Radio & Electronic Projects: What is a capacitor?

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Introduction

A capacitor is a device that stores energy in the form of electricity. Much less energy than a battery, and for a shorter time, however. The simplest form of capacitor takes the form of two flat metal sheets separated by air; connections are made to each plate, as Figure 1 shows. If you imagine a pair of these plates, 30 cm square and separated from each other in air by 1mm, the capacitance of this device would be almost exactly 80 picofarads (pF), i.e. 80 million-millionths of the unit of capacitance, the farad. Now this is quite a small value, you will agree, and it comes about because the farad is such a large unit. Nevertheless, as you will probably know, we may have capacitors of value 10000 microfarads (uF) in our radio equipment, and they can be smaller than your little finger, so they are obviously not made the same way!


Figure 1 Large and small.

We cannot go into the manufacture of capacitors here -- after all, we are users of the devices, not the designers! First of all, beware of incorrect statements; the words 'capacitor' and 'capacitance' are not the same. For example, a large capacitor would be a description of one the size of a toilet roll. It need not have as large a capacitance as one the size of your little finger. A 'large capacitor' is one which is physically big, a 'large capacitance' refers to a capacitor which can store a larger amount of energy when a certain voltage is applied between its plates. The capacitors in a mains power supply are usually big and have large capacitances. High-power RF amplifiers may have large capacitors with small capacitances!

Electrolytics

Electrolytic capacitors usually have capacitances of 1uF or above. They differ from other capacitors in that they must be connected the right way round (i.e. they have positive and negative connections, just like a battery).

They may explode if the connections are reversed!

. . . and the others

Other capacitors may be connected either way round, despite their names.

We have polystyrene, ceramic, silver-mica and tantalum. Each has its own advantages and disadvantages, and the parts list for a project will always tell you which type is best.

Storing energy

If you were to connect a large capacitance across a 12V power supply, nothing would appear to happen. Removing the capacitor from the supply and connecting it to a voltmeter would show that the capacitor has 12V between its ends. This shows that, while the capacitor was connected to the supply, energy flowed from the supply into the capacitor. We say that the capacitor was charged up by the supply. If you are using an analogue voltmeter (i.e. one with a meter and pointer), you will notice that the indicated voltage slowly drops until, eventually, there is no voltage across the capacitor. This is because the capacitor has discharged its energy into the voltmeter. If you had used a smaller capacitance, the same would happen, except that the voltage would drop to zero more quickly - the capacitor stores a smaller amount of energy because its capacitance is smaller. Capacitors behave like other things in life -- a small car can move more quickly than a large bus -- a small piccolo emits a higher note than a flute -- the voltages in a circuit containing a small capacitance will change more quickly than those in a circuit with large capacitance.

Varying the capacitance

Some capacitors are capable of having their capacitance changed manually; these are called variable capacitors. They work like the basic capacitor of Figure 1. Imagine moving the top plate of the pair a little to one side; the capacitance is determined, not just by the size of the two plates, but by their area of overlap. As this decreases, so does the capacitance. Such devices are limited in their capacitance, about 500 pF being the maximum value.

AC and DC

Because the plates of a capacitor do not touch each other, a direct current (DC) cannot pass between them. However, an alternating voltage on one plate can induce an identical alternating voltage on the opposite plate, and thus a capacitor appears to pass an alternating signal, even though currents as such, do not pass between the plates. This property of passing AC and not DC is very important, and a capacitor used in this way is called a DC blocking capacitor or, simply, a blocking capacitor . A blocking capacitor can be used at the same time, to couple a signal from one circuit to the next; here it would be known as a coupling capacitor . Decoupling capacitors are to be found where the capacitor is employed to remove an AC signal while retaining a DC component.

Finally...

Unlike resistors, the manufacture of capacitors renders them susceptible to excess voltage, so if you find a capacitor labeled 10uF 16V, it means that operating it above 16V may fatally damage the device (and the circuit around it). This voltage is called the working voltage of the capacitor; on some electrolytics, you may find it expressed as volts working (i.e. 8uF 450V WKG).

Many smaller capacitors have their properties marked on them in a color code, like resistors. Figure 2 shows these codes, and their meaning, and the table below summarizes the values of the colors.

Table 1



Figure 2 Some capacitors have colored bands or stripes, rather like resistors.

The color code, which is the same as the resistor code, is shown in Table 1.

The band shown on the chart as '1st' is the first number of the capacitor's value in pico-Farads, '2nd' is the second number and 'M' is the Multiplier or number of noughts. For example, a capacitor reads from the top: Brown, Black, Yellow, Black, Red. Its value is One, then Nought, then Four more noughts = 100 000 pF (also referred to as 0.1uF or 100 nF). Its tolerance (Black) is 20% and the working voltage (Red) is 250 V. The 'V' means the maximum working voltage. The band marked 'T' shows the tolerance, just like resistors, and the one marked 'TC' is only used on special capacitors designed to change their value with temperature.

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