# Chapter 2: Ohm Law DC and Resistors

## DC Voltage

We saw in the previous chapter that in order to get an electric current flow, it is required an energy source.

Electronic circuits usually work with a direct current voltage source. This means that voltage does not change its polarity (positive or negative) and its value remains relatively constant.

An example of DC voltage source is a battery or a regulated voltage source. In a circuit, we represent a DC voltage source with this symbol, where "+" and "-" are obviously the positive and negative terminals, and VDC represents the voltage value, for example 9 Volt.

## Resistor

We learnt about resistive materials in previous chapter.

This was a fanciful representation of the material interior. Here we can see how resistors

look like in the real world, and the symbol that is used to represent them in a circuit schematic. Later we will learn how to read the band color code. The main technical data that identifies a resistor is its resistance value, expressed in Ohms [ Ω]. When you go to a component shop, you will ask for a resistor of of 47 Ω, or 220 Ω, etc. The higher is the value in ohms, the higher is the resistance that the resistor offers to the current flow. For values of thousands ohms, the "K" prefix (Kilo) is used, for example 1000
= 1KΩ, or 22000 Ω = 22KΩ. For values of millions ohms, the prefix "M" prefix (Mega) is used, for example 1000KΩ=1MΩ and so.

## Ohm Law

There could be much more to be said about resistors, but it's time to introduce our first and most basic circuit, using a DC voltage source and resistor.

Here, the voltage V is applied directly to the resistor R terminals, generating a current flow of intensity I. Is it possible to know the level of current, knowing the voltage and resistance values? Fortunately there is a formula that relates these three dimensions: the Ohm Law:

Just that! And it is the most useful formula for electrical and electronic design. Let's assume for instance that we have a circuit with a 12 V battery and a 6 ohm resistor. What will be the current intensity?

Easy!  Now look this case.

We have a 220Ω resistor, and we want to supply a current of 25mA to it. Which voltage should we apply? In the ohm law ecuation, let´s isolate the voltage V:

So, with a 5.5V power supply, we obtain the required current.

There is a third case to consider. Let's assume we have a 15 V power supply, and we need to generate a current of 15mA.

What resistor will we choose in order to get this current? We need now to isolate R in the ecuation.

For the range of values used in electronics, is often more comfortable to express the current in mA and the resistance in Kilo ohm; keeping both units this way, you can use the ohm law in its three forms without doing conversions, and voltage results in Volts.

As simple as it looks, Ohm law is present everywhere in electronics.

## Power

The electricity involves energy, for example to generate light, movement or heat, the electric circuits deliver energy to a lamp, a motor or a heating resistor. There is a magnitude called Power that represents the amount of energy that a circuit or a component delivers per time unit. Electric energy over certain part of a circuit is calculated as de product between voltage and current, and its unit is the Watt.

When we have a voltage V applied over a resistor R, and circulates a current I, we can calculate the power over the resistor with the following two formulas:

For instance in the circuit below we know all the variables, so we can calculate the power in both ways, verifying the same result.

## Resistors Color Code

You may have seen that the resistors are not text-labeled, instead they have some bands with different colors.

Standard resistors for electronics have three color bands, and a fourth golden or silver band. In order to identify the resistance value, the first thing to do is to orientate the resistor with the golden or silver band to the right. This fourth band speaks of value tolerance, golden means 5% and silver 10% but this is not the most relevant data.

The resistance value (ohms) is coded into the color bands. As shown in the table, each color represents a digit from 0 to 9, (e.g. red=2 and green=5).

Starting from the left, the first band represents the first digit of the resistance value, the second band represents the second digit, and the third represents how many zeros we have to add to complete the number.

Complicated? Come on! It is very simple. Let's take the resistor above as an example. The first band is yellow, so the first digit is 4, the second band is violet, so the second digit is 7, till now we have formed the number 47. The third band is red, which means we have to append 2 zeros at the end of the number. The result: 4700 Ω = 4.7 K
.

We can see here some additional examples:

## Resistor Measurement

You can measure a resistor simply by setting the multimeter to resistance measurement, and contacting the terminals with the probes. It's the same thing in both ways, there is no polarity.

If the resistor is mounted on a board, the circuit must not be energized, and at least one terminal should be disconnected before applying the tester probes.

## Resistor Commercial Values

Although it is possible to buy resistors of many types and different values, the typical set to be found in shops is the 5% tolerance type, and the next series of resistance values:

On the next chapter we will go more in deep with resistors circuits.