new stir plate issue

[stever1000]

I just wired my stir plate and it only seems to run at one speed.
I have a 7.5V power supply, rocker switch, 5W 25 ohm rheostat and pc fan. The whole thing works and will spin my stirbar and create a vortex in the 2L flask, but I can't adjust the speed of the fan when I turn the rheostat. From what I understood it should vary the resistance from 0-25ohm thereby causing the fan to stop (25ohm) or run at full speed (0 ohm). I would like to reduce the speed so that the vortex is less intense.
Any thoughts on how I can wire it differently or change something? I can't tell if I'm by passing the rheostat because it looks like it slows and speeds up a small amount, so I'm assuming its correct

I'm only using the right two (of three) connections on the rheostat , which Other people did as well

Thanks!

 

[day_trippr]

Is one of the two connections to the center contact?

Cheers!

 

[stever1000]

Is one of the two connections to the center contact?

Cheers!

the red (pos) from my fan goes to the center contact on the rheostat.
I've noticed some wiring diagrams use all 3 of the contacts on the resistor, and some use only two of them. This adds to my confusion!

 

[stever1000]

I switched the outer contact that the power supply connects to and it changes the voltage now. But it doesn't fully stop the fan when switched to one side. Is this normal? i.e. the 25ohms isn't enough to fully halt the fan

 

[joebob296]

I switched the outer contact that the power supply connects to and it changes the voltage now. But it doesn't fully stop the fan when switched to one side. Is this normal? i.e. the 25ohms isn't enough to fully halt the fan

For certain fans yes, I have 2 of them that do the same thing. But I wouldn't worry about it.

 

[jekeane]

My build only fully stops when it is powered off.

 

[stever1000]

For certain fans yes, I have 2 of them that do the same thing. But I would worry about it.

Why should I worry about it exactly? Could I fry the fan or other parts?

 

[stever1000]

My build only fully stops when it is powered off.

How did you wire yours?

 

[joebob296]

Why should I worry about it exactly? Could I fry the fan or other parts?

Sorry, that was a typo. I corrected it. Don't worry about it. It's completely normal for some fans to do that.

 

[stever1000]

Thanks, I soldered it tonight so I'm happy how it is. With 2L of water it stirs it just fine.