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?

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

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

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

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

Step 6: 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

Drill four holes on a rectangular shape acrylic sheet.

Step 8: 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

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

- 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)

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!