Opherman47

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so im building a 3 unit stir plate, ive cut the lexan and have built the case, i purchased 3 95mm cpu fans and a variable voltage AC to DC power supply that has a switch to change from 3 all the way up to 12 volts 500 milli amps.... so what i want to do is chain them each will have its own switch and pot (for limited speed control.....

my question is what happens to the voltage with the 3 fans.... if all are turned on do they each get roughly 4 volts? if one is turned off does each fan then get 6?

should this config work ok? will it share the milli amps but not the volts????

CodeRage

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Opherman, Trim pots aren't going to be able to handle the large amounts of current. You might be able to get away with a larger one like these. http://www.radioshack.com/product/index.jsp?productId=2102789

wire the plus from each fan to it's own pot and then wire the other side of the pots to the +V from the power supply. Then bond all of the fan grounds to the power supply Gnd. If you are going to use a switch put it between the pots and power supply.

Since they're wired in parallel, they'll have the full 12V on each fan. The pot will act as a voltage divider for each fan.

I would try it with one fan first to make sure the pot doesn't burn up.

Otherwise, you could use a lm317 variable voltage regulator circuit for each fan.
http://www.circuit-innovations.co.uk/LM317.html

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Opherman47

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the 3 watt 25 ohm rheostat (musicians call them pots up here) is what i am using... i wired one up and it was fine.... is this gonna burn out after a while? i do have a voltage selector on the power supply i chose

SweetSounds

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I wouldn't worry about it too much - 1/2 an amp is more than enough to drive 3 muffin fans at full speed. You'll have to wire them in parallel so they all get the full voltage of the power supply.

For $24 in parts, though, you can have independent speed control of all 3 fans with the motor controller at Bakatronics - It's a PWM circuit. Basically, instead of increasing the resistance to the current flow (Dissipating power in the form of heat), it switches the fan on and off really fast. The longer the "On" cycle vs the "Off" cycle, the faster the fan turns. No heat Not that a muffin fan is going to roast a 3 watt pot - CPU fans draw less than 1/4 amp - or 3 watts at 12 volts.
You can run the speed controls with your current power supply, or just use a wall wart.

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Opherman47

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thanks fellas