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davesj1
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Registered: 04/11/05
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Proper rhiostat for DIY stirrer?
#5774033 - 06/20/06 08:42 PM (17 years, 7 months ago) |
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Ok so I have a supply that puts out 12VDC at 700mah and a fan that is 12DC at 190 mah. What kind of rhiostat do i need to be able to get the rpms down near 100 at a low setting.
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splifner180
Student


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Re: Proper rhiostat for DIY stirrer? [Re: davesj1]
#5774078 - 06/20/06 08:54 PM (17 years, 7 months ago) |
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Get yourself a 9VDC power supply. I find it works perfectly. Makes a minor (1/4 - 1/2") dimple.
splif
-------------------- First Grow: Ecuador -> LC -> HPoo/Straw -> Monotub Build a Do-It-Yourself Magnetic Stirrer in thirty minutes with no money.
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fastfred
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Re: Proper rhiostat for DIY stirrer? [Re: splifner180]
#5774644 - 06/20/06 11:06 PM (17 years, 7 months ago) |
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Any sort of pot should work fine. You would probably do better with a linear one rather than an audio (log) type. Any electronics you tear apart should have a few.
-FF
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davesj1
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Re: Proper rhiostat for DIY stirrer? [Re: fastfred]
#5775141 - 06/21/06 01:16 AM (17 years, 7 months ago) |
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what is a "pot". If i decide to buy one from say radio shack what should i ask for? THanks again
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fastfred
Old Hand



Registered: 05/17/04
Posts: 6,899
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Re: Proper rhiostat for DIY stirrer? [Re: davesj1]
#5775307 - 06/21/06 02:15 AM (17 years, 7 months ago) |
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pot = potentiometer It's the standard term for a variable resistor. If you go to radioshack just tell them that you want to use it to control a 12V computer fan.
Actually here... From: http://www.computerpoweruser.com/editori...3c02c.asp&guid= Hmmm.... Seems they want you to pay for that one. Try this: http://66.102.7.104/search?q=cache:tf-rv...us&ct=clnk&cd=4
"Picking the right pot is also fairly straightforward, again because typical case fans don't draw much current. There are two ratings to check when choosing a pot: power and maximum resistance. Our pots were rated for 5 watts with a max resistance of 100 ohms. To calculate how many watts your case fans draw, multiply the amount of current they draw by 12V. (This information is usually printed on the fan.)"
"You'll also need to consider the pot's maximum resistance. The higher the pot's maximum resistance, the more it will restrict the current flowing through the circuit. If the resistance is too high, the fans may not spin. If it's too low, the pots won't restrict enough current to noticeably affect the fan's speed. Pots rated between 50 and 100 ohms are best suited for the task; with anything higher or lower you may run into the problems mentioned earlier."
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creamcorn
mad scientist


Registered: 03/13/06
Posts: 2,962
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Re: Proper rhiostat for DIY stirrer? [Re: fastfred]
#5776377 - 06/21/06 12:34 PM (17 years, 7 months ago) |
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yeah, you want exactly that... a 5 watt pot at about 100 ohms. unfortunately radio shack doesn't sell such a beast. i think the one they carry is 75ohm/3 watts (which will still work, but they charge too much for it and even with it turned to the lowest setting your fan might be too fast... thats where you step down to a 9v adapter like mentioned.)
using any old pot isn't a great idea either... an audio pot for example can handle 0.1W. you'll burn it up in no time. most of the smaller ones are intended for low level circuitry, like preamplified audio signals, and the resistance is usually way higher in the 10k or 100k ohm range... meaning that if you picture the pot as a clock with 12 oclock being full speed, by the time you turn it to 1 o'clock your fan is already turned off. the rest of the range is useless for this purpose because the resistance jumps too high, so you'd get little to no adjustability out of it. and actually assuming an audio pot could handle the power, a log one would work better, because you'd have a larger range of low resistance to work with. (where a linear would have say increments of "100, 200, 300" a log one would be "1, 10, 100")
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RogerRabbit
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Re: Proper rhiostat for DIY stirrer? [Re: creamcorn]
#5776645 - 06/21/06 01:50 PM (17 years, 7 months ago) |
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I agree that an audio pot won't work because they're used to adjust the bias on a transistor and the resistance value will be in the k-ohm range since there's little to no base/emmitter current.
A 12 Volt motor that draws 190 milliamps will have an internal resistance of 63 ohms.(you don't have to worry about inductance, power factor, or impedence with a DC circuit because there is none) If the speed/current curve is linear, a 63 ohm rheostat or pot will cut the speed in half when reduced to minimum. I would say a 75 to 100 ohm pot will give you what you're looking for, so that's good advice. Cutting the voltage as suggested by creamcorn is also a viable option, so you have more than one way to go, or you could combine the two for even more control.
12 volts at 190ma (.19 amp) equals 2.28 watts, so you want at least a 3 watt rating on the pot you choose to avoid excessive heat buildup. If you can find one at a reasonable price get it, and if the motor is still too fast at the lowest setting, you can always get a fixed resistor from radio shack to put in series with the pot. If you get a 50 ohm fixed resistor, the new maximum current will be based on the 63 ohms of the motor plus the 50 ohm resistor for a total of 113 ohms. This will limit current to 10.6 ma which means maximum power will be 1.3 watts, so you could lower the rating on your resistor and rheostat to 2 watts. Confused yet? RR
-------------------- Download Let's Grow Mushrooms semper in excretia sumus solim profundum variat "I've never had a failed experiment. I've only discovered 10,000 methods which do not work." Thomas Edison
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creamcorn
mad scientist


Registered: 03/13/06
Posts: 2,962
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Re: Proper rhiostat for DIY stirrer? [Re: RogerRabbit]
#5776920 - 06/21/06 03:27 PM (17 years, 7 months ago) |
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with the resistor route you need one that can handle the power as well. the little common ones you see that you can pick up for basically pennies aren't going to cut it, they usually handle a fraction of a watt as well.
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davesj1
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Re: Proper rhiostat for DIY stirrer? [Re: creamcorn]
#5777344 - 06/21/06 06:00 PM (17 years, 7 months ago) |
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Why arent more people running into this problem. Ok so where would i find a 100 ohm/3watt pot. I looked on Ebay and didnt find anything.
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davesj1
Stranger

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Re: Proper rhiostat for DIY stirrer? [Re: davesj1]
#5777397 - 06/21/06 06:13 PM (17 years, 7 months ago) |
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Could i use the 75 ohm radioshack pot in series with a 25ohm resistor?
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RogerRabbit
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Re: Proper rhiostat for DIY stirrer? [Re: davesj1]
#5777716 - 06/21/06 07:26 PM (17 years, 7 months ago) |
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Yep, as long as they're both rated for three watts or more. In fact, it's probably cheaper to do the whole job with resistors. You could get four of the 25 ohm resistors and just put them in series to get the 100 ohms. This way you can use as many as needed to lower the current. When you find the sweet spot, solder or otherwise crimp the connections to make them permanent. RR
-------------------- Download Let's Grow Mushrooms semper in excretia sumus solim profundum variat "I've never had a failed experiment. I've only discovered 10,000 methods which do not work." Thomas Edison
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splifner180
Student


Registered: 03/24/06
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Re: Proper rhiostat for DIY stirrer? [Re: RogerRabbit]
#5778884 - 06/22/06 12:13 AM (17 years, 7 months ago) |
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Dude, I'm telling you. Get a 9VDC power supply. Why are you giving yourself an aneurysm? =P
Go to Radio Shack. Most of them have a bin in the back with wire and transformers they'd probably give you for free if you just ask. I poked my head in, asked for any spare 9VDC unit regardless of connector, polarity or amperage and they let me raid the whole box.
But if your geek side just won't let you accept a simple answer and your balls will explode if you don't rig up something cool, buy one of these. Connect it to a ripped-out PC power supply and you can control 4 stirrers independently. Radio Shack has those too, and they're already built to control a computer fan, so...
splif
-------------------- First Grow: Ecuador -> LC -> HPoo/Straw -> Monotub Build a Do-It-Yourself Magnetic Stirrer in thirty minutes with no money.
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Prisoner#1
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Re: Proper rhiostat for DIY stirrer? [Re: davesj1]
#5779378 - 06/22/06 02:44 AM (17 years, 7 months ago) |
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if you have an old stereo with knobs, shaft type car radios and such as well you can scavenge what you need from them, the volume and tuner nobs are both potentiometers that are capable of handling the task, most electronic equipment is stepped down from 115v to 12v. radio shack and maybe even home depot should have them as well now that HD carries a lot of low voltage junk for home theatres
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creamcorn
mad scientist


Registered: 03/13/06
Posts: 2,962
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Re: Proper rhiostat for DIY stirrer? [Re: Prisoner#1]
#5779886 - 06/22/06 09:17 AM (17 years, 7 months ago) |
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nope... we covered that a few posts back. volume knobs and such will burn to a crisp in short order, they handle a tiny fraction of the power a fan will pull through it. you could get lucky and find one that doesn't burn up when you try to use it, but its still not very safe and can become a fire hazard (especially if you leave your plate on 24x7). tuner knobs probably won't work, a rheostat (or potentiometer) is an adjustable resistor, but if you're pulling apart an old radio (especially an AM radio) you'll have an adjustable capacitor on your hands, which won't work at all.
i found this picture real quick to illustrate the difference, this is some home made radio offa some random webpage i dug up real quick.

the thing on the left is a pot (volume control) the thing on the right is a variable capacitor (the tuner)... so if you try salvaging things from a radio make sure it doesn't look like the one on the right.
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fastfred
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Re: Proper rhiostat for DIY stirrer? [Re: creamcorn]
#5780248 - 06/22/06 11:38 AM (17 years, 7 months ago) |
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There are plenty of web pages on controlling computer fan speed. The one I linked to earlier describes building a cool 3 or 4 fan controller with led lights and switches. So nobody's balls should explode from not being able to rig something fancy up. If that's not fancy enough for you then google "PWM fan controller" and you'll find some interesting pages on how to build a whole fancy circuit board to control fan speeds. I think that's way overkill though. Just get a 5 watt, 100 ohm pot and it will do the trick for you. If ratshack doesn't carry one then you're probably not looking hard enough, otherwise I'm sure they would order one for you. I always use Newark for my parts, but I think they have a minimum order around $15.
-FF
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cappa
Nerd
Registered: 02/12/06
Posts: 854
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Re: Proper rhiostat for DIY stirrer? [Re: fastfred]
#5780695 - 06/22/06 02:41 PM (17 years, 7 months ago) |
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Just buy the damned resistor kit. Then solder them together to get the resistance you want. RR gave you an excellent place to start on resistance values. Plus, you'll have an assload of resistors to play with for other projects.
-------------------- Their are 10 types of people. Those that understand binary, and those who don't. ~Cappa.
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davesj1
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Registered: 04/11/05
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Re: Proper rhiostat for DIY stirrer? [Re: cappa]
#5780884 - 06/22/06 03:39 PM (17 years, 7 months ago) |
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I want eh speed to be adjustable though. SO just using a few reistors is out of the question.
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fastfred
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Re: Proper rhiostat for DIY stirrer? [Re: davesj1]
#5791688 - 06/26/06 05:57 AM (17 years, 7 months ago) |
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> Just buy the damned resistor kit.
A non-variable speed stirrer would suck ass. Rather than buy a bunch of resistors and end up with a single speed stirrer just buy a single pot with the correct values or pull one from some junk electronics everyone usually has laying around.
-FF
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davesj1
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Re: Proper rhiostat for DIY stirrer? [Re: fastfred]
#5792013 - 06/26/06 10:21 AM (17 years, 7 months ago) |
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agreed. I tried a 25 ohm pot w/a 100 ohm reisitor. Unfortunately (i dont know why i didnt think of this earlier) since the pot is only 25 ohms it amkes only 25 out of the 125 toal ohms adjustable, which is effectively useless. Guess im gonna have to keep my eyes out for an appropriate pot.
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cappa
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Re: Proper rhiostat for DIY stirrer? [Re: davesj1]
#5804462 - 06/29/06 07:50 PM (17 years, 7 months ago) |
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If you wanted different speeds, you could also build a bus or simple multiswitch.
Think something like the letter H where the horizontal line can move up and down the other shafts, which have different resistor values along the way.
Or you can build a shunt. Look it up. It's pretty easy with a little magnet wire, a toilet paper tube, and some glue.
If you have money, and not much time, then just buy the correct pot.
Their are always alternatives.
-------------------- Their are 10 types of people. Those that understand binary, and those who don't. ~Cappa.
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