Stir Plate Wiring Help Needed!

[imp81318]

I'm building my own DIY stir plate, which I would think should be straight-forward enough but I seem to be having issue after issue... I have the most simple circuit possible - just the power supply, potentiometer, and fan. No toggle switch or anything. I have the black/negative wire from both the power supply and fan attached to the first prong of the potentiometer. The hot from power supply is on the middle prong, and the hot from the fan is on the third prong.

I wired it up this way and after about 10 minutes of testing it out the power supply died (confirmed with volt meter). So I scrounged up another old power supply and tested it out - by just holding the stripped wires from the power supply, the fan spins as it should but after a few seconds the potentiometer starts to smoke and I get the burning electronics smell.

Component stats are:
First (Dead) Power Supply: 15V 200mA
2nd Power supply: 12V 2.0 Amp
Potentiometer: 25 ohm
Fan: 12V 0.25 Amp

Do you think it is most likely that I overloaded the potentiometer while playing around with the first power supply?

 

[thumpersk_a]

If im understanding correctly i think you have the potentiometer wired wrong. You should only have one leg of the power supply running through the potentiometer to the fan and the other leg straight to the fan

 

[TallahasseeBrew]

you probably burned the potentiometer out. Smoking and burning smells are not your friend. You shouldn't have any negative wires hooked to the potentiometer at all. You should have the negatives from the fan and power supply hooked together, and the positive from the power supply hooked to the wiper (the moving conductor) lead of the potentiometer. Usually this is the middle pin. then hook the fan positive wire to either other prong on the potentiometer, it shouldn't matter which. That should get it running.

Oh and i wouldn't use a 2 amp power supply with a .25 amp fan. while true the fan should only draw the current it needs, but if something were to short in the fan, you have a lot of juice on tap for it. I would find another 12 volt .25 amp power supply.

 

[BigRob]

Sounds like it's wired incorrectly, also anything that smokes should be replaced.

 

[lschiavo]

 

[imp81318]

I have it wired according to the diagram above with a brand new potentiometer, and I cannot get it to run below 7.3 V with the potentiometer knob turned all the way down, and that speed throws my stir bar every time. When I first wired it up and plugged it in, the fan did not immediately start spinning until I turned the knob at which point it seemed to behave appropriately - as I turned the knob the fan started spinning slowly and the more I turned the knob 'up' the faster it spun, but when I tried to back it down I lost the ability to dial it all the way back to a slow speed. Is this likely just a limitation of the range of the potentiometer, or did I screw it up yet again?