Magnetic Stirrer Maintenance

I recently got this magnetic stirrer labeled “Fisherbrand Agitateur Magnétique 10510”. It had remains of silicone on the top.


It had some problems with the motor starting at low speeds, so I figured let’s check it out and try to fix it.

The circuit is unimpressive, it’s a small transformer, discrete bridge rectifier and a BD681 NPN transistor controlled by a potentiometer to regulate the motor speed. However the advantage here being, that is super simple to troubleshoot and repair. I definitely prefer this over uC controlled or PWM stirrers.

There was a lot of flux residue on the PCB, to be fair however it looked like someone messed with it at some point, so it’s unlikely to be out of the factory. I removed the flux residue and also the bridge on the line input, since the PCB track was intact.

PCB top PCB bottom

The capacitor lost some capacitance, probably because the circuit is essentially powered 24/7 when plugged in. The motor was struggling to start at lower speeds requiring to dial it to a high speed first and then lowering it. This indicates that the circuit cannot handle the startup current at lower speeds. To mitigate this I replaced the 220uF capacitor with a 470uF one to allow for more current capacity. This solved the problem, the stirrer now starts at very low speeds no problem.

I also added a sheet of plastic (overhead projector slides) over the PCB, to avoid the two magnets from shorting out anything (especially the primary side) in case they fall off.

PCB fixed The screw on the left back, is the chassis PE connection, so I didn’t fix the insulator with that one