What is a Resistor? Ohm's Law, Symbols, Code Reading

Resistor is a passive electrical component that resists the flow of electric current. It is designed to control and limit the current in a circuit. Resistors are made from various materials, including carbon, metal, and metal-oxide films. They come in different shapes and sizes, but their purpose remains the same: to manage electrical energy.

What is a Resistor?

Resistor in our daily life is generally directly referred to as resistance. It is a current limiting element, after the resistor is connected to the circuit, the resistance value of the resistor is fixed, generally two pins, which can limit the current size of the branch connected through it. A resistor whose resistance value cannot be changed is called a fixed resistor.

Variable resistance values are called potentiometers or variable resistors. An ideal resistor is linear, i.e. the instantaneous current passing through the resistor is proportional to the applied instantaneous voltage. Variable resistor for voltage division. On the exposed resistor body, one or two movable metal contacts are pressed tightly. Contact position Determines the resistance value between either end of the resistor and the contact.

Terminal voltage and current have a definite functional relationship, reflecting the ability to convert electrical energy into other forms of two-terminal device, denoted by the letter R, the unit is ohm. Actual devices such as light bulbs, heating wires, resistors, etc. can be represented as resistor elements.

Resistor Symbol

Resistors are represented by specific symbols in electronic circuit diagrams. These symbols provide a visual representation of the resistor's function and characteristics in a circuit. Understanding these symbols is essential for anyone working with electronic schematics. Here are the common resistor symbols and their variations:

Ohm’s Law of Resistors

Ohm's Law is a fundamental principle in electronics. It defines the relationship between voltage (V), current (I), and resistance (R) in a circuit. The law is expressed as following:

Voltage (V): The electrical potential difference between two points.
Current (I): The flow of electric charge through a conductor.
Resistance (R): The opposition to the flow of current.

Using Ohm's Law, you can calculate any of the three values if you know the other two. For instance, if you know the voltage and resistance, you can determine the current by rearranging the formula: I = V / R.

Types of Resistors

Resistors can be broadly categorized into two main types: linear and non-linear resistors. Understanding these categories can help you choose the right resistor for your application.

Linear Resistors
Linear resistors have a constant resistance regardless of the voltage or current passing through them. Their resistance value does not change with the variation in applied voltage or current, making them predictable and reliable for many applications. There are two main types of linear resistors:

1. Fixed Resistors
a. Carbon Composition Resistors: These are made from a mixture of carbon powder and a binding material. They are known for their stability and are used in high-voltage applications.

b. Film Resistors: These include carbon film, metal film, and metal oxide film resistors. They have a thin layer of resistive material deposited on a ceramic substrate, offering precise resistance values and good temperature stability.

c. Carbon Film Resistors: Made by depositing carbon on a ceramic substrate, these resistors provide better temperature stability than carbon composition resistors.

d. Metal Film Resistors: These resistors use a thin layer of metal alloy and offer high precision and stability. They are commonly used in applications requiring accurate resistance values.

e. Metal Oxide Film Resistors: Similar to metal film resistors, but made with metal oxide. They can handle higher temperatures and are used in power supplies and amplifiers.

f. Wire-wound Resistors: Constructed by winding a metal wire around a ceramic, plastic, or fiberglass core, these resistors can handle high power and are used in power supplies and high-power circuits.

2. Variable Resistors
a. Potentiometers: These three-terminal devices allow for adjustable resistance and are commonly used for controlling audio levels, tuning circuits, and as voltage dividers.

b. Rheostats: Similar to potentiometers but typically have two terminals. They are used to adjust current in circuits and can handle higher power than potentiometers.

Non-Linear Resistors
Non-linear resistors have a resistance that changes with the applied voltage, current, or temperature. This non-linearity makes them useful in specific applications where variable resistance is needed. The main types of non-linear resistors include:

1. Thermistors
a. NTC (Negative Temperature Coefficient) Thermistors: These resistors decrease in resistance as temperature increases. They are used in temperature sensing and compensation applications.
b. PTC (Positive Temperature Coefficient) Thermistors: These increase in resistance as temperature increases. They are used for overcurrent protection and self-regulating heating elements.

2. Varistors
Metal Oxide Varistors (MOVs): These resistors change resistance with applied voltage and are used to protect circuits from voltage spikes. They have a high resistance at normal operating voltages but become conductive at higher voltages, thus clamping the voltage to a safe level.

3. Light Dependent Resistors (LDRs)
These resistors change their resistance based on the intensity of light falling on them. They have high resistance in the dark and low resistance in bright light. LDRs are commonly used in light-sensitive applications such as automatic lighting controls and light meters.

4. Voltage Dependent Resistors (VDRs)
These resistors change their resistance with the applied voltage. Similar to varistors, VDRs are used to protect circuits from overvoltage conditions by limiting the voltage level.

What are the characteristics of resistors?

Resistors have several important characteristics, knowing them can help you to choose the right resistors for your projects. Here is a detailed explanation of each characteristic.

1. Resistance Value
Measured in ohms (Ω), the resistance value indicates how much the resistor will oppose the flow of current. Resistors are available in a wide range of values, from milliohms (mΩ) to megaohms (MΩ), allowing for precise control of current in various applications.

2. Tolerance
Tolerance indicates how much the actual resistance can vary from the stated value and is usually expressed as a percentage. Standard tolerances include ±1%, ±5%, and ±10%, with precision resistors offering tolerances as low as ±0.1% or ±0.01%.

3. Power Rating
The power rating, measured in watts (W), indicates the maximum amount of power a resistor can dissipate without being damaged. Common power ratings include 1/8W, 1/4W, 1/2W, 1W, and higher for power resistors used in high-power applications.

4. Temperature Coefficient
Measured in ppm/°C (parts per million per degree Celsius), the temperature coefficient indicates how the resistance value changes with temperature. Resistors with low temperature coefficients are more stable and suitable for precision applications.

5. Voltage Rating
The voltage rating specifies the maximum continuous voltage that can be applied across the resistor without causing damage or breakdown. In high-voltage applications, it is crucial to choose resistors with appropriate voltage ratings to ensure reliability and safety.

6. Noise
Resistors generate thermal noise due to the random motion of electrons, and some types can produce current noise. Metal film and wirewound resistors are known for lower noise compared to carbon composition resistors, making them suitable for sensitive analog and audio circuits.

7. Inductance and Capacitance
Some resistors, especially wirewound types, can have significant parasitic inductance, affecting their performance at high frequencies. Additionally, all resistors have a small amount of parasitic capacitance, which can impact high-frequency applications.

8. Physical Size and Package
Resistors come in various sizes and form factors, including through-hole and surface mount packages. Larger resistors can dissipate more power, while smaller surface mount resistors are suitable for compact designs.

How to read resistor color code?

The resistor color code is a method of indicating the value and tolerance of resistors using colored bands. It is worth noting that some resistors include four or five-band. We should know how to recognize them and read their right value. Here's a detailed guide to understanding this code:

Color Code Chart:

Black: 0
Brown: 1
Red: 2
Orange: 3
Yellow: 4
Green: 5
Blue: 6
Violet: 7
Gray: 8

White: 9

Color	Digit	Multiplier	Tolerance (%)	Temperature Coefficient (ppm/°C)
Black	0	10^0	-	-
Brown	1	10^1	±1	100
Red	2	10^2	±2	50
Orange	3	10^3	-	15
Yellow	4	10^4	-	25
Green	5	10^5	±0.5	-
Blue	6	10^6	±0.25	10
Violet	7	10^7	±0.1	5
Gray	8	10^8	±0.05	-
White	9	10^9	-	-
Gold	-	10^-1	±5	-
Silver	-	10^-2	±10	-
None	-	-	±20	-

Standard Four-Band Code Reading

First Band: Represents the first significant digit.
Second Band: Represents the second significant digit.
Third Band: Represents the multiplier (factor by which the first two digits are multiplied).
Fourth Band: Represents the tolerance (accuracy of the resistor's value).

For example, a resistor with bands of red, violet, yellow, and gold translates to:
- Red (2)
- Violet (7)
- Yellow multiplier (10^4 or 10,000): 270,000 ohms (270kΩ)
- Gold tolerance (±5%): The resistor's actual value can vary by 5% from 270kΩ.

Five-Band Code Resistance Reading

Used for precision resistors, it includes an additional significant digit:
1. First Band: First significant digit.
2. Second Band: Second significant digit.
3. Third Band: Third significant digit.
4. Fourth Band: Multiplier.
5. Fifth Band: Tolerance.

Let’s take a resistor with bands of brown, black, black, red, and brown as example, the reading value should be:
- Brown (1)
- Black (0)
- Black (0): 100
- Red multiplier (10^2 or 100): 10,000 ohms (10kΩ)
- Brown tolerance (±1%): The resistor's actual value can vary by 1% from 10kΩ.

What is the Main Function of a Resistor?

The primary function of a resistor is to limit the flow of electric current. This helps in protecting other components in the circuit from damage. Resistors also help in controlling voltage levels, dividing voltages, and biasing active elements like transistors.

Current Limitation
One of the primary functions of a resistor is to limit the flow of electric current within a circuit. By providing resistance, it controls the amount of current that can pass through. This is essential in protecting sensitive components from excessive current that could cause damage. For example, in an LED circuit, a resistor is placed in series with the LED to limit the current. Without the resistor, the LED could receive too much current, potentially burning out.

Voltage Division
Resistors are used in voltage divider circuits to create specific voltage levels from a single power supply. By arranging resistors in series and taking the output voltage across one of them, you can obtain a desired lower voltage. For instance, if you have a 12V power supply but need 6V for a particular component, a voltage divider circuit with two equal resistors can be used to step down the voltage.

Biasing Active Components
Resistors are used to set the operating point of active components like transistors and operational amplifiers. This process, known as biasing, ensures that these components function correctly and within their intended parameters. In a transistor amplifier circuit, for example, resistors are used to establish the correct base current, ensuring the transistor operates in its active region for amplification.

Pull-up and Pull-down Resistors
Resistors are used to ensure known voltage levels on digital circuits. Pull-up resistors connect to the positive supply voltage, while pull-down resistors connect to the ground. These configurations ensure that digital inputs are not left floating, which could cause erratic behavior. In microcontroller circuits, pull-up or pull-down resistors are used on input pins to ensure they read a known voltage level when not actively driven by an external device.

Filtering and Timing Circuits
Resistors, in combination with capacitors and inductors, are used in filtering and timing circuits. They help shape and control the timing of signals, as well as filter out unwanted frequencies. In an RC (resistor-capacitor) filter circuit, for instance, resistors help define the cutoff frequency, filtering out signals above or below a certain frequency.

Detailed Example: LED Circuit with a Resistor

Let's dive into a practical example to illustrate the main function of a resistor in detail. Consider a simple circuit with a power source, an LED, and a resistor.

Power Source: 9V battery
LED: Requires 2V and 20mA for optimal brightness
Resistor: To be calculated

Step-by-Step Process:
Determine the Voltage Drop Across the Resistor

The LED requires 2V, so the remaining voltage drop across the resistor will be:

Calculate the Required Resistance Using Ohm’s Law
The LED operates at 20mA (0.02A), so using Ohm’s Law (V = IR), we calculate:

Choose a Standard Resistor Value
Resistors come in standard values, so we select the nearest standard value, which is 360Ω.

The resistor limits the current to 20mA, ensuring the LED receives the correct current to operate safely and at optimal brightness. Without the resistor, the LED would likely receive excessive current, leading to overheating and failure.

FAQs

1. What happens if a resistor fails?

When a resistor fails, it can either open (create a break in the circuit) or short (provide no resistance). Both scenarios can affect the circuit's performance and potentially damage other components.

2. How do I choose the right resistor for my circuit?

Consider the required resistance value, power rating, and tolerance. Use Ohm's Law to determine the appropriate resistance value for your application.

3. Can resistors be connected in series and parallel?

Yes, resistors can be connected in series or parallel to achieve different resistance values. In series, the total resistance is the sum of the individual resistances. In parallel, the total resistance is calculated using the formula: 1/R_total = 1/R1 + 1/R2 + ... + 1/Rn.

4. Why do resistors get hot?

Resistors convert electrical energy into heat as they oppose the flow of current. The amount of heat depends on the power dissipated by the resistor, which is given by the formula: P = I^2  R.

5. What is a pull-up resistor?

A pull-up resistor is used to ensure a known state for a signal line. It "pulls" the voltage up to a high level (usually the supply voltage) when no other active device is driving the line.

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