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Old 04-05-2010, 04:36 PM   #1
mew
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Default Homebrew Stir Plate Potentiometer Issues: Assistance Appreciated!

I'm building a homebrew stir plate and I'm having potentiometer issues; the 25 ohm potentiometer I bought doesn't turn the fan down enough. This is how I wired things initially:

+>rocker switch>potentiometer(25 ohm)>fan
- >fan

So then I added various amounts of resistance (10-200+ ohms) in series with the potentiometer and this slowed the fan down, but reduced the range of the potentiometer to the point that it seemed to be a fixed resistor. So I connected the resistors in parallel with the potentiometer, which slowed the fan to a lesser degree and also reduced the range of the potentiometer. Here's a schematic:

________________>resistor (10-200+ ohms)>
+ >rocker switch>_______________________>fan
________________>potentiometer (25 ohm) >
- >fan

After that, I decided to connect two 25 ohm potentiometers in series. They didn't slow the fan much or speed it up much either. I also connected a 50 ohm potentiometer and it acts like a switch. It allows no current to flow until the very top of its range, then it's fully open.

I'm using a 12V 300MA power supply, which it what the fan is rated for. Any ideas on what to try next? I've read many previous posts about wiring homebrew stir plates, but I haven't found a solution.

One more thing, the rocker switch I would like to use has a lighted LED and four terminals. The top two are oriented horizontal to one another and the bottom two vertical. The top two are labeled +/-. I haven't been able to get this switch to work as I'm used to the three-terminal kind. Any ideas on this? This is how the terminals are arranged:
+ -
_?
_?

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Old 04-05-2010, 07:56 PM   #2
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25 Ohm is a pretty small potentiometer. I don't know what the resistance of your fan is (can you measure it?), but with a range of up to only 25 Ohm on the pot, you probably aren't doing much to the final voltage across the fan.

Adding more resistors in series will make the fan move slower over all, but your potentiometer is still only going to give you a small adjustable range on the speed.

Everything should be in series here. nothing in parallel.

(+)power ----> pot ----> fan ----> (-) power

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Old 04-05-2010, 07:57 PM   #3
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I'm definitely no electrician, but basically you may need a larger resistance potentiometer, thus allowing you to go lower. In theory, though, adding any level of resistor will lower the voltage over the line, and then mean your max power is lower, thus max fan speed is lower, not sure why that isn't working for you, all a potentiometer is a variable resistance device. You'd need to figure out w/ some math, and I can't remember it right now, what your input voltage is, and what you'd need for that system to lower the amperage down to 0 w/ resistance or something like that, sorry that I'm no genius here... I've not used this knowledge since college 20 yrs ago, so hopefully someone that does know can give you more help.

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Old 04-05-2010, 08:03 PM   #4
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Quote:
Originally Posted by mew View Post
One more thing, the rocker switch I would like to use has a lighted LED and four terminals. The top two are oriented horizontal to one another and the bottom two vertical. The top two are labeled +/-. I haven't been able to get this switch to work as I'm used to the three-terminal kind. Any ideas on this? This is how the terminals are arranged:
+ -
_?
_?
Forgot to address this.

When you say that you can't get the switch to work, do you mean that the switch really doesn't work, or you are just having problems with the illumination part of it?

It's possible that the switch you have is meant to pass your choice of AC or DC voltage through on the primary terminals, but requires a specific DC voltage on the other terminals to make the LED work (and it might even be a switch that is constantly illuminated rather than being illuminated only when turned "ON").

There is also the possibility that one of those 4 terminals is not connected to anything at all inside the switch, and the housing is just a generic one that the manufacturer packages many different types of switches in.

If you have a link or something for the switch, it would be easier to sort that out. Otherwise, your guess is as good as mine as to how the terminals are actually used by the switch and LED.
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Old 04-05-2010, 08:10 PM   #5
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I used a 5k ohm pot in my build, it will stop the fan completely, and give me a good adjustment range, your combination of fan and power supply may change the operation but it is a starting point.

here is my build

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Old 04-05-2010, 08:16 PM   #6
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This will cost you more but allows for you a full rpm range plus maintain torque.
I already have the 30 amp with variable Hz control version from a
fuel injector cleaning project and now use it for my stir unit.

http://www.electronickits.com/kit/co...or/CKMX033.htm

This is the unit that was used for my injector flushing and flow balance checking, it may also be used as the MLT and post boil whirlpool motor control, many other uses that has paid for itself 20 X over already.

http://www.electronickits.com/kit/co...or/CKMX067.htm

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Old 04-05-2010, 08:23 PM   #7
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Quote:
Originally Posted by mew View Post
One more thing, the rocker switch I would like to use has a lighted LED and four terminals. The top two are oriented horizontal to one another and the bottom two vertical. The top two are labeled +/-. I haven't been able to get this switch to work as I'm used to the three-terminal kind. Any ideas on this? This is how the terminals are arranged:
+ -
_?
_?
Looking at your text again, I am willing to bet that the + and - pins are SPECIFICALLY for the LED. If you want the LED to always be on, connect things like this:



If you want the LED to only be on when you have flipped the switch ON, connect things like this:

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Old 04-05-2010, 08:29 PM   #8
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Quote:
Originally Posted by BrewBeemer View Post
This will cost you more but allows for you a full rpm range plus maintain torque.
I already have the 30 amp with variable Hz control version from a
fuel injector cleaning project and now use it for my stir unit.

http://www.electronickits.com/kit/co...or/CKMX033.htm

This is the unit that was used for my injector flushing and flow balance checking, it may also be used as the MLT and post boil whirlpool motor control, many other uses that has paid for itself 20 X over already.

http://www.electronickits.com/kit/co...or/CKMX067.htm
Yeah, that's pretty pricey for the application. You can build a pulse modulator for about $3 for low amp use, or connect that modulator to a 40A SSR for heavy duty work for another $12-$15.
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Old 04-05-2010, 08:33 PM   #9
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Quote:
Originally Posted by BrewBeemer View Post
This will cost you more but allows for you a full rpm range plus maintain torque.
I already have the 30 amp with variable Hz control version from a
fuel injector cleaning project and now use it for my stir unit.

http://www.electronickits.com/kit/co...or/CKMX033.htm

This is the unit that was used for my injector flushing and flow balance checking, it may also be used as the MLT and post boil whirlpool motor control, many other uses that has paid for itself 20 X over already.

http://www.electronickits.com/kit/co...or/CKMX067.htm
oh jesus, we're talking about stir plate here.
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Old 04-05-2010, 08:34 PM   #10
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Quote:
Originally Posted by RevIP View Post
oh jesus, we're talking about stir plate here.
LOL. I know. Nothing like using a bazooka to kill a fly.
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