What is a Resistor?
A resistor is a passive electronic component that offers electrical resistance in a circuit. It’s a vital component in almost all electronic circuits, from the simplest to the most complex ones. Its main function is to limit the flow of electrical current.
Think of resistors like the water flow regulators in your household. Just as these regulators limit the flow of water in your shower, resistors control the flow of current in an electrical circuit.
The Science Behind Resistors
At the heart of resistor functionality is Ohm’s Law, which is represented by the equation: Where:
- is the voltage across the resistor.
- is the current flowing through the resistor.
- is the resistance value of the resistor.
Essentially, Ohm’s Law states that the voltage drop across a resistor is directly proportional to the current flowing through it.
Ohm’s law is without question the most important and most used formula in electronics. It is vital for calculations but also helps you to understand how current flows in a circuit and the effect of components such as resistors.
Different Types of Resistors
There are several types of resistors based on their function, composition, and tolerance:
- Fixed Resistors: These are the most common type and have a predetermined resistance value such as 330 ohms.
- Variable Resistors: These can change their resistance value. Potentiometers and rheostats are examples.
- Special Resistors: These have specific functionalities like temperature-sensitive resistors (thermistors) or light-sensitive resistors (photoresistors or LDRs). I have created many amazing circuits using these types of resistors.
How Do Resistors Work?
The working principle of a resistor revolves around the concept of “resistance”. When electrons flow through a conductor, they face opposition. This opposition is what we refer to as resistance.
Materials used in resistors have properties that make it harder for electrons to flow. This might be due to the atomic structure of the material, impurities added, or the physical dimensions of the resistor itself. When current flows through a resistor, energy is lost in the form of heat. That’s why resistors often get warm or even hot during operation. If you are able to get your hands on a thermal camera it offers an amazing opportunity to see where heat is generated in a circuit. This can be used to find faults in a circuit in the form of a short that will make something very hot.
Colour Codes and Resistance Value
Resistors usually come with colour bands, each representing a specific number. These colours help determine the resistance value of the resistor. The colour bands follow a standard sequence, and by reading them from left to right, you can determine the resistance value and tolerance of the resistor. Here is a chart which is used to determine the value of a resistor.
Applications of Resistors
- Voltage Division: In combination with other resistors, they can divide voltage in a circuit. I have provided an example at the end.
- Current Limiting: They prevent excessive current from flowing into sensitive components. An example of current limiting for an LED is at the end.
- Biasing: Used to establish predetermined voltages or currents at various points of an electronic circuit.
- Load: Acts as a component that consumes power, often used in testing scenarios.
- Timing: In combination with capacitors, they can create time delays.
Types of materials used in Resistors
The ability of a material to resist the flow of electrons is intrinsic to its atomic and molecular structure. Different materials have different resistances, and this property is exploited in the design and manufacturing of resistors. Here’s a deeper look into the types of materials commonly used in resistors:
1. Carbon-based Resistors
- Carbon Composition: These were among the first types of resistors commercially available. They’re made by mixing carbon powder with a binder (like clay). By adjusting the ratio of carbon to the binder, manufacturers can change the resistance of the end product. The mixture is then pressed into a cylindrical shape and leads are attached.
- Carbon Film: These are made by depositing a thin carbon layer on an insulating substrate. The resistance value is then adjusted by cutting a helical groove in the film. Carbon film resistors tend to have better tolerance and stability compared to carbon composition resistors.
2. Metal-based Resistors
- Metal Film: Similar to carbon film resistors but using a metallic film instead. Common metals include nickel-chromium (Nichrome) and tin oxide. They’re known for their high accuracy, stability, and low noise.
- Metal Oxide Film: These are made by depositing a layer of metal oxide (like tin oxide) on a ceramic rod. They have better temperature stability compared to pure metal film resistors.
- Wire-wound: These consist of a metal wire, often Nichrome or another alloy, wound around an insulating core. The resistance value is determined by the type of metal, its cross-sectional area, and the length of the winding. Wire-wound resistors can dissipate more power and are commonly used in high-power applications.
3. Semiconductor Resistors
- Made from semiconductor materials like silicon or germanium, these resistors are not as common as carbon or metal-based resistors in typical electronics but can be found in integrated circuits.
4. Ceramic Resistors
- These resistors use ceramics with semiconductive properties. Often, they are made with a blend of ceramic and metallic materials. They can handle a significant amount of power and are often used in situations where high resistance and high power dissipation are needed.
5. Thick and Thin Film Resistors
- These are made by depositing (via sputtering) a thin layer of resistive material on an insulating substrate. The film’s thickness determines the resistance. They are commonly used in surface mount devices (SMDs).
6. Conductive Polymer Resistors
- These resistors use a conductive polymer to provide resistance. They offer good thermal stability and a low temperature coefficient of resistance.
Material Characteristics and Their Importance
The choice of resistor material depends on several factors:
- Tolerance: Some applications require resistors with very precise resistance values.
- Temperature Coefficient: Some materials change their resistance more than others with temperature fluctuations.
- Noise: In sensitive electronics, like audio equipment, low noise is crucial. Some resistor materials inherently produce less noise than others.
- Power Handling: In high-power applications, the ability of the resistor to dissipate heat without changing its resistance value or breaking down is critical.
In conclusion, the realm of resistor materials is vast, with each material catering to specific applications and requirements. The choice of material plays a significant role in determining the performance, stability, and reliability of the resistor in various electronic applications.
Simple examples of resistor use
Resistors are found everywhere in electronic circuits. Here are some simple and everyday examples of their use:
- Volume Control in Radios and TVs:
- A variable resistor, known as a potentiometer, is used to adjust the volume. By turning the knob, you’re changing the resistance, which in turn controls the amount of current flowing to the speaker. Less current means lower volume, and more current means higher volume.
- Brightness Control in Lamps:
- Dimmer switches often use variable resistors to control the brightness of a light bulb. By increasing the resistance, less current flows to the bulb, making it dimmer. Conversely, decreasing the resistance allows more current to flow, making the bulb brighter.
- Battery Charging:
- When charging devices, resistors are used to limit the current flow to safe levels, ensuring that the battery isn’t damaged or overheated.
- Pull-up and Pull-down Resistors in Digital Electronics:
- These are used to ensure a well-defined voltage (either HIGH or LOW) on a digital pin. For instance, when a button is not pressed, a pull-up resistor can ensure that the input pin reads as HIGH. When the button is pressed, it connects the pin to ground, making it read as LOW.
- LEDs in Circuit Boards:
- If you connect an LED directly to a battery, it might draw excessive current and burn out. A resistor is used in series with the LED to limit the current to a safe level, ensuring the LED lights up without being damaged.
- Time Delays in Electronic Toys and Gadgets:
- By pairing a resistor with a capacitor, you can create a circuit that takes a certain amount of time to charge or discharge. This is used in toys and gadgets to create time-delayed effects or sounds.
- Temperature Sensing:
- Thermistors, which are temperature-dependent resistors, are used in appliances like coffee makers or air conditioners. As the temperature changes, the resistance of the thermistor changes, allowing the appliance to sense and adjust the temperature.
- Volume Control in Earphones:
- Just like in radios, the volume slider or wheel on earphones is often a small potentiometer. As you adjust the volume, you’re changing the resistance and, hence, the loudness of the sound.
- Toaster Heating Elements:
- The heating element in a toaster is essentially a high-resistance wire. When electricity flows through this resistor, it heats up, toasting the bread.
- Defrosting in Cars:
- The rear windshield of many cars has a resistive wire embedded in it. When electricity flows through this resistor, it heats up, melting frost or ice on the window.
These are just a few examples of how resistors play a role in our daily lives. They might be small components, but their impact on the functionality and performance of electronic devices is immense.
Applications of resistors examples
Voltage Divider Circuit with Two Resistors
Imagine you have a 9V battery, but you need to power a component that only requires 6V. You can achieve this using two resistors in series.
- Connect two resistors and in series across the 9V battery.
- Measure the voltage across to get your desired voltage.
Using Ohm’s Law and Kirchhoff’s Law:
We want to find (the voltage across ).
The formula for a voltage divider is:
Plugging in the values:
So, by selecting resistors and of 1kΩ and 2kΩ respectively, you’ve divided the original 9V down to 6V across .
This is a basic example of a voltage divider circuit. It’s worth noting that the actual voltage can vary slightly due to resistor tolerances and other real-world factors. Always remember, the current in a voltage divider remains constant, so it’s essential to ensure the divided voltage doesn’t draw too much current, which could result in excessive power dissipation across the resistors.
Current Limiting for an LED
Current limiting is a fundamental concept in electronics used to protect components from excessive current that could potentially damage them. One of the most common scenarios where current limiting is vital is when driving LEDs.
LEDs are diodes that emit light when current flows through them. However, they’re sensitive to the amount of current passing through them. Too much current, and they can burn out.
- You have an LED that operates at 2V and needs 20mA (0.02A) to work correctly.
- You’re connecting this LED to a 9V battery.
What resistor value should you use to limit the current to 20mA?
First, calculate the voltage across the resistor. The LED drops 2V, so the resistor needs to drop the remainder:
Using Ohm’s Law
, we can rearrange to solve for :
Plug in the voltage across the resistor and the desired current:
Thus, you’d want a resistor close to 350Ω to limit the current to 20mA for the LED. The closest standard resistor value is 330Ω. Using a 330Ω resistor would make the current slightly higher than 20mA, but it’s generally safe and within the tolerable limits for most LEDs.
Always remember: It’s better to have a slightly higher resistor value (resulting in less current) than a lower one, which might pass too much current and damage the LED.
This example underscores the importance of resistors in safeguarding sensitive components, like LEDs, from the potentially harmful effects of excessive current.