Voltage sources and resistors

Introduction

When developing for embedded systems, having knowledge about electronics gives much more freedom on what we can create. Instead of using only shields or demonstration boards, we can develop custom solutions for our needs, allowing for a greater efficiency and a lower cost.

This post explains some very basic principles about electronics, namely DC voltage sources and resistors. I’ll try to make some posts about more basic electronics every once in a while, in order to complement the other posts about embedded systems.

Voltage sources

A voltage source is one of the most basic elements of a circuit. An ideal DC voltage source has two terminals and can maintain a fixed voltage difference between those terminals, independently of the load at which it is applied.

Typically, a DC voltage source is represented in an electronic schematic as shown in figure 1. In that example, the power source imposes a 5 V difference between points A and B, meaning that any load that we connect between those points will have a voltage difference of 5 V between its terminals.

Figure 1 – Representation of a 5 V DC voltage source.

An ideal voltage source is able to provide an infinite amount of current to the circuit where it is applied. Nevertheless, we need to keep in mind that ideal voltage sources are mathematical representations that don’t exist in reality, meaning that we always need to take into account the maximum current our source will be able to supply. Real world examples of power supplies are batteries.

Resistors

Resistors are passive elements of the circuit that are typically used to limit current or to divide voltages.

Resistors are measured in ohms (unit represented by the symbol Ω) and the relationship between voltage and current at its terminals is given by Ohm’s law, as shown below. In this equation, U represents voltage (in volts), I represents current (in amperes) and R represents resistance (in ohms).

There are two typical representations or resistors in a schematic, as shown in figure 2. Nevertheless, I prefer to use the one on the left, since the one of the right can be used to represent an impedance, a more generic load.

Figure 2 – Two typical representations of resistors in a circuit.

A basic circuit

A simple circuit to illustrate both components described before is shown in figure 3.

 Figure 3 – Simple resistive circuit.

Since the resistor is connected to the terminals of the voltage source, it has a voltage difference (U) of 5 V across its terminals. Since we also know R, we can solve Ohm’s law in order to I, to find the current flowing in the circuit:

From the equation above, we know that if we increase R and maintain the voltage source, there will be less current flowing in the circuit, thus explaining why resistors are used as current limiters.

Note: The schematics presented were created using an online free tool available here.