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I will have this 47k 50W wire wound resistor which will be across line and neutral which means 230 VAC rms. So the electrical power in theory will be V^2/R = 1.2 Watt.

How should one interpret this datasheet and the below curve whether a heat sink is needed? (Above which electrical power a heat sink is needed needed).

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Similarly a 4.7k 50W resistor will electrically consume 1W. Can we say no heatsink required looking at its datsheet as well?

Im trying to find a method or rule of thumb ect., thats why i gave two examples-

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The derating curve really does not tell you anything about the need for a heatsink unless you are operating in a situation a 1W where you will be over 250°C. In room temp running 1W through a 50W resistor you will not need a heat sink.

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  • \$\begingroup\$ Does that mean a resistor rated 15W never requires a heat-sink if the electrical power is less than 15W? Even 12W power for such resistor will not require any heatsink? \$\endgroup\$ – cm64 Nov 2 '18 at 14:37
  • \$\begingroup\$ It depends on who's rating the resistor. That resistor is very conservatively rated -- I've seen parts (including resistors) that are rated at X Watts at room temperature with good airflow, and derated for anything more strenuous than that. You have to pay attention to how brands (and product lines within brands) shade their numbers. \$\endgroup\$ – TimWescott Nov 2 '18 at 15:03
  • \$\begingroup\$ But theres no info about those. \$\endgroup\$ – cm64 Nov 2 '18 at 15:06
  • \$\begingroup\$ Later in the data sheet you have a chart for Surface temp rise. This talks about temp rise per power dissipation. You can use this cart to determine if you need a heat sink. As I said looking at 1w you will see less that a 20° increase in temp and therefore you will be fine without a heat sink. At 20W you will see an 80° change and you may need a heatsink depending on temp starting and airflow as you will need to look at the derating curve when you start hitting that temp. \$\endgroup\$ – Robert Fay Nov 2 '18 at 15:25

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