What potentiometer to use for my stir plate

[TheCubbyHole]

I've been reading for awhile but this is my first post. I'm making my stir plate and Ive got everything hooked up except for my potentiometer. I accidently bought a 100k Ohm pot from the shack and that was obviously way too strong and essentially worked as an on/off switch and then I replaced it with a 25 ohm pot (huge jump I know) and that one is obviously too weak and doesn't really control the speed. I'm using a 15v power supply that I got from my dads work (he helps program modems) so I'm getting quite a bit of juice. I was thinking I probably need something like a 100 ohm pot but radioshack doesnt sell them and didn't want to go pay for a $2-$3 piece and then pay $5 in shipping and then found out that it isn't right. Does anyone have suggestions?

 

[mbbransc]

You'll probably need a resistor in line with the 25 ohm, from what I've read. Otherwise, it will spin too hard and constantly throw your stir bar. I built one over the weekend using the 25 ohm pot from RS along with a 6V power cord and I'd say it's about perfect.

 

[TheCubbyHole]

My problem was I tried to use a 5v Power supply from an old cell phone charger and it seemed too weak. How much of a resistance do you think I would need?

 

[jflongo]

I am also using a 25 ohm with a 6V power and it's fine.

 

[TheCubbyHole]

Has anyone else used a pot with a 15v power source

 

[jflongo]

Has anyone else used a pot with a 15v power source

Maybe you'll get an answer, but 15v is way too high. Look around, ask friends or family, I can almost guarantee, someone has a 5 - 6 v laying around they are no longer using from cell phone upgrades in the past.

 

[TheCubbyHole]

I had a 5v power supply and it seemed way too weak. I could stop the fan very easily by just nudging a blade. Maybe I'm expecting it to spin faster then it should? This fan is also a 12v fan, is that what everybody else is using?

 

[tracyk]

It seems like the newer fans do not spin as easy as the old ones from a couple of years ago. I build a stir plate about a 1 1/2 years ago, then went one to buy the exact same fan to built a stir plate for a buddy. On a side by side test the older one is definately spinning faster. Everything else is the same. Same pot, same magnet, same power supply, unless there is something wrong with the pot. It still works, just not as efficently as the older one. I just told my buddy if he found a better fan to trade it out.

 

[jflongo]

I had a 5v power supply and it seemed way too weak. I could stop the fan very easily by just nudging a blade. Maybe I'm expecting it to spin faster then it should? This fan is also a 12v fan, is that what everybody else is using?

Also, you have a knob on your potentiometer right to adjust the speed? So you are saying even at full power, it didn't spin fast? Also, I just doubled checked, I'm using a 8V not a 6V supply, with my 25ohm. If I turn my up to 75% or higher, it will spin the bar off the magnets.

 

[TheCubbyHole]

Also, you have a knob on your potentiometer right to adjust the speed? So you are saying even at full power, it didn't spin fast? Also, I just doubled checked, I'm using a 8V not a 6V supply, with my 25ohm. If I turn my up to 75% or higher, it will spin the bar off the magnets.

Correct with the pot at wide open (no resistance) and a 5v power supply it seemed to move way to slowly. Now I've got a 15v power supply and the 25 ohm (full resistance) is still too fast. It looks like I'm just going to have to buck up and pay $5 shipping on a $2 pot to get a 100 Ohm

 

[jflongo]

Correct with the pot at wide open (no resistance) and a 5v power supply it seemed to move way to slowly. Now I've got a 15v power supply and the 25 ohm (full resistance) is still too fast. It looks like I'm just going to have to buck up and pay $5 shipping on a $2 pot to get a 100 Ohm

Or ask family/friends if they have a 6V or 8V laying around, probably 50% of the people do, from old phones/routers/etc.