Units of Electrical Resistance
Electrical resistance is measured in ohms (Ω). Because resistors are manufactured in a very wide range of values, larger and smaller units are commonly used to make resistor values easier to read and write. Understanding these units is essential when reading circuit diagrams, selecting components and using a digital multimeter.
The Ohm (Ω)
The ohm, represented by the Greek letter Ω (Omega), is the SI unit of electrical resistance.
A resistor with a value of 1 Ω allows one ampere of current to flow when one volt is applied across it.
Common Resistance Units
| Unit | Symbol | Equivalent Value |
|---|---|---|
| Milliohm | mΩ | 0.001 Ω |
| Ohm | Ω | 1 Ω |
| Kilohm | kΩ | 1,000 Ω |
| Megohm | MΩ | 1,000,000 Ω |
| Gigohm | GΩ | 1,000,000,000 Ω |
Unit Conversions
| Value | Equivalent |
|---|---|
| 470 Ω | 0.47 kΩ |
| 1,000 Ω | 1 kΩ |
| 2,200 Ω | 2.2 kΩ |
| 10,000 Ω | 10 kΩ |
| 100,000 Ω | 100 kΩ |
| 470,000 Ω | 470 kΩ |
| 1,000,000 Ω | 1 MΩ |
| 2,200,000 Ω | 2.2 MΩ |
| 10,000,000 Ω | 10 MΩ |
Typical Resistor Values
| Resistance | Common Use |
|---|---|
| 0.1–1 Ω | Current sensing and shunt resistors. |
| 10–330 Ω | LED current limiting. |
| 470 Ω–10 kΩ | General electronic circuits. |
| 10 kΩ–100 kΩ | Biasing and pull-up resistors. |
| 100 kΩ–10 MΩ | High-impedance sensor circuits. |
Reading Resistor Markings
Manufacturers often use letters instead of decimal points on resistor markings.
| Printed Marking | Actual Value |
|---|---|
| 4R7 | 4.7 Ω |
| 10R | 10 Ω |
| 220R | 220 Ω |
| 1K | 1 kΩ |
| 2K2 | 2.2 kΩ |
| 4K7 | 4.7 kΩ |
| 1M | 1 MΩ |
| 2M2 | 2.2 MΩ |
Measuring Resistance
- Turn off power to the circuit.
- Set the multimeter to the resistance (Ω) range.
- Place the probes across the resistor.
- Read the displayed value.
- Compare it with the resistor's marked value.
Applications
- Choosing the correct resistor value.
- Reading circuit diagrams.
- Component identification.
- Electronic troubleshooting.
- Multimeter measurements.
- Power supply design.
- Audio amplifier circuits.
- Microcontroller projects.
Common Mistakes
| Mistake | Explanation |
|---|---|
| Confusing kΩ and MΩ | 1 MΩ equals 1,000 kΩ. |
| Ignoring the unit symbol | 10 Ω is very different from 10 kΩ. |
| Measuring in-circuit | Other components may affect the reading. |
| Using the wrong meter range | May produce inaccurate measurements. |
Key Points
- The SI unit of resistance is the ohm (Ω).
- Large resistance values are commonly expressed in kilohms (kΩ) and megohms (MΩ).
- Correct unit conversion is essential when selecting resistors.
- Digital multimeters measure resistance directly in ohms.
- Understanding resistance units is one of the first skills every electronics student should master.