Lesson 02: Basic components and Ohm's Law

  1. Identify the main components present in almost every electronic circuit.
  2. Briefly describe its main characteristics and purpose.
  3. Know its limitations, specifications, and classic circuits that use them.

This is a basic lesson

No previous knowledge is requiered.

If you find it too easy, move on to the next lesson, or check our basic tutorials on electronics here.

The most basic element: the resistor

I’m pretty sure you have heard this term more than once. But, do you actually know what is a resistor? A resistor is a device that limits the current flowing through it and offers resistance against it (this is why it is called resistor :D). To understand a resistor, we need to know a little bit more about how the electricity behaves. We have already introduced the concepts of voltage and intensity in the previous lesson (Lesson 01). The physical law that relates those two magnitudes is called Ohm’s Law. Ohm’s law states that the relationship between voltage (energy of the electrons) and intensity is directly proportional:

V=I·R

US Resistor symbol vs EU resistor symbol
US vs EU resistor symbol. Both of them mean exactly the same.

The resistor, allows us then to select that constant to get the desired magnitudes on our electronic circuit. The resistance R is measured in Ohms (\Omega) and can be bought from a wide range of standard values and precisions, according to design needs. The symbol that we use in schematic drawings to identify those components is a rectangle with two terminals (in Europe) and a zig-zag wire  (in the US).

Let’s put an example that will clarify the what I’ve just explained. If we know the current that is flowing through a resistor, we can calculate  the voltage drop across the resistor by multipliying the measured intensity by the resistance value (provided by the manufacturer).

Consider the following circuit:

200 mA current source connected in series to a 2 Ohm resistor.
Current source connected to a resistor.

We know that the current source is providing 200 mA, which are flowing to the top of the resistor and going back to the current source after passing through the resistor. The resistor is rated as a 2 \Omega resistor (nominal value).  What voltage difference will appear between the top and the botton of the R1 resistor? We can use Ohm’s law to calculate it:

V=I·R=200mA · 10^{-3} \frac{A}{mA} · 2\Omega=0.4 V

Note (and this is particularly important) that what this means is that there is a voltage  drop of 0.4V across the resistor. Now, if we set an arbitrary reference point of 0 V called ground, earth, or mass (although they are not exactly the same thing), at the negative pole of the current source, we can determine that the positive pole of the current source and the part of the resistor that connects to that pole ( as they are connected by the same wire), have a voltage of

V^+=V^-+\Delta V_{R1}=0+0.4V · 10^3\frac{mV}{V} = 400 mV

Other basic elements: capacitor, inductance and LED

So, we’ve seen resistors so far, but there are more elements that behave in a different way and can be incorporated in our circuits. A capacitor is a very common element that allows us to store energy inside of it. The usages are huge and range from line filtering, to timming applications. We can put a conveniently placed capacitor and create a circuit that activates when the capacitor is charged, but not before. As we can calculate (by means of a very simple formula) the time that will take to fully charge that capacitor, we can set a delay in our circuit. Physically, the most popular ones (electrolitic capacitors) look somewhat like a cell; but we have many types of them depending on the particular application that we need them for.

Colorful LEDS
LED of different colours and sizes.

Inductances are more or less the same than a capacitor, but they work with current, rather than voltage. They consist of a wire wrapped around air or a metallic core, and can have one or more turns. Their utility is very reduced in DC (direct current) circuits (that is, circuits in which the current flows consistently in one direction, as opposed to AC circuits).

Last, but not least, LED stands for “Light Emitting Diode” and it is a very special type of diode that emits light while in operation. They’ve recently gained popularity due to their reduced energy consumption and high endurance. A diode is an electronic component that only lets the electricity flow in one direction (directly polarized). If you polarize it the other way around, the component will block the circuit, and this will behave as if someone had cut the cable right at the diode. LEDs, of course, also show this behaviour, although this is not (generally) their main purpose.

The symbols that represent those components in schematic drawings are shown below:

Main symbols that represent the basic electronic components. From left to right: LED, diode,, capacitor (non-polarized), capacitor (polarized) and inductance.
Main symbols that represent the basic electronic components. From left to right: LED, diode, capacitor (non-polarized), capacitor (polarized) and inductance.