Linear Regulator Heat and Heatsink Calculator

What this tool does

A linear regulator drops the excess voltage by turning it into heat, so 12 V in, 5 V out at half an amp means 3.5 W of heat and an efficiency barely above 40 %. Without a heatsink a TO-220 package typically rises around 50 °C for every watt, so it shuts down long before that. The last result is the highest thermal resistance a heatsink may have — choose one with a lower figure. If the number comes out negative or tiny, no practical heatsink will do: drop the input voltage or use a switching regulator.

Formula

P = (V_in − V_out) × I + V_in × I_q · T_j = T_a + P × θ_JA · θ_SA ≤ (T_j,max − T_a) ÷ P − θ_JC − θ_CS

Variables

Symbol	Meaning	Unit
`vi`	Input voltage	V
`vo`	Output voltage	V
`io`	Load current	A
`iq`	Quiescent (own) current	mA
`ta`	Ambient temperature	°C
`rja`	Junction-to-air thermal resistance (no heatsink)	°C/W
`tjm`	Maximum junction temperature	°C
`rjc`	Junction-to-case thermal resistance	°C/W
`rcs`	Case-to-heatsink thermal resistance (pad, paste)	°C/W
`P`	Power turned into heat	W
`TJ`	Junction temperature without a heatsink	°C
`NH`	Needs a heatsink?	—
`HS`	Largest heatsink thermal resistance that will do	°C/W
`EF`	Efficiency	%

Worked example

Input voltage12 V
Output voltage5 V
Load current0.5 A
Quiescent (own) current5 mA
Ambient temperature25 °C
Junction-to-air thermal resistance (no heatsink)50 °C/W
Maximum junction temperature125 °C
Junction-to-case thermal resistance5 °C/W
Case-to-heatsink thermal resistance (pad, paste)0.5 °C/W

Power turned into heat3.560 W
Junction temperature without a heatsink203.0 °C
Needs a heatsink?Yes
Largest heatsink thermal resistance that will do22.59 °C/W
Efficiency41.3 %

Limitations

Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.