A Quick Reference to the most Common Electronic Components

In this section we will briefly refer to the most significant electronic parts that, among others, are also used into PSUs. For those of you that don't have an electronics background, you should read carefully this section since it will help you understand better the current article.

Inductors - Transformers

An inductor, or induction coil, stores electrical energy in a magnetic field. The inductors are used almost everywhere and especially in PSUs, they play a role of high importance. An inductor is simply a coil of wire wrapped around a core (iron, ferrite or simply air). Depending on their usage they have several names: coils, chokes, solenoids etc.

Now how they work? Actually is very simple, if current passes through an inductor then a magnetic field is created around the wire. Every change in current affects the magnetic field which by its turn induces voltage across the inductor. That voltage creates a current flow opposite to the initial current. The above property is known as inductance (L) and is measured in henrys. Because the latter is a quite large unit you will usually come across millihenrys (mH) or microhenrys (µH).

Some interesting facts about inductors:

• They store electrical energy in magnetic fields
• They act as open circuit at first when we apply DC (Direct Current) to them, but after a while they freely allow it to pass.
• They oppose to current changes

Now let's take a quick look at transformers. Inductors usually are shielded so their magnetic fields do not interact with other components in the same circuit. However, if we place two unshielded inductors side by side and feed one of them with AC (Alternating Current) then its magnetic field induces a voltage not only in the current inductor but also in the other. The process of inducing voltage in the second inductor is called mutual inductance. So if you pass current from one inductor then you create voltage in the near inductor. A transformer is nothing more than two inductors/coils wounded up around the same core material in a way that mutual inductance is at maximum level. The coil that lets current pass is called primary and the coil that is induced with voltage is called secondary. A transformer can electrically isolate two circuits and also step up/down voltages.

Capacitors

Capacitors can be used to smooth out voltage, as reservoirs for electrical energy storage, block DC current etc. A capacitor is consisted of two metal plates which are separated by an insulator, the dielectric. One of the most notable features of capacitors is their oppose to voltage changes, meaning that if suddenly the voltage applied to a capacitor is changed the latter cannot react immediately and the voltage across the capacitor changes more slowly compared to the applied voltage.

Capacitors allow DC to pass very briefly before they block it. On the contrary AC passes freely from them (but with changed, rectified, shape). We calculate the charge a capacitor can store, called capacitance, in Farads. However a Farad (F) is a very large unit so you will usually meet micro-Farad (µF or uF) or pico-Farad (pF). Besides their capacity the two most significant features of a capacitor are: its working voltage and the temperature rating (and for those that have polarity the negative lead marking). In PSUs the best capacitors are the ones labeled at 105 °C since they have increased lifespan compared to the ones labeled at 85 °C. Of course the manufacturer plays a key role and Japanese made capacitors are always preferred.

There are various types of capacitors depending on their construction and the materials used. Some of the most common types are dielectric, film, ceramic, electrolytic, glass, tantalum, polymer etc. In PSUs we mostly meet electrolytic and polymer and in the transient filtering/APFC stage Y (ceramic) and X (metalized polyester) capacitors. Y capacitors are placed from line to earth and always come in pairs while X capacitors are placed across the line. If the first short-circuit there is a high risk for an electric shock to the user and if the latter short there is a fire risk.

If we place two or more capacitors in parallel then their capacitances are added (1). On the contrary if we connect them in series then their total capacitance is reduced (2)

(1) C_Parallel = C₁ + C₂ + C₃...
(2) C_Serial = 1/(1/C₁ + 1/C₂ + 1/C₃...)

Resistors

Resistors are the most commonly used electronic component. Their role is to simply restrict the flow of electrical current when necessary and make sure that the correct voltage is supplied to a component. We measure resistance in Ohms; however an Ohm represents a very small resistance so in most cases you will meet resistance measured in KO (1000 O) or MO (1,000,000 O).

When we combine several resistors in series then we simply add their resistance (3). Note that the same current flows through all resistors connected in series but there is some voltage drop at each resistor.

(3) R_Series = R₁ + R₂ + R₃...

When we combine several resistors in parallel then we decrease the overall resistance (4). In addition, when there are multiple resistance branches in a circuit the current flowing into each of them is inversely proportional to the resistance of the branch.

(4) R_Parallel = 1/(1/R₁ + 1/R₂ + 1/R₃...)

Since we made it so far we should mention the famous Ohm's law: Voltage equals current multiplied by resistance (5). Another equally famous law is Joule's one (6) which gives the relation of power (P) with voltage (V) and current (I).

(5) V = I × R
(6) P = V × I = (I × R) × I = I² × R