Stir Plate Question...way too HOT

So. Finally got around to making a make-shift stir plate (turned a coffee can upside down...curve on the bottom fits a flask perfectly!) Had all components except the potentiometer which I bought from radio shack. I have a power supply (550 mA) to a computer fan and when I plug it in with the potentiometer, it gets warm. Really warm then hot to the touch. Even the nob is hot. Something is wrong, but don't know what...?

Catnip_X07 said:

So. Finally got around to making a make-shift stir plate (turned a coffee can upside down...curve on the bottom fits a flask perfectly!) Had all components except the potentiometer which I bought from radio shack. I have a power supply (550 mA) to a computer fan and when I plug it in with the potentiometer, it gets warm. Really warm then hot to the touch. Even the nob is hot. Something is wrong, but don't know what...?

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What value pot are you using and what is it's power rating? Is the power supply 12 volts and is the fan a 12 volt fan?

I bet your pushing the pots max mA capacity beyond what it's designed vs what the muffin fan draws. Get a larger pot, better yet a rheostat as they can handle higher current loads or the best a PWM for your speed control.
Carl's Electronics is a cheap PWM supplier, best part the PWM can be used for many other projects just add a low voltage power socket.
http://www.electronickits.com/kit/complete/motor/CKMX033.htm

OOPS. Don't sure if makes a difference, but it is actually a 25-Ohm 3-Watt Rheostat. Fan is 12V. charger is 7.5V

Catnip_X07 said:

OOPS. Don't sure if makes a difference, but it is actually a 25-Ohm 3-Watt Rheostat. Fan is 12V. charger is 7.5V

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The missing number is the fan current draw, it should be marked in mA.
If no tag use a digital meter and read the mA it draws.
I have 120 mm 12 VDC muffin fans that draw 720 mA for 8.64 watts that will smoke
many of the low wattage pots.

The fan says .13A

Catnip_X07 said:

The fan says .13A

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The rheostat would dissipate less than 1/2 watt at that current.

How is it connected?

With your 7.5 VDC the wattage should be less than if running at 12VDC which is 1.56 watt, you should be good with that pot operating at 1 watt at 7.5 VDC. Something is not right here as that pot should handle this wattage the fan is using.
I'll play with 720 RPM muffin fans when they arrive, my thinking that controlling a 720 RPM down to 0 RPM should be easier than a 3,000 RPM to 0 RPM muffin fan for a yeast starter build. I already have a couple spare PWM units to control it.
http://www.amazon.com/dp/B001BMGRMG/?tag=skimlinks_replacement-20

BrewBeemer said:

With your 7.5 VDC the wattage should be less than if running at 12VDC which is 1.56 watt, you should be good with that pot operating at 1 watt at 7.5 VDC. Something is not right here as that pot should handle this wattage the fan is using.
I'll play with 720 RPM muffin fans when they arrive, my thinking that controlling a 720 RPM down to 0 RPM should be easier than a 3,000 RPM to 0 RPM muffin fan for a yeast starter build. I already have a couple spare PWM units to control it.
http://www.amazon.com/dp/B001BMGRMG/?tag=skimlinks_replacement-20

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Carl,

If the fan draws only 130 milliamps and he has a 3 watt pot and the power supply is 7.5 volts as you and I both suspect he has to have it connected up wrong to make the pot get so warm.

Guy,
kind of hard to connect a pot up the wrong way, ending up with fixed low speed using the two outside taps for full fixed resistance or off the center tap be it CW or CCW speed adjusting. Yeah my thinking also that the power supply is connected directly across the pot making for a small heater as well the fan wired in for two seperate resistance loads.
This can lead to letting the smoke out.

A true rheostat has only two terminals while a pot has three. If your device has three terminals, connect the center tap to one of the other terminals. Then connect this now two-terminal device in series with the fan. You should barely feel it getting warm.

Hmm...I thought I had wiring backwards. I changed it up, just to make sure.
charger has black and black/white stripe. Fan has red and black. Did red to black/white and black to black. fan worked. Thought this was causing the overheating, so I changed it up. red to black and black to black/white. Works also.
My next thought is, maybe I need directions on wiring this thing because I didn't think the fan would work if +/- were hooked up wrong.

What ever I received from radio shack is here:
http://www.radioshack.com/product/index.jsp?productId=2062299

Quaffer said:

A true rheostat has only two terminals while a pot has three. If your device has three terminals, connect the center tap to one of the other terminals. Then connect this now two-terminal device in series with the fan. You should barely feel it getting warm.

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I beg to differ my friend; in my hand is a Ohmite MFG. Co. Skokie Ill. 100 ohm 0.71 amp wire wound rheostat with three terminals w/ center tap.

Catnip; all the muffin fans i've played with will only run with the correct polarity, if reversed it will not run at all.
Many muffin fans will have four wires, BLK, R to fan, two whites for the thermistor.

BrewBeemer said:

I beg to differ my friend; in my hand is a Ohmite MFG. Co. Skokie Ill. 100 ohm 0.71 amp wire wound rheostat with three terminals w/ center tap.

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Well, the key word here is true. Most "rheostats" you buy out of a catalog will have three terminals because there is no benefit in having just two. It makes it more useful if it has three. It does not change the fact, though, that a rheostat is always connected by two terminals. Think a 1960's car with a infinite fan speed control. It is a variable series resistor. In the end it makes little difference what we call it. It just has to be connected right or else we let the smoke out.

Catnip_X07 said:

Hmm...I thought I had wiring backwards. I changed it up, just to make sure.
charger has black and black/white stripe. Fan has red and black. Did red to black/white and black to black. fan worked. Thought this was causing the overheating, so I changed it up. red to black and black to black/white. Works also.
My next thought is, maybe I need directions on wiring this thing because I didn't think the fan would work if +/- were hooked up wrong.

What ever I received from radio shack is here:
http://www.radioshack.com/product/index.jsp?productId=2062299

Click to expand...

The important part is what you did with the three terminals on that rheostat, not so much what the color of the wires were. It is real important that whatever you did with it was in series with the fan, nothing in parallel. If you can do that we are almost done. It will either work or not, but nothing will get hot. We'll take it from there. Let us now.

Hook it up like this:

Sawdustguy said:

Hook it up like this:

Click to expand...

Brief and to the point. Perfect.

After redoing everything, it now works. I think. Rheostat/potent no longer gets hot. I think there was a bare wire touching somewhere it shouldn't have. However, now when i plug it in, the fan starts slow, then speeds up on the turn. When it was getting hot, the fan would stay stationary and then speed up on on the turn. Likewise, it would slow down to a stall on the opposite turn. I'm happy in that i can now use the stir plate and no fears of melting my coffee can stir plate or burning down the house. Although vortex isn't that great. I do have other PC fans lying around, so I may try a new fan.