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Old 10-28-2009, 03:15 PM   #1
tcMonk
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Default Stirplate build......voltage question.

I am building a stirplate in the likes of the cheap and easy stirplate idea that was posted by Anthony Lopez started a few years ago.
I have a 6v 700milli amp wall plug (this configuration runs at 4.2 watts)?, using a 25 ohm 3 watt pot to control the speed of my 12v computer fan.
What size resistor do I need to put in place to get the "power" running at 3 watts into the pot? Basically I need to drop from 4.2 watts to 3 watts. Any help would be greatly appreciated.

TONY


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Old 10-28-2009, 03:39 PM   #2
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Not sure why what you have won't work w/o mods. I've hooked up a 12v, 9v, and currently 4v cell phone charger to my fan and they've all worked fine. I used the same directions I think and the adjuster knob (I can't remember what its called) works great for regulating the speed.


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Old 10-28-2009, 04:13 PM   #3
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I am just worried that the extra power will heat up the pot maybe short it out. Is this not right? I do not know that much about electricity and circuits ect.

Thanks

PS everything works great otherwise.......
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Old 10-28-2009, 04:20 PM   #4
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The .7 amps is the max the wall pug will supply,b ut not neccesarly what the circuit will draw. What's the wattage on the fan? 3 or less watts and you should be fine. You can also build it, and put your multimeter in series to check the running amps. More than .5 amps & you could have issues with the pot.
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Old 10-28-2009, 04:23 PM   #5
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The pot will be OK. Your load (i.e. the fan) won't draw more than about 100mA (check it). That, times the 6V, is your max power. That is less than 1W.
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Old 10-28-2009, 04:24 PM   #6
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I believe the fan is 12v dc and 1.4 amps. Which is 16.8 watts?
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Old 10-28-2009, 04:30 PM   #7
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That would be a BIG fan. Something is likely off on your numbers. I'm with passedpawn: the pot is OK. Do you have a multimeter? If so I would just build it and double check the power afterwards.
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Old 10-28-2009, 05:56 PM   #8
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Quote:
Originally Posted by tcMonk View Post
I am building a stirplate in the likes of the cheap and easy stirplate idea that was posted by Anthony Lopez started a few years ago.
I have a 6v 700milli amp wall plug (this configuration runs at 4.2 watts)?, using a 25 ohm 3 watt pot to control the speed of my 12v computer fan.
What size resistor do I need to put in place to get the "power" running at 3 watts into the pot? Basically I need to drop from 4.2 watts to 3 watts. Any help would be greatly appreciated.

TONY
If you used a 25 ohm 3 watt potentiometer you have nothing to worry about. You don't "drop" power in an electric circuit. The fan should be connected to the wiper arm of the potentiometer on end of the potentiometer shoul be connected to the "+" side of your power source the other end should be connected to the "-". If you have nothing connected to the wiper arm the potentiometer would dissipate about 1.4watts. To calculate the power needed by the circuit, say at midpoint on the potentiometer you are using 12.5 ohms on the potentiometer in series with the internal resistance of the fan (about 15 ohms). The total resistance would be about 27 ohms at midpoint so Power= Voltage squared/Resistance, then 36/25=1.3 watts. Even with the potentiometer dead ended to one side (only the fan in the circuit) your maximum power requirement is 2.4 watts. Kapish?
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Old 10-28-2009, 06:46 PM   #9
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Guy
Thanks for the information (all).
A few questions, which arm is the wiper arm, the middle? Also pictures from the said post has just the red (+) wire from switch/power running to the pot, the red (+) wire from the fan on the middle (wiper arm), then black (-)/ground wire back from the fan to the power/switch to complete the circuit. Is this correct ?

Thanks for your patience for a noob.



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