Heat Sink Calculator

Determine required thermal resistance for cooling.

Power Dissipated (W)

Max Junction Temp (°C)

Ambient Temp (°C)

Rth Junction-Case (°C/W)

Rth Case-Sink (°C/W)

Max Rth Sink -- °C/W

What is Thermal Resistance?

Thermal Resistance (R_th) is a measure of how difficult it is for heat to flow through a material. It is the thermal equivalent of electrical resistance.

In electronics, we want heat to flow easily from the silicon chip (Junction) to the outside air (Ambient). A heat sink acts like a wide highway for heat, lowering the total resistance of the path.

The Thermal Formula

The fundamental equation of heat transfer in electronics is analogous to Ohm's Law (V = I × R), where Temperature is Voltage and Power is Current.

T_J = T_A + (P_diss × R_{th_total})

Where:

T_J: Junction Temperature (The silicon chip itself). Usually max 125°C or 150°C.
T_A: Ambient Temperature (The air usually 25°C, or 50°C inside a box).
P_diss: Power dissipated as heat (Watts).
R_{th_total}: Sum of all thermal resistances (Junction-to-Case + Case-to-Sink + Sink-to-Air).

Practical Applications

CPU Cooling: keeping a 100W processor under 85°C. This requires a massive heatsink with heat pipes and fans to achieve an extremely low R_th (e.g., 0.2 °C/W).
Linear Regulators (LM7805): Dropping 12V to 5V at 1A creates 7 Watts of heat. Without a heatsink, the TO-220 package (R_th ≈ 65°C/W) would reach 480°C and instantly fail!
LED Lighting: High-power LEDs degrade quickly with heat. Aluminum PCBs are used to conduct heat away to a metal chassis.

FAQ

Why do I need thermal paste?

Microscopically, metal surfaces are rough. When you press a component against a heatsink, there are tiny air gaps. Air is a terrible conductor of heat. Thermal paste fills these gaps, lowering the "Case-to-Sink" (Contact) resistance drastically.

Active vs. Passive Cooling?

Passive cooling relies on natural convection (air moving on its own as it heats up). It is silent but requires large fins. Active cooling uses a fan (force air) to strip heat away much faster, allowing for smaller heatsinks but adding noise and a failure point.

What is R_th-JC?

Junction-to-Case resistance. This is internal to the component, set by the manufacturer. You cannot change it. A large package like TO-247 has a low R_th-JC (good heat transfer), while a tiny SOT-23 has a high R_th-JC.