Calling on anyone who knows how to wire a potentiometer!!

[snowtires]

Hi im working on a stir plate build and looking around for items i have around the house i found a pair of computer speakers so i got the volume control knob off it but I dont even know if it would work for a stir plate. I have found it online but dont know how i would wire it up. So if anyone out there knows if this would work and how, that would be much appreciated.

http://www.alibaba.com/product-gs/346285679/16mm_DIP_Rotary_potentiometer_with_switch.html

 

[ajdelange]

I guess I've wired a pot or 2 in my day. It's not as simple as you are hoping, however. There are dozens of ways to control speed with a pot. Putting it in series with the field of a motor is one way. Using it to control the firing angle of an SCR is another and using it to control the frequency of an inverter's output yet another. The particulars depend on the design and you must have the proper pot to match the rest of the design.

 

[DanH]

I think it will work fine. What's your power source? I'll post a pic of mine wired up when I get home tonight cuz it looks very similar.

BTW Radio Shack has a ton of those to choose from.

 

[snowtires]

I do have various resisters and some voltage regulators and i can solder, but don't know much about how it all works. I can sure put it together tho once told. Oh the fan is 12v 400ma and i have various 5 volt 700ma adapters as well as a 12v 300ma I have a bunch of stuff Thanks

 

[Hamsterheadsalem]

[ame]http://youtu.be/28X8XckvUTM[/ame]
This is the best YouTube video I have found. There are many more that give step by step instructions. I built mine for about $20. I used a project box from Radio Shack and gutted an old PC tower and used the fan and magnet from the hard drive. I would spend the few bucks at RS for a 3 pronged potentiometer, mine works well with a 1000 ml flask!
Good luck

 

[dmcman73]

Simplify it. Use a separate on off switch along with a three prong POT from radio shack. Here is how to wire it up: http://www.electro-tech-online.com/...-how-do-i-wire-potentiometer-hobby-motor.html Note that you don't need a wirewound high current POT they are describing in that web page, that's to control a larger motor. Something like this will do the trick: http://www.radioshack.com/product/i...5&filterName=Type&filterValue=Potentiometers#. The shaft of the POT is long so you can cut it to the length you need it to be, determined by the thickness of the project box you bought.

 

[Randomguy18]

How does the math work for safely running a 12V DC, 0.14A computer case fan on, say a 9V, 0.5A power source? What kind of resistance do I need from a pot?

I thought it was all about the voltage and that these things wouldn't draw more amps than needed, but frying the cheapest 12V brushless fan from dx.com in seconds on a 9V battery makes me question that.

 

[dmcman73]

How does the math work for safely running a 12V DC, 0.14A computer case fan on, say a 9V, 0.5A power source? What kind of resistance do I need from a pot?

I thought it was all about the voltage and that these things wouldn't draw more amps than needed, but frying the cheapest 12V brushless fan from dx.com in seconds on a 9V battery makes me question that.

You can safely provide more AMPS from the source (power supply) than what the device calls for, the device will only use what it needs but do not supply less than what the device is rated or it will try and pull what it wants and blow out the power supply. As for voltage, you cannot go over the rated voltage it requires or you will burn it out. Lower voltage means it will spin slowly already and I would match the Votage the fan is rated at to the power supply.

Now regarding the size POT you need, it's not a simple "cut and dry" answer, you are going to have to do some math in order to find the answer.

First you'll need to find the resistance of the fan itself (this will only work for DV fans): R = Voltage / Current. So for example, if your fan is rated at 12V @ .08 amps with a .96 Watt consumption then R=12/.08 which will give you a resistance of 150 Ohms. This means you'll need a POT with nominal resistive value of 150 Ohms and at minimum can handle .96 watts. The higher the wattage the POT can handle above the draw, the cooler it will stay.

EDIT: Going by your fan rating, you will need an 86 Ohm POT. They really do not make an 86 Ohm one so a 100 Ohm POT will do. Do not use the 9 volt supply, get a 12V supply with the same current rating or a little higher.

 

[Randomguy18]

Thanks! I eventually wound up buying a pre-built fan speed controller from ebay, which was cheaper than the pots i could find.

 

[dmcman73]

Also, if any one else is looking, if you have a Microcenter or similar computer parts stores, they sell fan speed controllers for cheap like this: http://www.microcenter.com/product/391591/Twister_35_Front_Panel-PCI_Slot_Fan_Speed_Controller_Black. You still need a power supply though.

 

[Dawai]

Find you a "brush type motor" like a old sewing machine motor.. or?? drill motors are brush motors too. A Harbor freight router speed control, has a plug, a receptacle.. you just put it inline.. away you go. I got one on sale for $5. Used it for things like "heat control for a plastic welder, speed control for various things"

I could go into detail how to build a 556 pulse-width modulator.. but you probably don't want to etch a circuit and build it from scratch. On the back of that pot you will see a "wattage" probably 1/4 watt.. most motors, 24 volts and five or more amps.. so no direct hook up.. it'd be like Amps x volts = watts.. and you see that adds up to more than you got of a pot. A motor I have here off a 25watt pot runs a hand massager, It is about four inches in diameter and three inches thick.

Now you could of course go to a variac, a transformer you turn to adjust windings.. variable voltages.. or a DC power supply like I used to use tattooing (off ebay for $30)

A light dimmer, it is a triac/diac (driver) with a built in pot, cut's phase angle to clip voltage and current. Costs more than the HF speed control did.

The HF router speed control was a "less work" and simpler method for me.
NOW find us a source for magnetic stirrers..

 

[DanH]

Here's mine. I just hooked up the potentiometer in-line between an AC Adapter and the fan. Then I connected the neutral wires of the adapter and fan together. The AC adapter has a switch to choose a range of voltages from 3 to 12. What works best for me is 7.5V; any higher and it will throw the bar. The lower ones don't allow much of a range of speed with the potentiometer.

 

[Dawai]

Okay, that is a tiny motor, I am still interested how "hot" the potentiometer gets.

Last one of these I "tore down" and repaired had a motor the size of my wrist.
The CPU fan is a new trick for a old "dog here". Inventive.

(edit)..

Some of those "cpu fan motors" are pulsed DC, some are AC (will not work like this example) they did that to stop the brushes from wearing out. TO logically control a regular brush type motor, instead of doing the BRUTE" force method of limiting resistance, (that pot is a variable resistor with a sweeping brush) you'd be better off to build a small breadboard with a 3904, or 3906 transistor and a couple of resistors and "that pot" to control gate of the transistor". OR look up a "chopper circuit". It cuts the dc voltage into bites limiting the current and voltage.

You could turn over a kids toy car or go to Robotstore.com and find a current limit circuit. I may dig through the junk drawer here and see if there is anything I can send you. I have cases full of New electronics I've picked up over the years.

(Edit) for example, off ebay listing. This one still requires a small wall bug, transformer.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=181112543103&fromMakeTrack=true&ssPageName=VIP:watchlink:top:en