Magnetic Stirrer

Needed a magnetic stirrer to prepare solutions. Folks seemed to be making these out of axial cooling fans from computers and hard drive magnets. I had a bunch of axial fans and a few hard-drives that I had to discard. Commercial magnetic stirrers were finicky as you had to adjust the rotation speed till the magnetic bar in your solution "caught" and then spun smoothly. If the motor speed was too fast the magnetic bar would jiggle but not rotate. So I knew I would need good speed control.

As I checked the fans that I had, one of them had an interesting specification. It was rated for 12V-30V DC with different current draws. I tested this fan with an adjustable lab supply and it seemed to handle voltages from 12-30V quite well. I ordered a DC motor control board from an ebay vendor based on a 555 Pulse Width Modulation Circuit. DC motors like PWM rather than decreasing voltage. I also had boost power supply boards that would increase voltage from 5V or higher input to an output of up to 28V. So the magnetic stirrer took shape pretty quickly. I would feed the fan motor 24V through the pulse width modulation speed control board which would give it more power and more speed to handle big stirring jobs.

I added a switch so that the motor would run from 12V for lighter loads or from 24V for higher loads. Both the 12V and 24V were switched through the PWM motor control board so I could still vary the motor speed at low or high power settings.

The end product turned out quite well. Nice smooth speed control. Compact device but can still take a heavy liquid load on top without falling apart. The video shows it in operation.

I drew up a concept of the magnetic stirrer based on the parts I had. Decided on a 6 inch square surface as still small enough to store but big enough to support big flasks on top. I cut the parts out from Pergo laminate flooring panels that I had got from a freecycler. I harvested a magnet from an old hard drive. The process is relatively simple if you bend the magnet support plate rather than trying to pry the magnet out from its support plate with a chisel or screw driver. Bending the plate breaks the epoxy bond and the magnet can be easily removed. You can see the two support plates. The bottom one still has the magnet stuck on it (though released from its epoxy bond). The top plate is missing its magnet but the two glue tabs can be seen.

The harvested magnet was placed on a bead of polyurethane glue placed on the top of the fan rotor. The motor was spun at speed for the magnet to move and self balance on top of the rotor. The glue then cured and held the magnet in place.

The base plate had holes drilled in radially so that air could be sucked in from the bottom and escape from holes in the sides of the box. To this base plate the motor was glued and screwed. The two sides were also glued to the base plate. The wiring diagram shows how I connected the different parts together including a switch that would either directly pass 12V to the PWM board or channel the 12V through the voltage boost board and then pass the up-converted 24V to the PWM board. The output of the PWM board was directly connected to the motor.

The DC jack that I was using to supply power to the Magnetic stirrer was harvested from some old electronics and it already had a series diode on the DC input jack.

Holes for the control knob and the switch were drilled in the 6 inch wide front panel. A small piece of wood was glued perpendicularly to the front panel to support the PWM board. I also glued a small piece of aluminum to serve as a heat sink for the voltage boost board. And to hold the voltage boost board I had to glue to other pieces of wood to which the boost board would be screwed into. The way the board was attached, the converter IC would be pressed against the heat sink. The last figure shows the two boards wired together.

I had to cut the wires and remove the voltage boost board as I had decided to add the three-way switch (Off-low power-high power) later on. I adjusted the output voltage on the boost board to deliver 24V and then re-wired the +12V input and +24V output from this board through the switch. The 0V line from the DC jack was soldered directly to the input of the boost board and the 0V line from the boost board output was wired to the PWM board. The + input to the PWM board came from the switch. Hopefully, the photographs and the little diagram will make sense.

I taped the wires to clean up the insides and then attached the front panel to the sides with nails that went into pre-drilled pilot holes. You can just about see the nail heads in the second picture.

For the top I wanted a thin and stiff panel. Thin so that I would not lose the magnetic force which falls off dramatically with distance and stiff so that it would not bend under the weight of heavy flasks and touch the rotating magnet. I had discarded pieces of fiberglass paneling from which I cut a 6 inch square. Just to be safe I glued wooden pillars to provide more support to the top panel. These pillars also provided places to screw the top panel. Drilled holes into the top panel, countersunk these holes and then glued and screwed the top panel.

I painted the whole box with black acrylic paint after roughing up the surfaces with a wire brush, I then glued labels and will apply a coat of acrylic varnish or sealant to cover the surfaces and protect the labels. I ran the unit with a steel rod and the speed control was good enough to dial the optimum speed to get a good vortex going.

Overall happy with the way this simple but useful device has turned out.