What is thermal resistance θja?

θja (junction-to-ambient) is the total thermal resistance from the component junction to ambient air, in °C per watt. Lower θja means better heat dissipation. It depends on the package, PCB layout, and airflow.

What is the maximum junction temperature?

Most silicon components are rated to 125-175°C junction temperature. 150°C is a common design limit. Operation above this causes accelerated aging and eventual failure.

Why does ambient temperature affect power rating?

Higher ambient temperature leaves less "headroom" for junction temperature rise. At 85°C ambient, a component rated for 1W at 25°C may only handle 0.43W (linear derating to 150°C junction).

How do heatsinks help?

Heatsinks reduce θja by providing a larger surface area for heat transfer. A TO-220 package with θja = 65°C/W may drop to 5°C/W with a good heatsink, allowing 15× more power.

What about duty cycle?

For pulsed loads, average power determines steady-state temperature, but peak power determines instantaneous junction temperature. For fast switching (MHz), average power is the key metric.

How does PCB copper affect dissipation?

More copper area around a component reduces θja. For 0603 SMD resistors, increasing copper pad area from minimum to 100mm² can reduce θja from 250°C/W to 150°C/W.

Power Dissipation Calculator

Calculate electrical power dissipation, junction temperature, and thermal derating from voltage, current, and resistance. Includes package thermal reference table.

Calculate From

Voltage (V)

Current (A)

Ambient Temperature (°C)

Thermal Resistance θja (°C/W)

Power Dissipated

40.00 mW

Heat generated

Voltage

2.0000 V

Across component

Current

20.00 mA

Through component

Resistance

100.0000 Ω

V/I

Temp Rise

2.6 °C

P × θja

Junction Temp

27.6 °C

✓ OK

Max Power

1.92 W

At 25°C, Tj_max=150°C

Power Utilization

2.1%

P / P_max × 100

Junction Temperature

28°C / 150°C max

Package Power Ratings

Package	Max Power	θja	Notes
0201 SMD	0.05 W	~640 °C/W	Tiny chip resistor
0402 SMD	0.063 W	~400 °C/W	Common chip size
0603 SMD	0.1 W	~250 °C/W	Popular general use
0805 SMD	0.125 W	~160 °C/W	Standard chip
1206 SMD	0.25 W	~100 °C/W	Larger chip
¼W Axial	0.25 W	~150 °C/W	Through-hole standard
½W Axial	0.5 W	~100 °C/W	Larger axial
TO-220	1−75 W	~1.5−65 °C/W	With heatsink
TO-247	25−150 W	~0.5−40 °C/W	Power package

Planning notes, formulas, and examples

About the Power Dissipation Calculator

Power dissipation is the conversion of electrical energy to heat in electronic components. Every resistor, transistor, IC, and conductor dissipates power as heat, and this heat must be managed to prevent failure. The fundamental equations are P = VI = I²R = V²/R.

This calculator computes power dissipation from any combination of voltage, current, and resistance. It then uses the thermal resistance (θja, junction-to-ambient) to estimate the component's junction temperature, maximum allowable power at the given ambient temperature, and power utilization percentage.

Five calculation modes cover the most common scenarios: LED current limiting, MOSFET switch losses, resistor power ratings, voltage regulator dropout, and heater elements. A comprehensive package reference table lists thermal resistances and power ratings for SMD chips (0201 through 1206), through-hole axial resistors, and power packages (TO-220, TO-247).

Understanding power dissipation is essential for PCB thermal design, component selection, reliability engineering, and system-level thermal management.

When This Page Helps

Thermal failures are the leading cause of electronic component failures. It provides a first-pass thermal analysis from basic electrical parameters.

It prevents over-temperature damage by clearly showing junction temperature and power utilization relative to limits.

How to Use the Inputs

Select the calculation mode based on what parameters you know.
Enter the voltage, current, resistance, or power values.
Enter the ambient temperature and thermal resistance of the package.
Read the power dissipation, junction temperature, and max power rating.
Check whether the junction temperature is below the safe limit.
Use the package table to find thermal resistance for your component.

Formula used

P = V × I = I²R = V²/R.
Junction temp: Tj = Ta + P × θja.
Max power: P_max = (Tj_max − Ta) / θja.
Utilization: P / P_max × 100%.

Example Calculation

Result: P = 40 mW, ΔT = 10°C, Tj = 35°C, utilization = 8%

P = 2 × 0.02 = 0.04 W. Temperature rise = 0.04 × 250 = 10°C. Tj = 25 + 10 = 35°C. Max power = (150 − 25)/250 = 0.5 W. Utilization = 40/500 = 8%.

Tips & Best Practices

Derate components to 50-70% of maximum power rating for reliable long-term operation.
MOSFET switching losses can exceed conduction losses at high frequencies.
For linear voltage regulators, P_dissipated = (Vin − Vout) × I_load — this is pure waste heat.
Thermal resistance adds in series: θja = θjc + θcs + θsa (junction→case→sink→ambient).
Infrared cameras can verify calculated temperatures in real PCBs — always validate critical designs.

When To Use This Calculator

Calculate electrical power dissipation, junction temperature, and thermal derating from voltage, current, and resistance. Includes package thermal reference table. Use it when you need a repeatable calculation in the physics / general category and want the setup, result, and supporting values kept together. This is especially helpful when small input changes, unit choices, or rounding decisions can change the final number.

How To Check The Result

Start by confirming that the inputs match the formula shown on the page. Then compare the main output with the worked example and any secondary values shown by the calculator. If the result will be used in another calculation, keep extra precision until the final step and record the assumptions beside the number.

Practical Notes

Treat the result as a calculation aid rather than a substitute for context. For schoolwork, include the formula and substitution steps. For planning, technical, financial, or health-related decisions, verify important numbers against primary records, current rules, or a qualified professional before acting on them.

Sources & Methodology

Last updated: January 15, 2025

Frequently Asked Questions

θja (junction-to-ambient) is the total thermal resistance from the component junction to ambient air, in °C per watt. Lower θja means better heat dissipation. It depends on the package, PCB layout, and airflow.