Homebrew Stir Plate Potentiometer Issues: Assistance Appreciated!

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mew

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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:
+ -
_?
_?
 
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
 
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.
 
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.
 
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
 
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/complete/motor/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/complete/motor/CKMX067.htm
 
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:

switch1.jpg


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

switch2.jpg
 
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/complete/motor/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/complete/motor/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.
 
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/complete/motor/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/complete/motor/CKMX067.htm

oh jesus, we're talking about stir plate here.
 
Your forgetting I purchased a unit needing not only PWM but Hz control plus able to handle a 24 VDC 30 amp disability stair climber motor.
This being a 4 brush high torque ball bearing motor that will also swing a 9"
bronze outboard racing prop for my HLT and BK for whirlpooling the IC and trub.
I needed 12.7 amps alone on the injector bank flow testing unit hence
a higher amp capacity unit. Purpose puchased for many uses not limiting myself.
True a low amp PWM unit can be built rather cheap if your only driving a muffin fan.
A multi use power supply not a bunch of different power supplies, cheaper buying one unit that fill my needs.
My muffin fan stir motor draws .72 amps, small pots went up in smoke plus less torque vs a PWM power.
 
Your forgetting I purchased a unit needing not only PWM but Hz control plus able to handle a 24 VDC 30 amp disability stair climber motor.
This being a 4 brush high torque ball bearing motor that will also swing a 9"
bronze prop for my HLT and BK for whirlpooling the IC and trub.
I needed 12.7 amps alone on the injector bank flow testing unit hence
a higher amp capacity unit.

I'm not saying that YOU didn't have a use for it.

You're forgetting that the OP wants to spin a little metal rod in circles. :D
 
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


I also have a 5k ohm potentiometer but it acts almost like a switch because it's all off and then suddenly all on.


From all the replies it sounds like I need to figure out the resistance of the fan and match that to the resistance of the potentiometer.
 
I also have a 5k ohm potentiometer but it acts almost like a switch because it's all off and then suddenly all on.
You aren't by chance using a potentiometer that is meant for use as a volume knob, are you?. These are often designated as "audio taper" (as opposed to a "linear taper").

Here's a graph that shows how wildly different the different types of pots can behave when you twist the knob.

If you have an audio pot (curves 2, 3, or 4 below), you could see behavior like you are describing..... twisting it seems to do nothing at first and then it suddenly causes a full speed run of the fan.

pottaper.gif



From all the replies it sounds like I need to figure out the resistance of the fan and match that to the resistance of the potentiometer.

Yes. I've never built a stir-plate, but you probably want a pot that is 3x or 4x the amount of resistance of the fan so that you can swing the voltage across the fan within the range of (nearly) 100% down to something like 25% or 20%.
 
Walker, several of the potentiometers I've tested act like the volume potentiometers you described, including the 5k ohm one, but it is labeled "Linear." I need to read the packages of these things more carefully, I guess.

I have soldered everything in place except for where the potentiometer should go, where I've got alligator clips attached. I'm going to go to the local electronics store and hook up potentiometers until I find "the one."
 
my pot must be a linear one, it is touchy but not so much so that it is almost on or off, full sweep from stop to full is about 3/4 of the range of motion
 
Check into a Ohmite's wire wound variable resistance resistors, they are larger plus can handle the higher amperage like a giant pot or the hollow tube type with the sliding center tap clamp with a set screw clamp (Radio Shack item). The round rotary type you can adjust like a giant pot. Both can be used to get within the rpm range of your stir motor plus add a low ohm pot for your fine tune speed adjustments. For a speed control the larger rotary Ohmite units I bet would cost as much or more than a small PWM unit which gives you 100% from zero rpm's to 100% of your muffin fans speed rpm's. A MOSFET unit, at times is it really worth fighting it going too cheap on an item at times I must ask?
 
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