Ohm's Law Explained with Practical Examples

Ohm's Law is the single most important equation in electronics. Whether you're designing a power supply, debugging a faulty circuit, or choosing the right resistor for an LED, you'll use Ohm's Law constantly. This guide breaks it down with practical, real-world examples.

The Formula: V = I × R

Ohm's Law states that the voltage (V) across a conductor is equal to the current (I) flowing through it multiplied by its resistance (R). This relationship was discovered by Georg Simon Ohm in 1827 and remains the foundation of electrical engineering.

• V = Voltage in volts (V)
• I = Current in amperes (A)
• R = Resistance in ohms (Ω)

You can rearrange the formula to solve for any variable:

• V = I × R — Find voltage when you know current and resistance
• I = V / R — Find current when you know voltage and resistance
• R = V / I — Find resistance when you know voltage and current

Example 1: Calculating LED Current

Suppose you connect a red LED (forward voltage 2V) to a 5V supply through a 150Ω resistor. What current flows through the LED?

The voltage across the resistor is 5V - 2V = 3V. Using Ohm's Law:

I = V / R = 3V / 150Ω = 0.02A = 20 mA

This is the typical operating current for a standard LED — perfect.

Example 2: Choosing a Resistor

You need to limit current to 100 mA through a 12V motor from a 24V supply. What resistance do you need?

Voltage across resistor: 24V - 12V = 12V

R = V / I = 12V / 0.1A = 120 Ω

Don't forget to check power dissipation: P = V × I = 12V × 0.1A = 1.2W. You'd need at least a 2W resistor.

Example 3: Measuring Unknown Resistance

Your multimeter reads 9V across a component and 45mA through the circuit. What's the resistance?

R = V / I = 9V / 0.045A = 200 Ω

The Power Triangle

Ohm's Law extends naturally to power calculations:

• P = V × I — Power equals voltage times current
• P = I² × R — Useful when you know current and resistance
• P = V² / R — Useful when you know voltage and resistance

Common Applications

Voltage Dividers

Two resistors in series create a voltage divider. The output voltage depends on the ratio of the resistors, derived directly from Ohm's Law.

Current Sensing

A small "sense resistor" in a circuit develops a voltage proportional to current flow. By measuring this voltage, you can determine the current without breaking the circuit.

Troubleshooting

If a circuit draws more current than expected, Ohm's Law helps you identify whether a resistance has decreased (short circuit) or if a voltage source is misbehaving.

Limitations of Ohm's Law

Ohm's Law applies perfectly to ideal resistors and to many real-world conductors under normal conditions. However, some components are "non-ohmic" — their resistance changes with voltage, current, or temperature:

• Diodes and LEDs have a nonlinear voltage-current relationship
• Thermistors change resistance with temperature
• Varistors change resistance with applied voltage
• Superconductors have zero resistance below critical temperature

For these components, Ohm's Law still applies instantaneously, but the effective resistance is not constant.

Tips for Beginners

1. Always convert units before calculating (mA to A, kΩ to Ω)
2. Draw the circuit diagram and label known values before solving
3. Double-check your answer — does the result make physical sense?
4. Remember that power dissipation matters as much as the resistance value

For quick calculations on the go, use our online tool that handles all the unit conversions and power calculations automatically.