Stir plate doesn't have enough control

[forgetaboudit]

I built my own stir plate with a PC fan, laptop charger, magnets, and a potentiometer a year ago.
I am now finally getting sick of it, and want to know how to fix it.
The potentiometer seems to only have a tiny amount of adjustment on the speed of the fan. It's pretty much either on, or off with a very minute amount of movement to speed up/slow it down.

What could the issue be? Potentiometer not correct size? Fan not correct?

 

[gromitdj]

Supply voltage is probably too high. Try a 5 volt wall wort. If that doesn't solve your problem, you might want to look into one of the PWM or LM317 circuits.

 

[day_trippr]

A pot with too high a resistance range would behave like that...

Cheers!

 

[piojo]

Does your fan have 4 wires? If so, you can do something even better than PWM power reduction: you can give it a PWM signal that tells it how fast to run, and the fan will control its own speed based on what you tell it. I'm not sure how deep down the rabbit hole you'll want to go, but this would require soldering and buying a few extremely common electronics components.

 

[forgetaboudit]

Does your fan have 4 wires? If so, you can do something even better than PWM power reduction: you can give it a PWM signal that tells it how fast to run, and the fan will control its own speed based on what you tell it. I'm not sure how deep down the rabbit hole you'll want to go, but this would require soldering and buying a few extremely common electronics components.

Like what?

 

[piojo]

Like what?

I think you need a 555 timer, any potentiometer, a transistor, a couple resistors and ~2 capacitors. Whatever is currently driving the fan will stay the same, but you'll hook up this new circuit to the speed-controlling wire. (If it's a 2-wire fan, it won't have a speed control wire, and a 3-wire fan probably won't.)

 

[Toxxyc]

Sounds like you have a logarithmic potentiometer installed and not a linear one. Linear one is a cheap and quick fix.

 

[forgetaboudit]

The potentiometer is 10K ohm, 8.5V power supply, and 12V 200mA fan.

 

[day_trippr]

Regardless of the taper a 10K ohm pot is suboptimal by two orders of magnitude. This is why your control is reduced to just the very last bit of travel.
What would work better is a linear pot (or rheostat) in the ~200 ohm range with a few watts power capability...
This is a bit overkill (by about 5X) but it's cheap enough to splurge https://www.amazon.com/dp/B013JKQLQY/?tag=skimlinks_replacement-20

Cheers!

 

[Toxxyc]

The potentiometer is 10K ohm, 8.5V power supply, and 12V 200mA fan.

Doesn't matter what capacity you get, if it's a log pot it's going to cause issues. The reason is because it works on a logarithmic scale to increase/decrease the resistance. If you look at the graph of how it works, you'll see the difference between logarithmic and linear:

Now if the motor requires X amount of power to turn and the potentiometer only provides this level of power at 90% (which is around the last 8% of the rotation according to the graph), you're going to struggle to get the correct speed you want. With a linear pot you get at least double the play. If you play with the resistance (like @day_trippr above mentioned), you can get even better with it. I made this mistake once by buying the cheapest pot I could get to use as a volume control on a DIY amp. It was annoying that 90% of the rotation had no volume and then suddenly all the volUME IS THERE IN YOUR FACE!!!

Buy the pot mentioned above. Just check with them to make sure it's LINEAR, and not LOGARITHMIC.

 

[day_trippr]

Right, the actual problem with a 10K pot regardless of taper is the voltage drop is humongous until you're down under a hundred ohms or so. Decades of experience with 12V dc fans says somewhere around 5.5V is the average threshold for reliably starting a 12V fan, and the OP is starting with an 8.5V power source. With a 200ma draw it doesn't take much resistance to drop 8.5V below that threshold.

Also, 10K signal pots don't carry 200ma well for long...

Cheers!

 

[forgetaboudit]

I replaced the 10k with a 100ohm potentiometer and all is good with the world again!