DC Electrics - Circuit Protection and Capacitors — Page 42, Lesson 43

I want to walk you through capacitors — a fundamental component in DC electrical circuits. Let's start with the symbols you'll see on circuit diagrams. Figure 3.5 shows the electrical circuit symbols for various capacitors. There are four main types. First, a fixed non-polarized capacitor — it has two parallel lines of equal length, both straight. Then a variable capacitor — that's the fixed symbol with an arrow through it diagonally, indicating its value can be adjusted. Next is a preset capacitor — similar to the variable symbol but with a small line across the arrow, meaning it's set once during manufacture or installation and not adjusted in normal operation. Finally, a fixed polarized capacitor — this one has one straight line and one curved line. The curved line represents the negative terminal, and the straight line the positive terminal. With the polarized capacitor it is important to connect the positive terminal to the positive supply. Non-polarized types can be connected either way round. Now let's talk about what a capacitor actually does. The capacitance (C) of a capacitor measures its ability to store an electrical charge. The unit of capacitance is the farad (F). The farad is a very large unit, so in practice we subdivide it into smaller, more convenient units. One microfarad (1 µF) equals one millionth of a farad — that's 10⁻⁶ F. One nanofarad (1 nF) equals one thousand millionth of a farad — 10⁻⁹ F. One picofarad (1 pF) equals one millionth millionth of a farad — 10⁻¹² F. So you'll almost always see capacitors rated in µF, nF, or pF. What determines how much capacitance a capacitor has? There are three factors. First, the area of the plates — a large area gives a large capacitance. Second, the distance between the plates — a small distance gives a large capacitance. Third, the material of the dielectric — different materials have different values of capacitance. The dielectric is the insulating material between the plates. Examples include paper, mica, air, and fuel. The value of the dielectric is referred to as the dielectric constant (k). For example, waxed paper has a k value of about 3, whereas air has a k of 1. So a capacitor having waxed paper as its dielectric would have 3 times the capacitance of the same capacitor having air as its dielectric. There's also a critical rating called the working voltage. This is the largest voltage — either DC or peak AC — which can be applied across the capacitor. It is often marked on the case of the capacitor. If it is exceeded, the dielectric may break down and permanent damage result. So you must always choose a capacitor with a working voltage higher than the maximum voltage it will see in the circuit. Now let's look at what happens when you put a capacitor in a DC circuit. Figure 3.6 shows a capacitor in series with a battery and a switch. If the switch is closed, electrons are pushed by the battery onto one plate — let's call it plate Y — building up a negative charge. This charge exerts a repelling force across the dielectric which causes electrons to leave the other plate — plate X — and be attracted to the positive plate of the battery. While this charging action is taking place, electrons are passing through the connecting wires, but no current flows through the dielectric. That's a key point: the dielectric is an insulator, so no electrons actually cross between the plates. The current in the wires is just the movement of electrons onto one plate and away from the other during charging. Once the capacitor is fully charged, the current stops.