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I am making a stir plate out of an old PC fan, variable resistor, and DC power supply from this tutorial.

I have two power supply's to choose from, their outputs are:

PS1 = 12Vdc, 2A

PS2 = 12Vdc, 300 mA

Question #1: Which power supply should I use?

Question #2: What size potentiometer should I select given the power supply such that I can control the speed of the PC fan?

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  • \$\begingroup\$ What is the specification of your fan? If it's 12 VDC at less than 300 mA then use PS2, if it's 12 VDC at > 300 mA and < 2 A use PS1. \$\endgroup\$ – Colin Jan 10 at 14:38
  • \$\begingroup\$ The PC Fans to choose from are: PCF1 = 12V, 0.88A PCF2 = 12V, 4.4W PCF3 = 12V, 0.48A \$\endgroup\$ – andrewrmunro Jan 14 at 14:25
  • \$\begingroup\$ Since both your fans draw more than 300 mA (0.3 A), you need to use the 2A supply. \$\endgroup\$ – Peter Bennett Sep 23 at 15:41
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Question #1: Which power supply should I use?

That depends on the rating of the fan you're going to use. If it needs more than 300mA then use the PSU that can push 2A. Usually there's a sticker on the center of the fan that gives the rating. If there's no sticker, using PS1 is okay, the fan will only take as much current as it needs.

Question #2: What size potentiometer should I select given the power supply such that I can control the speed of the PC fan?

Usually you find out the resistance of the fan and use a rheostat in multiples of that value. (PC fans usually start at 50ohms). If you can't find that out though I would suggest using a variable voltage regulator like the LM317 to vary the output voltage to the fan.

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Q1, Use the higher amperage supply.

Q2, far more complicated Broadly speaking, voltage controls max speed, where current controls torque. The torque load on a fan blowing air is going to be less than that of a liquid mixer for a given speed (viscosity differences). This is all to say you are going to be using the fan well out of its design spec, this is not inherently bad, its likely just going to be inefficient and not necessarily work the ways you would expect.

By using a potentiometer you are more directly controlling voltage than current, it's not going to be as easy to predict what potentiometer will work best. I will note you will need to use a relatively high wattage resistor, the fan you spec is a ~10 Watt model. To get low speeds you will have to sink almost all of that power into your resistor. 10 Watt potentiometer are going to be expensive. A better, more controllable option is to get a variable voltage supply, or en even better option is a variable current supply (probably not possible.

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