Introduction:
Every electrical consumer has an (internal) resistance. An electrical conductor, such as wiring, indeed has a low resistance value, yet there is still a specific resistance that depends on the material, dimensions, and temperature. Every consumer also has a resistance value. The resistance value ultimately determines how much current flows through it.
Resistors are found as components in almost all electronics. In automotive technology, resistors are also used in electrical circuits on, for example, circuit boards. A resistor limits the electrical current through a circuit and converts electrical energy into heat:
- increasing resistance: a progressively smaller current flows through the circuit;
- decreasing resistance: the current increases.
The resistor on a circuit board is connected in series with a component where the current must not become too high.
The unit of resistance is ohm and is denoted by the Greek letter omega 9a. We use the letter and symbol R for resistance (derived from the English translation: Resistor).
A resistor can be designed as a fixed resistor or an adjustable resistor. In the adjacent image, the symbols of these two types of resistors are shown. The symbol consists of a rectangle with a line on each side. Often, in a diagram, the letter R is mentioned in or next to the rectangle with the resistance value in ohms.
- Resistors with a fixed resistance value are often recognizable by the colored rings around the housing. The resistance value can be determined based on the colored rings;
- Resistors with a variable value are usually adjustable with a knob. This type of resistor can also be designed as a potentiometer, which is often used as a position sensor.
The following paragraph shows the different types of resistors that we can encounter in the automotive sector.
Types of Resistors:
The image below shows an overview of twelve different types of resistors. Below the image, the construction and application of each type of resistor are described by category.
The commonly used resistors in automotive and automotive training are shown below. For each resistor, the construction of the materials from which the resistors are made is described.
Carbon Composition Resistor:
This resistor has a cylindrical shape and features color bands used to identify the resistance value. The resistance element consists of carbon powder or graphite powder mixed with ceramic clay. The resistor is coated with a molded plastic housing. These resistors are known for a poor temperature coefficient and low reliability in terms of noise and precision. This type of resistor has been replaced by the film type.
Carbon Film Resistors:
The carbon film resistor consists of a ceramic substrate with a thin layer of carbon film over it. The resistance value is determined by the groove.
Metal Film Resistor:
The metal film resistor is very similar in construction to the carbon film resistor. However, in this type of resistor, a metal film is applied to a ceramic substrate.
Metal Oxide Film Resistor:
The construction of this resistor has many similarities with a metal film and carbon film resistor. Instead of metal or carbon, a metal oxide film is deposited on the ceramic substrate.
Wirewound Resistor:
The wirewound resistor contains a metal resistance wire that is wrapped around ceramic material. The resistance depends on the thickness of the metal wire. The accuracy of the wirewound resistor is high. The temperature resistance coefficient is so low due to the resistance wire that this resistor is highly suitable for applications where high power is required.
SMD Resistor:
The SMD resistor is often called a “chip resistor” and consists of a metal alloy (made of metal oxide or a metal film) with a three-layer electrode structure on each side. The length, thickness, and material used determine the resistance value. The inner electrode is connected to the metal alloy. The middle electrode is made of nickel and functions to ensure heat resistance during soldering. The outer electrode is a tin layer and makes the resistor suitable for direct soldering onto the circuit board.
Potentiometer:
The potentiometer acts as a variable resistor. The resistance value depends on the position of the wiper on the carbon track. When power (often 5 volts) and ground are connected, the output voltage ranges between 0.5 and 4.5 volts, depending on the position of the dial.
More information can be found on the page: Potentiometer.
Properties of Resistors:
When we want to use resistors, we need to choose the type of resistor that suits the required properties: is high accuracy at low power needed, or is high power required where system noise is not important?
- Maximum voltage: the maximum voltage of a resistor must not be exceeded. If it is, breakdown can occur. This can affect the resistance value;
- Maximum power: if the power of the resistor is exceeded, the temperature will become too high. The resistance value can change. The power of carbon resistors often ranges from 0.25 Watts to 1 Watt, and of wirewound resistors from 3 Watts to 20 Watts.
- Tolerance: a resistor never has exactly the value specified on the housing. However, a percentage is specified on the housing indicating the deviation. This percentage deviation is caused by the precision in the manufacturing process. A resistor of 120 ohms with a tolerance of 5% can measure between 114 ohms and 126 ohms.
Color Coding:
The value and tolerance of a resistor are indicated on a carbon or wirewound resistor using a color coding (color bands) on the resistor’s housing. It is important to start reading from the correct side:
- the first ring is often closer to the end of the housing;
- the first ring is often wider;
- the last rings can be silver or gold. These colors are not used for the first rings.
When a resistor has four rings, the meaning of the rings is as follows:
- Ring 1 and 2: resistance value;
- Ring 3: multiplication factor;
- Ring 4: tolerance.
In the illustration, we see a resistor with the 1st ring brown, 2nd ring black, 3rd ring red, and the fourth ring gold. In the table, we read the numbers: 10*100 ± 5%. The resistance value is 1000 9a (1 k9a) with a tolerance of 5%. The actual value is between 950 and 1050 9a.
Resistors come in series. We often encounter the E12 series, in which the resistance values increase as follows:
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82.
These values can be divided or multiplied by ten, for example, 100, 120, 150, 180. Or, 1000, 1200, 1500, 1800. There are no resistors of 130 ohms.
