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Home Brew Forums > Home Brewing Beer > DIY Projects > Chillers and Stir Plates > Stir plate not changing speeds
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Old 04-17-2013, 09:41 PM   #1
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Default Stir plate not changing speeds

So I've been reading for like 2 hours now and I can't find a solution to my problem. I used the schematic below.

Voltage regulator - LM317T
Resistor - 330 ohm
Pot - 5K-Ohm 0.5W linear-Taper from Radio Shack
Capacitor's - 0.1uF 50WVDC Ceramic Disk (both)
Fan rating - 12V 200mA
Power supply output - 12.5V 0.64A

When I plug the fan in it starts up and my pot doesn't change the speed in the least bit. What am I missing?

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Old 04-17-2013, 10:15 PM   #2
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Have you tested the pot? Remove it, and run a resistance check on it. Pretty simple to do with a multimeter.

MC

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Old 04-17-2013, 10:33 PM   #3
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FWIW, if the pot has three legs, there will be a fixed resistance between two of them equal to the pot's value. The third leg is attached to the wiper and provides the variable resistance relative to either of the other two. The wiper is usually the center leg.

That is why the circuit layout indicates that two of the legs are shorted together. Make sure you have shorted the wiper leg to only one of the other legs. And not the two fixed legs together as this will simply short across the entire pot and result in no variable resistance.

In addition, make sure that the legs of the LM317 are correctly connected and isolated.

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Old 04-18-2013, 12:21 AM   #4
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The Pot is good (tested with a multimeter, goes from no resistance to around 5000). I checked, and the middle leg is the wiper which is what's jumped. The LM317T is connected right.

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Old 04-18-2013, 12:33 AM   #5
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You dont want to short the center leg to one of the other legs like you have it. you want to use the center leg and 1 of the 2 side legs. Try just removing the resistor you have across both legs and just attach it to the center leg. I bet that will fix the issue.

EDIT: Also wanted to add if after you do that the knob works 'backwards' you can switch the wire you have on the side leg to the other side to have it go the other way.

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Old 04-18-2013, 02:27 AM   #6
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Connecting the wiper to one of the fixed contacts has nothing to do with why the OP's implementation isn't working. Indeed, that is a recommended way to connect a potentiometer, as the resistance across the pot will never go infinite as can happen when a wiper doesn't make good contact with the resistance surface.

The pot wiring is fine. Something else is wrong...

Cheers!

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Old 04-18-2013, 02:14 PM   #7
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Is the DC voltage measured across Cap2 changing with Pot1 adjustment? If so, what is the range?

The circuit is predicted to produce about 12 volts when Pot1 is adjusted for 2000ohms, which is less then half of the available adjustment It will not rise above that, given your input voltage. In addition, it will take about 6 to 8 volts to get the fan spinning, which is at about 900 to 1300 ohms. Therefore, most of your speed(voltage) adjustment is during a fairly limited area of Pot1 adjustment.

FWIW, I fine tuned my LM317 circuit by making changes to Res1 and Pot1 plus an additional fixed resister in series with Pot1. I determined what minimum and maximum voltages were necessary and adjusted the circuit to approximate them at the full range of Pot1.

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Old 04-18-2013, 04:05 PM   #8
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It turned out to be a defective regulator. I replaced it and it's working now. As you said raouliii it's very sensitive to changes in the pot and maxes out after about a quarter turn. What resistor are you using, my calculations tell me that a 550 ohm resistor would give me a more desirable range. Thoughts?

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Old 04-18-2013, 04:55 PM   #9
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There are LM317 calculators online which use a 1.25(1+Pot1/R1) approximation such as this one.

A few considerations to maximize adjustability: You really don't need any outputs below maybe 5v and you can't obtain outputs over 12v. A fixed resistor in series with Pot1 will raise the minimum.

Something like R1=1K, Pot1=5K and Rfixed=3.5K results in a theoretical output range of 5.6v (Pot1=0) to 11.9v (Pot1=5k). Adjustments can be made depending on available values and actual results.

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Old 04-18-2013, 05:49 PM   #10
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This is a great idea. Do you have a diagram of where the "in series" resistor should go? Thanks!

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