Required Heat Sink
8.00 °C/W
Tj
125.0°C
Margin
0.0°C
From device datasheet (typically 0.5–5 °C/W)
Thermal pad: 0.2–1 °C/W
Enter to compute actual Tj with your chosen heat sink
Required θsa (Sink to Ambient)
8.00 °C/W
Junction Temp
125.0°C
Case Temp
110.0°C
Margin
0.0°C
Total θ
10.00
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Price as of Jul 24, 2026. Product prices and availability are accurate as of the date indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
Inputs
Result
θsa = (125-25)/10 - 1.5 - 0.5 = 10 - 2 = 8.0°C/W.
Inputs
Result
θsa = (150-40)/25 - 0.5 - 0.3 = 4.4 - 0.8 = 3.6°C/W.
θsa = (Tj_max - Ta)/P - θjc - θcs. For 10W, 25°C ambient, 125°C max junction, θjc=1.5, θcs=0.5: θsa = (125-25)/10 - 1.5 - 0.5 = 8.0°C/W.
| Power (W) | Ambient | Max Tj | Required θsa |
|---|---|---|---|
| 5W | 25°C | 125°C | 18.0 °C/W |
| 10W | 25°C | 125°C | 8.0 °C/W |
| 20W | 25°C | 125°C | 3.0 °C/W |
| 50W | 40°C | 150°C | 0.2 °C/W |
θjc is the thermal resistance from the semiconductor junction to the outside of the package. It's fixed by the IC design and listed in the datasheet. Typical: TO-220 = 1-3°C/W, TO-92 = 80-150°C/W.
θcs depends on the thermal interface between IC and heat sink. Thermal grease: 0.2–0.5°C/W. Thermal pad: 0.5–1.0°C/W. Dry contact: 1–2°C/W.
A negative θsa means no heat sink can keep the junction below Tj_max. Solutions: reduce power, improve airflow, use a lower θjc package, or increase acceptable Tj.
Electronics, outdoor, and everyday tools
Last Updated: Jul 24, 2026
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