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I working on mini project to create a magnetic drink stirrer out of an old PC Case Fan.

(For those that don't know, a magnetic stirrer is basically a spinning magnet under your glass and a magnetic bar in the glass that spins with it and mixes the drink.)

The Fan I'm using is a old 12V DC PC Fan, one of the two wire types, I tried connecting it to a 5V connection and it span but was too slow to mix the drink properly and so I'm now using a 12V power supply but that spins too quickly for the magnets.

I tried connecting a 100K B Potentiometer as a variable resistor to the circuit to adjust the speed but all that does is has the fan off for 95% of the pots range and then full speed for the last bit. Is there any way I can adjust the circuit easily to control the fan speed?

I don't want to replace the fan with a 3 or 4 wire type and I ideally want to keep using the 12V supply, I'm looking for a solution that will allow me to do this with only one or two component additions or changes such as a different or additional potentiometer etc. I'm pretty new to electronics so don't know if I'm doing something stupid here.

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Although you can use a resistor to control a motor's speed, usually the impedance/resistance of the motor is too small for this to be useful. You end up having to use a resistance less than 100 ohms, which can heat up a lot and might get destroyed if you don't have them rated at the right wattage. At 12 volts, most 100 ohm resistors don't stand up very well and generate a lot of heat, and you'd probably need a much smaller resistance for the motor. If you do have a resistor with a high power rating and a low resistance, you could throw it into the circuit and see how it works, although this won't work with most resistors.

You could use a couple diodes in series, if their power ratings are high enough. Since a diode always has a voltage drop over it of around 0.7 volts, 3 or 4 of them in series may drop the voltage enough to slow the motor down. To calculate how much power will be across each of the diodes you can multiply the current flowing through the circuit with no diodes in it times the voltage drop across each diode, or 0.7 Volts X __ Amps. If the power rating of the diode is at least that large, then you should be able to experiment with a few of them in series to try and find the right number.

Both the above methods are fairly unconventional; generally to control motor speed people use what's called "pulse width modulation" or "PWM" for short. Basically what you want to do is turn the motor on and off quickly (around a hundred times a second usually, although the frequency doesn't matter too much) with varying amounts of on and off times.

For example, if you wanted to turn the motor slowly, you might have the motor on for 1ms, then off for 5ms. If you wanted to turn the motor faster but not to it's maximum speed you might adjust the cycles to be on for 4ms and off for 2ms.

Now unfortunately you can't really do this with just 1 or two components, the simplest way to do this is to use a chip like the 555 timer, or a microcontroller like the Arduino, so it's probably not the best method for you. If I were you, I would try out the diodes to see how well they work if you have any lying around, but in the end you may have to dip into microcontrollers or digital electronics in order to do what you want.

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  • \$\begingroup\$ Thanks very much. I'll probably use the diodes idea for a quick fix and then look into a microcontroller as I slightly longer project; I've been looking for an excuse to get into those anyway. \$\endgroup\$ – Kersakov Aug 10 '15 at 10:09
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I assume that you have the pot connected as a rheostat (variable resistor - 2 connections). The fact that the motor is off for 95% of the pot's rotation should tell you that the pot is at least twenty times two much resistance. Likely the resistance needs to be less than 100 ohms. If the motor's rated current is known, divide that into 5 to get a better resistance for a starting point.

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Get a handful (no more than 10) of cheap 1A silicon diodes such as 1N4004. Connectas many as you need in series to drop the voltage to what works for you. The speed will be more stable than if you use a pot in this application.

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