Introduction: Variable Speed Magnetic Stirrer From a PC Power Supply

Picture of Variable Speed Magnetic Stirrer From a PC Power Supply

I found quite lot of simple designs proposed by hobbyists on the web for turning a PC fan to a magnetic stirrer and some projects to build a bench top power supply from and old computer power supply.
I think the power supply can act as base for the magnetic stirrer project! Let’s combine the two and build a simple magnetic stirrer that looks like a professional one.

Commercial magnetic stirrers are equipped with heater and rotation speed controller for stir bar. I have added PWM speed controller to this project but not the hot plate.
You may like to build a more professional one by adding a hotplate as well to your design.

Step 1: How It Works?

Picture of How It Works?

Magnetic stirrer is a mixer for low viscosity liquids like water. An immersed magnet bar (stir bar) that rotates very quickly in responses to an external switching magnetic field creates the rotation in liquid.

Step 2: Open Up the Power Supply and Cut Extra Wires

Picture of Open Up the Power Supply and Cut Extra Wires

Multiple output wires are connected to PC power supply to power different PC units. You may use them if you decide to turn the PC power supply to bench top power supply. But in this project we only need the power supply fan to run. Keep green wire, two red wire and three black wires and cut all extra output wires. You can use thermal glue to cover remain part of wires gently. Make sure that stripped wires with diffract color are not in touch after cutting.

Green=Power on/off
Red = 5 Volts
Black= Ground

Step 3: Connect a 3w Thermal Resistor to 5V Output

Picture of Connect a 3w Thermal Resistor to 5V Output

Most of PC power supplies require a minimum load on their output to run. Connect a thermal resistor between a red and a black wire. You can salvage a typical 3-5 watt resistor from a junk PCB board.

Step 4: Add Power On-off Switch

Picture of Add Power On-off Switch

The power supply wouldn’t start without connecting the green wire to ground. This could be used as on off switch. I opened a rectangle on side of the enclosure for mounting the switch. I drilled a hole for LED indicator in this stage while the power supply is open. You better to remove the board from the box before cutting the holes. Small debris from cutting may short cut the board later. You can also cover the board by something before cutting.

Step 5: Add LED Power on Indicator

Picture of Add LED Power on Indicator

Step 6: Attach Two Small Strong Magnets to the Fan

Picture of Attach Two Small Strong Magnets to the Fan

I used thermal glue. You may use any available glue that you have. 

Step 7: Top Plate

Picture of Top Plate

Drill four holes on a rectangular shape acrylic sheet.

Step 8: Immerse Magnet Bar (stir Bar) and Run

Picture of Immerse Magnet Bar (stir Bar) and Run

Immerse the stir magnet bar before turning the stirrer. Make sure the stir bar is located at the center of the two magnets attached to the fan. Always have some water in the beaker. The magnet stir bar may crack the glass wall of the beaker if it runs away from the center position suddenly in an empty beaker.

The next stops are related to the speed controller.

Step 9: Speed Control With PWM

Picture of Speed Control With PWM

To control the speed of fan we need a PWM speed controller. Initially I was thinking to build a PWM speed controller PCB board  but I realized that I can get one on eBay as cheap as 4$. Less than the price for electronics components!
You just need to cut the fan wires and connect them to the output of the speed controller. The input of the speed controller would be the output of the power supply for the fan.
Note that we cannot reduce the fan speed by reducing its voltage. It requires the 12 V to run. That is why by pulse width modulation (PWM) controller, we supply the same 12 V voltages to the fan in on-off manner with high frequency.

Step 10: Install the PWM Inside the Enclosure

Picture of Install the PWM Inside the Enclosure

- Drill a hole on the  front panel for speed control potentiometer
- Add insulator to the back side of the PWM PCB.
- Cut the fan wires and connect them to the output of the PWM controller.
- Connect the input of the PWM to the output of the power supply for the fan

Step 11: Final Assemble (The Rabbit and the Turtle)

Picture of Final Assemble (The Rabbit and the Turtle)

Print small rabbit and turtle images and attach them to the front panel to show the direction of speed increment. You may come up with other ways to show this like arrows or plus and mines.

Hope you like this project!

Have Fun with your projects!



amiir made it! (author)2015-07-14

i use a hammer switch to change voltage of fan

slow side connect fan to 5V and Fast side connect it to 12V

and fan is 12V so it will spin slow if connect to 5V

PWM is expensive and big for my stirrer because my PSU is a mini one.

u also can use 100K Ohm potentiometer to reduce fan speed :D

thanks to share this great idea

M Allahkarami (author)amiir2015-08-21

Hi Amiir,

Glad you made it! Thanks for sharing ideas and suggestions.

BurgersBytes (author)2014-01-26

Why such a small resistor? The power supply doesn't require .7 amps of load.

5 volts / 7 ohms = .714 ma

The fan will only pull 300 ma at most, maybe a bit more with resistance, so the load is now over 1 amp.

Also the fan power is those yellow wires you said to cut already...I wouldn't use the existing power supply fan as the stir fan cannot cool anything. The entire project could be made using a 1 amp 12 volt supply. What's the 5 volts for?

Thanks for the calculations!

I want to add that the PC Power supply's need a load to work properly which tricks the power supply into thinking its powering something. I just tried some of thermal resistor that I had (from old TV PCB) and just chose the one that worked!

What you could do is replace that resistor with a light. it might useful to have a light on your work bench. Or you could replace the resistor with a heating element. That way you could stir and heat at the same time. (although it wouldn't be the most controlled heating)

Tardisrepairman (author)2014-02-22

Isn't there a risk that liquids could spill over the lip of the beaker and get in through all the holes in the ATX power supply? You would be better with a wall wart so there are no high voltages on the bench.

If you want a professional looking base, you could use the case from the ATX PSU with the speed controller inside in another sealed box.

For a professional "Weighty" feel you could hot-glue a half brick inside! :-)

I really like your suggestions and the way that you looked at the problems involved with this project. Thanks for providing solutions to the concerns! :-)

powerspan (author)2014-01-26

General comment. I love looking at all Instructables, however I have never come across a 'Magnetic stirrer' - I have no idea what it is used for. Can someone please explain the uses for this * THING *

M Allahkarami (author)powerspan2014-01-26


Hope this will help,

"A magnetic stirrer or magnetic mixer is a laboratory device that employs a rotating magnetic field to cause a stir bar (also called "flea") immersed in a liquid to spin very quickly, thus stirring it."

powerspan (author)M Allahkarami2014-01-27

Thanks, I understand what it is and how it works, but what is it used for ???

thormj (author)powerspan2014-02-13

Stirring things ;) You can use it to keep things stirred and not settle (eg, paint, abs-juice, etc), or you can use it when you're supposed to be slowly adding one thing to another while stirring (diluting acids, other chemical reactions) -- especially handy for this as you're trying to be careful and holding one flagon with your hands -- stirring and pouring is a lot like rubbing your belly and patting your head. Make sure your fluid doesn't eat the magnet (eg, don't use an ABS-covered magnet with acetone).

msadewicz (author)powerspan2014-01-28

I'll be using mine for yeast starters in brewing beer, to constantly aerate the wort overnight.

fossilmaniac (author)2014-01-26

Great instructable. I build a lot of things in the shop but I am not that familiar with the inside of a computer. A picture of the power supply unit in the computer would be helpful. Also, where did you get the 3w thermal resister? My point is that your instructable is geared for an electronics savvy person. There are many of us out here that don't know that much about electronics but are trying to learn from posts like yours. Please don't assume we already know this or that. Peter

Very good help :-) Thanks!

LynxSys (author)fossilmaniac2014-02-02

I wrote an Instructable a while ago that's intended as a reference
guide for anyone who is hacking an ATX power supply. It explains a lot
of the basics that you need to know, without
being focused on a specific project. At risk of self-promotion, I think
that you might find it helpful:

Let me know if I can answer any questions for you!

Hi Peter,

Indeed it is my honor that I am getting suggestions from people
like you that build a lot of things in the shop. J
Sorry that I have not included all the details!

I found the following links about using PC power supply very
helpful useful.

I salvaged thermal resistors from an old TV electronic
board. People want to get rid of their old TV for free! Everybody buys LCD or LED TV. You can also use
any kind of load. Just the power supply needs to power something to be
functional. It is a part of feedback system. Masoud

M Allahkarami (author)2014-01-30


10K pot

The 10k potentiometer is a part of PWM speed controller that I got from eBay.

If you are good in making PCB your self and want to build the PWM, the following link would be helpful:

  1. 0k pot

Hope this will help :-)

ppsailor (author)M Allahkarami2014-01-31

Very good help thanks

blackepyon (author)2014-01-31

In my experience (jury-rigging computer Power Supply Units for general +5v and +12v electronics usage), you shouldn't need a load resistor at all. Just short pin-14 (Green wire) on the motherboard connector with common negative (black) as shown in step 4. If the fan spins, you're good.

The green-black trick only works on ATX style PSUs (which have been in use for over a decade, since the Pentium II era).

I still have some old AT-style PSUs which had the old push-button switches. Those don't have that green wire. In that case, just reduce the cable leading to that switch as much as you need to. DO mind the wire colors on the switch as live AC runs thru that particular cable, where the green pin-14 is just a low voltage signal.

uncle frogy (author)2014-01-27

I understand using a PC power supply there are a lot of them so modifying them is good practice. I just used a salvaged wall wart to power a pc fan used to insure some room ventilation using one could I use one to power a fan to do the same job of magnetic stirrer?

Great Idea!

Thanks for the suggesstion.

ppsailor (author)2014-01-28


In what value is the potentiometer?

and how it connects?

NitroRustlerDriver (author)2014-01-26

Very clever using the fan as the "spinning" part. I expected it to be more complicated than that.

Leila S Faraji (author)2014-01-26

Nice project !

Thanks Leila :-)

topperud (author)2014-01-26

Far better to just use a cooling fan for/from a computer (you can find lots of guides here on instructables or other places on the net).

To use a complete power supply like that when you really just need the fan will waste LOTS of energy, that is why the PSU needs a fan in the first place, because it generates a lot of heat - aka wasted energy.

M Allahkarami (author)topperud2014-01-26

Nice comment!

Yes, you are right! This is not an efficient product as you explained. Like all other home made projects that costs more than buying them from eBay or Amazon, lets build these things for fun!

shizumadrive (author)2014-01-26

cool. be careful with that capacitor

Consider My Solution (author)2014-01-26

"magnet stir bar" on Amazon. They are encased in plastic and won't react to liquids or break the beaker.


Thanks for your suggestion!

Nate Cougill (author)2014-01-26

Cool project! This reminds me of this milk frother:

I think the mechanism is very similar.

starphire (author)2014-01-26

This is a pretty neat idea for stirring low-viscosity fluids, as the author suggests. If a large stir bar is needed, or the liquid has a higher viscosity, the fan probably won't have enough torque to turn the stir bar...commercial magnetic stirrers have more powerful motors rotating a much larger magnet.

One thing worth noting for those using a 12V computer-type fan and wanting to control the speed of rotation: these fans use a type of brushless DC motor inside which MUST have the rated voltage (i.e. 5V or 12V, typically) to operate at all. So you cannot control the speed by varying the voltage using an LM317 or similar, as you might with a traditional DC motor. The PWM controller is absolutely the way to go if you want to control the speed of brushless motors!

starphire (author)2014-01-26

Topperud, I think you're missing the fact that old PC power supplies are basically free from obsolete computers destined for the scrap heap. Just using the fan would require a separate power source for this project, which one would have to buy if they don't happen to have spare 12 Volt power supplies sitting around.

I disagree with your claim that they are wasteful! Actually they're about as efficient as any other consumer-grade low voltage power supply. The only reason they have a cooling fan is that they had to be able to provide HUNDREDS of Watts to power all of the hardware inside a typicaly tower-style PC, so at higher loads the fan becomes necessary. For this project, the total load is only 10-20 Watts, so the supply shouldn't get hot at all.

starphire (author)2014-01-26

BurgersBytes, actually the fan is running off a different (12 Volt) output from the PC power supply. Because these supplies won't function at all without *some* load on the 5 Volt output, the resistor is connected to that output instead of the 12 Volt output, to get the power supply unit to operate at all. This is a sneaky way to make use of an otherwise free old piece of computer scrap.

If the container is as large as the fan, it could block airflow meant to cool the power supply. However, with such a small load for this project it's very unlikely that anything is going to overheat in there.

One thing I would caution though, the instructions don't specify a suitable range of resistance for the 3-5W resistor. Too low, and the resistor will overheat and/or waste power unnecessarily. Too high, and it won't draw enough current to start the PC power supply. This should be noted in the instructable!

bezo88 (author)2014-01-24

hey, nice project. do you have a video?

tutdude98 (author)2014-01-24

you can use lm317 for voltage or speed control

M Allahkarami (author)tutdude982014-01-24

Thanks for your suggestion! I just checked the one that I got from eBay. It has LM324 operational amplifier.

Leth Bomholt (author)2014-01-24

you Can use the led in series with the switch instead of using seperate wires

Thanks! that is a better idea.

M Allahkarami (author)2014-01-24


rmalgazhdarov (author)2014-01-24

Haha) i liket

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