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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 supplies to choose from, their outputs are:

  • PS1 = 12Vdc, 2A
  • PS2 = 12Vdc, 300 mA
Questions:
  1. Which power supply should I use?
  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
    Commented Jan 10, 2019 at 14:38
  • \$\begingroup\$ The PC Fans to choose from are: PCF1 = 12V, 0.88A PCF2 = 12V, 4.4W PCF3 = 12V, 0.48A \$\endgroup\$ Commented Jan 14, 2019 at 14:25
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    \$\begingroup\$ Since both your fans draw more than 300 mA (0.3 A), you need to use the 2A supply. \$\endgroup\$ Commented Sep 23, 2019 at 15:41

3 Answers 3

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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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Q1 Which power supply should I use?

Most Fans of 12V 80 x 80mm used for auxiliary cooling of PC cases, demand typically 100mA.
When the fan is blocked to spin, the consumption can be as high as 150-180mA.
The maximum voltage these brushless motors can run is 15V, some of them burn its electronics above 16~18V.
A1: That 12V x 300mA would suffice.


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

Some preliminary information for this project:

Driving a Stir-plate creates a Demanded Torque curve that is different than just running a Fan.
Static friction either caused by the magnetic ‘pill’ inside the fluid, or when fluid contain particulates or powdered components, results in non-Newtonian fluid behavior. Any of these cases, create a higher friction to start moving the fan rotor - being magnetically coupled as a higher holding torque.

Assuming the fan is 12V x 150mA in Stall and 100mA at full running speed, Equivalent resistance is R= 12V/i
R_stall = 12/0.15 = 80R.
R_run = 12/0.1 = 120R.


Possible options commented - from simplest to more adequate:

  • Resistor limiting the current is the simplest but it is not good, even for a fan in the original use. For higher holding torques, it will likely be a bad solution. A wire-wound potentiometer would be the ‘variable’ resistor solution.
    A2: Best guess would be to use 100-150R wire-wound (5W) potentiometer.
    But I don’t recommend this solution.

  • Zener in series - reproduces a voltage drop that would fan see. Has the advantage of providing a the same voltage drop, regardless it is in stall or run status - something that a resistor (or potentiometer) would not allow.
    Most fan motors start running at 4-5V, so a Zener of 6V2 x 1W can dissipate up to 150mA and be, even if holding torque does not allow fan spin (high friction).

  • Adjustable Series regulator - An LM317-based circuit (see Figure 16 of Datasheet) could allow voltage range from a minimum as 3~4V to 12V nominal. It is like the previous option, but with an ‘adjustable’ Zener. In one hand it is better because here, the Output voltage is properly maintained. On the other hand, it can be worse, as the LM317 requires a minimum voltage drop (Vin - Vout = V_dropout) of ~2V for 150mA. There are LowDropOut (LDO) alternatives as AMS1117, or LM1117. The point here is that maximum Output Voltage will be lower than 12V nominal, as your 12V is the Input Voltage, unless your power supply is higher as 15V~20V: in this case, a 19V Laptop would be an overkill (too big) but provides a high enough voltage.

  • Variable PWM-based circuit: This is the most adequate, eventually the only good solution for non-newtonian or highly viscous fluids. An example would be using an LM555 timer circuit in Astable (Figure 4), PWM (Figure 9) or PPM (Figure 10) - each one you could adapt a potentiometer to change the duty cycle. The advantage of doing this is that when On, motor gets full voltage to overcome friction and start to run.
    I suggest running frequencies of 2-4 Hertz, but not faster than 10Hz, to allow sufficient initial torque “kick”. The higher the duty cycle, the higher the final run speed.
    A Bipolar or MOSFET transistor would be the final control element, switching between Cut & Saturation, so even small TO-92 cased ones would work; for peace of mind, I would spend a few cents more and get a TO-220 sized transistor.

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