we measure across and read a voltage difference between the two because of the build up of electrons. We still get this reading even when we disconnect the battery.

If you remember with magnets, opposites attract and pull towards each other. The same occurs with the build up of negatively charged electrons, they are attracted to the positively charged particles of the atoms on the opposite plate, but they can’t ever reach them because of the insulating material. This pull between the two sides is an electric field which holds the electrons in place until another path is made.

If we then place a small lamp into the circuit, a path now exists for the electrons to flow and reach the opposite side. So the electrons will flow through the lamp, powering it and the electrons will reach the other side of the capacitor. This will only last a short duration though until the build up of electrons equalises on each side. Then the voltage is zero, so there is no pushing force and no electrons flow.
Once we connect the battery again, the capacitor will begin to charge. This allows us to interrupt the power supply and the capacitor will provide power during these interruptions.

Examples

We use capacitors everywhere. They look a little different but they’re easy to spot. In circuit boards they tend to look something like this and we can see them represented in engineering drawings like this. We can also get larger capacitors which are used for example on induction motors, ceiling fans or air conditioning units and we can even get enormous ones like this which are used to correct poor power factor in large buildings.

On the side of a capacitor we will find two values. These will be the capacitance and the voltage. We measure the capacitance of the capacitor in the unit of Farads which we show with a capital F, although we will usually measure a capacitor in microfarads so we have a micro symbol just before this which looks something like the letter U with a tail.

The other value is our voltage which we measure in volts with a capital V, on the capacitor the voltage value is the maximum voltage the capacitor can handle.

This capacitor is rated at a certain voltage and if I exceed this value then it will explode.

Most capacitors have a positive and negative terminal. We need to make sure that the capacitor is connected correctly into the circuit.

Why we use them

One of the most common applications of capacitors in large buildings is for power factor correction. When too many inductive loads are placed into a circuit, the current and voltage waveforms will fall out of sync with each other and the current will lag behind the voltage. We then use capacitor banks to counteract this and bring the two back into alignment.

Another common application is to smooth out peaks when converting AC to DC.
When we use a full bridge rectifier the AC sine wave is flipped to make the negative cycle flow in a positive direction, this will trick the circuit into thinking it’s getting direct current.

But, one of the problems with this method is the gaps between the peaks. So we use a capacitor to release energy into the circuit during these interruptions and that will smooth the power supply out to look more like DC.
 

Q: What's a capacitor?

A: A capacitor, or cap, is an electronic component that can take up, store, and discharge electrical energy. Because they can do all that quickly, capacitors are used to filter or buffer any sudden changes in a circuit's voltage, smoothing the ensuing signal.

Q: What's a capacitor used for?

A: In car audio, large outboard capacitors, sometimes called stiffening caps, are used to prevent lights from dimming when loud bass notes play. They accomplish this by supplying the amplifier with a quick jolt of power.

Q: How do I hook up a capacitor?

A: If you don't have the instructions that came with your capacitor, you should know first off that a cap can be dangerous. It can charge and discharge so much power so very quickly that it can weld metal objects, like tools and jewelry, and melt its own insides out.

A new cap comes completely discharged, so it's safe. A resistor or wired light bulb usually is included in the package. Wiring the bulb or resister across the cap's terminals allows the capacitor to discharge slowly and safely. The same bulb or resistor, wired differently, also gets used to charge up the cap safely.

As in all car electronic installations, start by disconnecting the ground cable from the car battery. In this installation, also take out the in-line fuse on the amp's power wire next to the battery.

A capacitor should be mounted as close to the sub amp as possible using the shortest wires possible. This is so the extra charge doesn't have far to go to get to the amp quickly. Make sure the cap gets mounted securely and won't become a dangerous flying object in the event of an accident.

A capacitor has two poles: a positive and a negative. They should be clearly marked on the capacitor. The positive connects to the same positive power lead that goes to your sub amp's positive, 12 volt, connection. Use the same gauge wire as the amp uses for its power. This can be accomplished with a distribution block. Or, sometimes, the cap comes with multiple connection terminals that make it easier to wire it into your system. The multiple terminals act just like a distribution block so, for instance, the power wire coming from your battery can connect directly to the cap's positive terminal while a short cable connects from there to the amp's positive power connection. The negative pole of the capacitor connects to your chassis ground, just like the amp. The best practice is to use the same bolt the amp uses for ground. Make sure all the paint is scraped off around where you put the chassis ground and the connections are clean and tight.

Next, you need to charge up your capacitor. If done too quickly — it could "pop," destroying the cap.

If you don't have the original charging/discharging resistor or light, you'll need to get one. An automotive 12-volt test light, the kind with a bulb, not a small LED, will do nicely. Otherwise, you can use a high-wattage, low resistance resistor, available at most electronics parts stores. The exact value doesn't matter, but get one with a value of 10-1,000 ohms along with a rating of 1-20 watts. The lower the resistance, the higher the wattage should be.

Take the test light or resistor and connect it to the two terminals of the amp's in-line fuse holder (where you took the fuse out earlier). Re-connect the car battery's ground cable. The resistor will get hot, or the bulb will light up, while the cap charges. After 10 to 30 minutes, the bulb will fade out, or the resistor will start to cool. Remove the light or resistor carefully — they can get very hot. As you replace the fuse, you may experience a small spark — that's okay, but should remind you of how powerful the electric forces involved are. Your capacitor is now installed.