If you’re just getting started with electronics, chances are you’ve come across resistors. They might look small and simple, but these humble components are essential in almost every electronic circuit. So what exactly do they do, and how does a resistor work?
In this beginner-friendly guide, we’ll answer that question and explore everything you need to know about resistors—including their types, uses, testing methods, and how they behave in different circuit configurations.
Table of Contents
- What Is a Resistor?
- How Does a Resistor Work?
- Types of Resistors
- Resistor Color Code: How to Read Resistor Values
- How Does a Resistor Work? : Resistor Uses in Electronics
- How Does a Resistor Work in Series and Parallel Circuits
- Power in a Resistor (Watts)
- How Does a Resistor Work? — How to Test a Resistor
- Surface-Mount Resistors (SMD Resistors)
- Conclusion: How Does a Resistor Work?
What Is a Resistor?
A resistor is a passive electrical component that resists the flow of electric current. In simple terms, it limits how much current can pass through it.
Resistors are defined by their resistance, measured in ohms (Ω). The higher the resistance, the less current flows. Think of it like narrowing a pipe to reduce water flow.
Electronic resistors are used in virtually all circuits to control voltage, protect components, and manage power distribution.
How Does a Resistor Work?
At a basic level, a resistor converts electrical energy into heat by opposing the flow of current. This opposition is what we call resistance.
Ohm’s Law defines the relationship between voltage (V), current (I), and resistance (R):
V = I × R
This means if you know any two of the three values, you can calculate the third.
For example, if you apply 5V across a 1,000-ohm resistor:
I = V / R
= 5 / 1000
= 0.005A or 5mA
The resistor “slows down” the current to 5 milliamps.
Types of Resistors
There are many types of resistors, each suited to different applications. Here are the most common:
1. Fixed Resistors
These have a set resistance value that doesn’t change. They’re the most widely used type.
2. Variable Resistors (Potentiometers)
Allow you to adjust the resistance manually. Great for volume controls and dimmers. See our article How Do Potentiometers Work? for more information on these electronic devices.
3. Power Resistors
Designed to handle high power and heat. Used in power supplies and motor controllers.
4. SMD Resistors (Surface Mount Resistors)
Tiny resistors used in compact electronics. They’re soldered directly to the surface of PCBs.
5. Wire-Wound Resistors
Made by winding a wire around a core. They’re durable and used in high-power applications.
Resistor Color Code: How to Read Resistor Values
Most through-hole resistors used in hobby electronics don’t have printed numbers. Instead, they use colored bands to indicate their resistance value and tolerance. Each color corresponds to a specific number or multiplier.
Here’s a standard color code chart:
| Color | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | ×1 | — |
| Brown | 1 | ×10 | ±1% |
| Red | 2 | ×100 | ±2% |
| Orange | 3 | ×1,000 | — |
| Yellow | 4 | ×10,000 | — |
| Green | 5 | ×100,000 | ±0.5% |
| Blue | 6 | ×1,000,000 | ±0.25% |
| Violet | 7 | ×10,000,000 | ±0.1% |
| Gray | 8 | ×100,000,000 | ±0.05% |
| White | 9 | ×1,000,000,000 | — |
| Gold | — | ×0.1 | ±5% |
| Silver | — | ×0.01 | ±10% |
| None | — | — | ±20% |
4-Band Resistors (±5% Tolerance)
A 5% tolerance resistor used in hobby electronics is typically a 4-band resistor:
- Band 1: First digit
- Band 2: Second digit
- Band 3: Multiplier (number of zeros)
- Band 4: Tolerance
The body of these resistors is usually a light tan or brown color. The following image shows a 1k (1000Ω) resistor. This resistor has a power rating of 1/4W and a tolerance of 5%. A tolerance of 5% means that it can be between 5% above and 5% below its stated value. 5% of 1k is 50. This means that the actual resistor value can be between 1050Ω (1000 + 50) and 950Ω (1000 – 50).
Example:
Brown – Black – Red – Gold
→ 1 (brown), 0 (black), ×100 (red), ±5% (gold)
= 1,000 ohms (1kΩ) ±5%
OR
Brown = 1
Black = 0
Red = 2 (number of zeros is 2 or 00)
Gold = 5% tolerance
Put these all together or next to each other and we get 1, 0, 00 or 1000Ω
These resistors are usually from the E12 series – a set of standard values spaced in a logical sequence.
5-Band Resistors (±1% Tolerance)
A 1% resistor is typically a 5-band resistor:
- Band 1: First digit
- Band 2: Second digit
- Band 3: Third digit
- Band 4: Multiplier
- Band 5: Tolerance
The body of these resistors is usually a blue color, making these resistors more difficult to read. This is because they do not have as much contrast between the body color and colored bands.
The following image shows a 1k (1,000Ω) resistor. This resistor has a power rating of 1/4W and a tolerance of 1%. 1% of 1,000 is 10. This means that this resistor’s actual value is between 1,010Ω and 990Ω.
Example:
Brown – Black – Black – Brown – Brown
→ 1 (brown), 0 (black), 0 (black), ×10 (brown), ±1% (brown)
= 1,000 ohms (1kΩ) ±1%
OR
Brown = 1
Black = 0
Black = 0
Brown = 1 (1 zero or 0)
Brown = 1% tolerance
Put all these values next to each other and we get 1, 0, 0, 0 which is 1000 or 1,000 which can also be written as 1k or 1kΩ
These resistors are often from the E24 or E96 series – finer increments for precision use.
Understanding Standard Resistor Values (E-Series)
Resistors are manufactured in standard values grouped into E-series, which are logarithmically spaced so each value is about a certain percentage apart:
- E6 (20% tolerance) — 6 values per decade
- E12 (10% tolerance) — 12 values per decade
- E24 (5% tolerance) — 24 values per decade
- E48, E96, E192 — for precision resistors (1%, 0.5%, 0.1%, etc.)
Example: E12 values (for each decade):
10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82
This means you’ll commonly see resistors like 1.5kΩ, 3.3kΩ, 6.8kΩ in kits and circuits, because they follow these standardized values.
How Does a Resistor Work? : Resistor Uses in Electronics
Resistors have many practical applications in electronics:
- Current Limiting – Protect LEDs and components by controlling current.
- Voltage Division – Create reference voltages using voltage dividers.
- Pull-Up or Pull-Down Resistors – Ensure stable logic levels in digital circuits.
- Biasing Transistors – Set operating points in amplifiers.
- Power Dissipation – Convert excess energy into heat.
- Current Shunt – For measuring current in a circuit.
How Does a Resistor Work in Series and Parallel Circuits
How Does a Resistor Work in Series
Resistors in series: When resistors are connected end-to-end, their resistance adds up:
R_total = R1 + R2 + R3 + ...
Useful when you need a higher resistance value.
How Does a Resistor Work in Parallel
Resistors in parallel: In parallel, the total resistance decreases:
1 / R_total = 1/R1 + 1/R2 + 1/R3 + ...
Great for lowering resistance or adding redundancy.
Adding Parallel Resistors Example
If you connect two 100Ω resistors in parallel:
1 / R_total = 1/100 + 1/100
= 2/100 → R_total = 50Ω
Power in a Resistor (Watts)
Resistors don’t just resist current—they also dissipate power as heat. The power (in watts) is calculated by:
P = V × I
or
P = I² × R
or
P = V² / R
Make sure your resistor’s power rating (e.g., ¼ watt, ½ watt, 1 watt) exceeds the amount of power it needs to handle in your circuit. Otherwise, it might overheat or fail.
How Does a Resistor Work? — How to Test a Resistor
To test a resistor:
- Turn off power to the circuit.
- Remove the resistor or isolate it from the circuit.
- Set your multimeter to resistance (Ω) mode.
- Measure the resistance across the two leads.
Compare the reading to the expected value (from the color code or datasheet). A resistor with significantly higher or lower resistance may be damaged.
Surface-Mount Resistors (SMD Resistors)
Surface-mount resistors are compact, rectangular components designed to be soldered directly onto the surface of a PCB. As a type of Surface-Mount Device (SMD), they are commonly used in modern electronics like smartphones, laptops, and wearables where space is limited.
Instead of color bands, SMD resistors use numerical codes to indicate their resistance values — for example, “103” means 10 kΩ (10 followed by 3 zeros = 10,000 or 10k). Despite their small size and handling difficulties, they function the same way as traditional through-hole resistors in a circuit.
The following image shows some small surface mount resistors and surface mount transistors on a circuit board. These components are known and surface mount device (SMD) components.
Conclusion: How Does a Resistor Work?
So, how does a resistor work? It simply resists current—yet that simple action has endless uses in electronics.
From lighting an LED to regulating voltage in a microcontroller circuit, resistors are the quiet heroes of the electronics world.
Now that you understand what a resistor is, how to read resistor color codes, the types of resistors, and how they behave in series and parallel, you’re well on your way to mastering electronics basics.
Next time you pick up a resistor, you’ll know exactly what it does—and why it matters.
Learn more about other electronic components in our components category.