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I'm looking at reducing the power of a 24V 0.1A fan by half. I've calculated that I need a 240 ohm resistor for this purpose. But I also need to choose the wattage.

I'm using the following website for my calculations:

https://www.blackfiveservices.co.uk/fanspeedcalc.php?Voltage=24&Current=0.1&Target=12

It summarizes:

Power dissipated by resistor: 12V * 0.05A = 0.6W

I've read that when choosing a resistor I should double the required wattage on the part.

Is a 2W resistor sufficient?

I'm trying to do this as inexpensively as possible as high watt resistors are expensive.

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  • \$\begingroup\$ You can use series / parallel combinations of 1/4 W resistors as well. Then you'd have the option of going for 1 W, 1.5 W, etc. as required. \$\endgroup\$
    – Transistor
    Commented Oct 26, 2020 at 18:15
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    \$\begingroup\$ Fans are not resistive loads : you can't assume it takes 0.05A at 12V (you might want to measure it). That affects both the value and power rating of your resistor. \$\endgroup\$
    – user16324
    Commented Oct 26, 2020 at 18:16
  • \$\begingroup\$ I think, rather than "can this resistor survive wasting a bunch of power and generating heat", perhaps you should be asking "how do I reduce a fan's power using PWM, which is both common and efficient"? \$\endgroup\$
    – Reinderien
    Commented Oct 26, 2020 at 18:20
  • \$\begingroup\$ It's also worth noting that - depending on a lot of things, including the resistor's derating curve, expected ambient temperature, airflow, heatsink contact and thermal resistance - if your resistor is in the air flow path of the fan, you might get away with a smaller resistor. But resistors are so cheap that it's generally a better expenditure of engineering time to simply over-spec the resistor. \$\endgroup\$
    – Reinderien
    Commented Oct 26, 2020 at 18:30
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    \$\begingroup\$ I've read that when choosing a resistor I should double the required wattage on the part. So you're saying that when a manufacturer states that a resistor can dissipate 1 W, we can only use it up to 0.5 W? In 35 years of dealing with electronics, I've never heard that one before. Because it is nonsense. If you want to have some margin, then yes, take a higher rated resistor. But always doubling the figure is silly. Next time: ask yourself why that would be needed. \$\endgroup\$ Commented Oct 26, 2020 at 20:38

3 Answers 3

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For things less than 5W, I typically double the wattage of what I calculate. 1W will work, but I do this because the resistor won't get as hot if it's a 2W compared to a 1W. Which means it's more likely to last longer.

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You should be ok. A good rule of thumb (and a requirement in many companies) is that electronic components not be operated at greater than 50% of their ratings. Obviously this cannot be held true in every situation, but in general -

Transistors - 50% of Vce (continuous) Transistors - 50% of rated continuous collector current Diodes - 50% of continuous or average forward current Capacitors - 50% or rated voltage Everything - 50% or rated continuous power dissipation

etc.

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Yes, the 2W will be cooler, because it's bigger, so has a larger thermal exchange surface with air.

If you don't have a 2W 240 ohm resistor in stock you can use four 1kOhm 0.25W resistors in parallel. If you put them in the airflow of the fan they should stay well below finger-burning temperature.

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