Stir Plate Fan Rheostat/Potentiometer Hairline Speed Control Problem

[BrotherBock]

Working on my stir plate. I have it all wired up and works fine. Power turns it on and off and my rheostat cuts down the speed. However, if I turn it too far (which is barely anthing) it cuts the fan speed completely. I need to use hairline adjustments to moderate the speed. When open full it spins pretty damn fast.

It a 12v DC power supply and a old PC fan. Don't know any stats on the rheostat at the moment. I can find them if it helps diagnose the issue.

Advice? Do I need some kind of capacitor or resistor installed?

 

[BrotherBock]

Shameless bump. Anyone?

 

[raouliii]

A simple potentiometer, in series with a DC fan, speed control is the least workable option because of the exact problem you mention. I would suggest upgrading to a voltage regulator based or a PWM type speed control. Purchased or handmade.

I use a LM317 voltage regulator based circuit on my stirplate.

 

[MardyBum]

A simple potentiometer, in series with a DC fan, speed control is the least workable option because of the exact problem you mention. I would suggest upgrading to a voltage regulator based or a PWM type speed control. Purchased or handmade.

I use a LM317 voltage regulator based circuit on my stirplate.

Do you have a wiring diagram of your stir plate? I think i wired mine wrong because the diagram i used included resistors and capacitors. It also included the LM317 in the diagram and i wired it all up but it just would seem to work.

 

[raouliii]

Do you have a wiring diagram of your stir plate? I think i wired mine wrong because the diagram i used included resistors and capacitors. It also included the LM317 in the diagram and i wired it all up but it just would seem to work.

Incorrect wiring will always result in a non-working circuit.

Here is a stock circuit diagram and pinout diagram.

Vin is from the DC supply, through a switch.
Vout is to the positive lead on the fan.
The negative lead from the DC supply is connected to the bottom, ground buss, as is the negative lead of the fan.

I built mine with a little experimentation along with using combinations of resistor values I happened to have on hand.
I used a 12VDC wall wart power supply and a 12VDC fan (if I recall correctly).
I settled on R1=3kohm (3 - 1kohm in series) and R2=5kohm(fixed) + 5.1kohm(potentiometer) in series.
This resulted in an actual adjustable output voltage to the fan of 3.7vdc to 6.2vdc.
In use, I find that only half of the potentiometer's adjustment is useful during stirring which equals to about 5.0vdc to 6.2vdc to the fan.

There are many useful LM317 calculators on the web.

 

[BrotherBock]

View attachment ImageUploadedByHome Brew1418919820.276906.jpg I essentially used this diagram. With the exception of the rocker switch. I had one of those copper lever things.

It's definitely wired up correctly. At least in the sense that everything works. It's just that the speed control is temperamental and could be more effective. But I'm not sure the best route. Or the easiest. It doesn't need to be pretty.

 

[raouliii]

...... At least in the sense that everything works. It's just that the speed control is temperamental and could be more effective. But I'm not sure the best route. Or the easiest. It doesn't need to be pretty.

As I mentioned in my earlier post, a rheostat/potentiometer, of a resistance value about equal to the fan, in series with the fan is a workable and simple circuit. However, it presents the exact issues you are having. When the pot is at 0ohms, the full voltage is applied to the fan and when the pot is at max, it essentially divides the voltage to a value related to the fan and pot resistances.

In my testing I have found that the voltage provided to the fan, for stir plate operation, has a relatively narrow usable range, well above 0v and well below 12v. Therefore, a circuit that provides better voltage regulation is a better solution.

Of the methods discussed on HBT, from worst to best, imo: Pot in series with fan, LM317 based voltage regulator, PWM. The level of complexity of the circuit also happens to be the same order.

 

[Westermans]

You can also use a lower voltage power supply. I run my 12 volt fan with a 5 volt power supply. The rheostat then controls at lower speeds.