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Old 11-14-2012, 01:52 AM   #1171
TBaGZ
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So I am taking my turn and putting together a stir plate... Originally I was going to use a 9v power supply, but it won't turn the fan. The fan is marked 12v, but I figured it would just spin slower. The only other PS I have is 19.5 DC, this spin the fan just fine. The problem is when I wire in the rheostat for speed control. It doesn't do anything. Is a 25ohm 3W too small for this PS, or did I wire it wrong? I went neg on fan to neg on PS, with pos on fan to outer leg of stat, and center leg of stat to pos on PS.

I want to get the wiring all figured out before I start mounting anything.
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Old 11-17-2012, 04:55 PM   #1172
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OK.. I found a 9v PS that was able to power the fan, but my rheostat still is not working. I have power from the PS going to the middle post, and the leg on the right going to the fan (this all the positive side), with the neg going directly from the PS to fan. Should this not work?
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Old 11-18-2012, 04:44 PM   #1173
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Quote:
Originally Posted by TBaGZ
OK.. I found a 9v PS that was able to power the fan, but my rheostat still is not working. I have power from the PS going to the middle post, and the leg on the right going to the fan (this all the positive side), with the neg going directly from the PS to fan. Should this not work?
Sounds right but the rheostat is probably providing too high a resistance so it stops the fan completely. You can wire in an LM317 voltage regulator to provide more voltage but allow the rheostat to dial the voltage down from the full voltage. This thread has such a wiring diagram.

Note that some fans require a higher minimum voltage than others.

 
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Old 11-18-2012, 08:07 PM   #1174
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It's not that the rheostat stops the fan, it doesn't seem to slow it down enough.
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Old 11-19-2012, 03:50 AM   #1175
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So yeah, this rheostat is a dud, going to get another one. I picked up a few resistors and and if I put one in line instead of the rheostat it slows down like it should.
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Old 11-19-2012, 04:21 PM   #1176
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Quote:
Originally Posted by TBaGZ
So yeah, this rheostat is a dud, going to get another one. I picked up a few resistors and and if I put one in line instead of the rheostat it slows down like it should.
Oh, then it doesn't provide enough resistance, it's not defective. Look into ohm's law to figure out how much resistance you need.

 
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Old 11-19-2012, 04:50 PM   #1177
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I was thinking that, but when I put a 10ohm 1w resistor inline it slows down. So why wouldn't the 25ohm 3w stat work?
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Old 11-20-2012, 03:37 AM   #1178
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Quote:
Originally Posted by TBaGZ
I was thinking that, but when I put a 10ohm 1w resistor inline it slows down. So why wouldn't the 25ohm 3w stat work?
It probably did, just not enough to notice the difference. Testing my basic understanding of ohm's law....

Let's say your fan is 12V and .5A. To drop the current in half (.25A), you'd need a 48 ohm resistor. Just the 25ohm rheostat cut the current to .48A (12 / 25), so it barely did anything, only 4%. Adding a 10ohm resistor cut the current to .34A (12 / 35), or 32%, which may be enough to notice.

 
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Old 11-20-2012, 12:35 PM   #1179
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I'm talking the 10ohm resistor without the rheostat made a noticeable difference.
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Old 11-20-2012, 05:25 PM   #1180
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Quote:
Originally Posted by TBaGZ
I'm talking the 10ohm resistor without the rheostat made a noticeable difference.
Don't know what to tell you then. Maybe you're right and the rheostat is defective, although that seems unlikely if it's new. Or maybe the 10ohm resistor is a 100ohm resistor (translating those color bands can be error-prone)?

 
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