Capacitors: Power supply

A capacitor is a passive electronic component that stores electric charge: it is an energy store. Its storage capacity is measured in Farads, in honor of the British physicist and chemist Michael Faraday, to whom we owe the principles that make electric motors and electrolysis processes (such as chrome plating or anodizing) work. But the Farad, which incidentally is written with a capital letter like all units named after a person (such as the Watt or the Joule, and unlike the meter or the kilogram), is an enormous unit: it represents the capacitance of a capacitor that, when polarized (connected) to a potential difference of one Volt, acquires a charge of one Coulomb, meaning that approximately six million trillion electrons are set in motion (a six followed by 18 zeros). For this reason, fractions of this unit are always used; the most common are micro-Farads (one millionth), nano-Farads (one billionth) or pico-Farads (one trillionth of a Farad).

After this salad of units and figures, let's come back down to earth. We saw that power supplies need, on the one hand, to filter the rectified current (from alternating, coming from the socket, to direct, which is what components need) and, on the other, to have a certain amount of stored energy to meet rapid current demands (when reproducing the strike of a hand on a drum, for example).

FILTERS

Remember how a power supply works: a transformer stepped down the mains voltage (220V) to the level suitable for the device (normally between 5 and 18V), and a rectifier converted that alternating current into the direct current needed by the components. But such a basic power supply still delivers a very impure voltage: that direct current still contains a large AC component and is noisy, so it needs to be filtered, and for that purpose components capable of storing and returning energy quickly and sequentially are used (taking energy when the still irregular direct current presents a peak, and returning it when it presents a trough). That is why capacitors are used in this role, since that is precisely their specialty (storing and delivering energy), and just by adding a relatively small capacitor to that power supply we will notice a significant difference in the quality of the current supplied.

A side effect in these cases is a slight increase in voltage, because the capacitor brings it closer to the peaks of that sequential wave, all the more so the greater its capacitance. This is something anyone who has played with Scalextric using an old power pack (which were unregulated) knows well: fitting a capacitor to a car makes it noticeably faster, because a higher voltage reaches the motor.

ENERGY

Capacitors are wonderful: not only do they achieve the miracle of drawing cleaner direct current from the power supply, raising its voltage slightly in the process, but they also allow that power supply to deliver, at specific moments, more energy than it was designed for, thanks to their ability to store and rapidly deliver energy.

This capability is widely used in audio, and when looking at a photo of any amplifier, our gaze should first go to the main power transformer (as we discussed at the time, this is where the "myth" that the heavier, the better comes from, something that changes with switch-mode power supplies). But second, and no less important, we should look at the size and number of capacitors in the power supply: a good capacitor bank will ensure perfect dynamics and that, at least as far as the power supply is concerned, the component will never run out of breath...

Is it possible to calculate the energy stored in a capacitor? Of course: the formula tells us that it is directly proportional to its capacitance and proportional to the square of the charging voltage. That is why it is important to look at the capacitance of the power supply capacitors, but also (in fact, more so) at their voltage. And all of this will be reflected in their size: just go to any electronics store and compare capacitors of identical capacitance but higher voltage (much larger), whereas when capacitance increases, size does not increase in the same proportion.

TYPES OF CAPACITORS

There are many ways of making a capacitor, but the principle is always the same: two metal plates separated by an insulating layer. When we apply a voltage to these plates, they acquire an electric charge: positive on one side and negative on the other (the overall charge remains balanced at zero). The larger the surface area, the greater the capacitance, which is why they are made from materials that allow some kind of "rolling up".

In power supplies, electrolytic capacitors are used above all, so called because one of the "plates" is an electrolyte and the other is the container, usually made of aluminum; the dielectric is a thin layer of aluminum oxide between the two, and this makes it possible to have high-capacitance capacitors of reasonable size, perfect for power supplies.

Because of their construction they are polarized, that is, their terminals (positive and negative) are always marked, and if they were connected incorrectly, when current flowed the oxide layer would disappear, a short circuit would occur between the electrodes, and the capacitor would heat up until it exploded.

Relatively much larger in size are paper capacitors (which use paper, or oil-impregnated paper, as the dielectric) and those that use plastic materials (polypropylene, Teflon, polystyrene...), as well as ceramic capacitors. They are less relevant in power supplies, and we will look at their applications in the next issue.

COMBINATIONS

A group of capacitors will also behave as a capacitor, but how do we calculate its capacitance? Although it may seem complicated, it is actually very easy to know how to combine several capacitors: when they are connected in parallel, that is, in the case of power supply electrolytics, always positive to positive and negative to negative, the equivalent capacitance of the group is simply the sum of the individual capacitances. They should preferably have the same voltage rating, and you should never use a capacitor in an application where it may be exposed to a higher voltage than its marked maximum (the one indicated on its casing).

To improve high-frequency filtering, advanced power supplies (either in the supply itself or locally, near the circuit being powered) commonly use several different capacitors in parallel: rather than to add up capacitances (the largest is an electrolytic and the others are much smaller film capacitors), the idea is to take advantage of the qualities of each type at those high frequencies.

Series combinations of capacitors (positive to negative, or in line) are more complex to calculate and result in a capacitor whose capacitance is the inverse (1/C) of the sum of the inverses of the individual capacitances. A simple example: two 10-microFarad capacitors in series would result in one of five.