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Old 03-01-2013, 04:27 PM   #1
jflongo
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Default Trying to use a resistor with my stir plate isn't working

Ok, I feel like I'm brain dead right now and stupid

I am using a 8V 500ma output adapter, and a 80mm computer fan, it runs it as is, but too fast.

I figured I could just hook in a resister to slow it down for now.

I have a 330 ohm resistor, so i figure V = (0.0005)(330) = 0.165 voltage drop

I must be doing this wrong, when I hook the resistor up, the fan won't spin at all, I'm a little confused. I know there are better solutions, I'm just wondering what I'm missing here. I have tried it on both wires, same result.


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Old 03-01-2013, 05:21 PM   #2
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I still would like to know what's wrong, but I think i'll scrap the resistor idea and buy this instead.

http://www.amazon.com/Zalman-Fanmate...fan+controller


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Old 03-01-2013, 05:28 PM   #3
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Generally, afaik, voltage dividers use two resistors, like the diagram here: http://en.wikipedia.org/wiki/Voltage_divider
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Old 03-01-2013, 05:59 PM   #4
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Quote:
Originally Posted by afr0byte View Post
Generally, afaik, voltage dividers use two resistors, like the diagram here: http://en.wikipedia.org/wiki/Voltage_divider
I'm just trying to drop the voltage by x amount, so not sure how to use that diagram with no ground?
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Old 03-01-2013, 06:03 PM   #5
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Perhaps use figure 3-64 as a reference: http://www.tpub.com/neets/book1/chapter3/1-35.htm
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Old 03-01-2013, 06:18 PM   #6
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So is this saying by adding the resistor in, i'm unbalancing the circuit?
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Old 03-01-2013, 06:21 PM   #7
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Quote:
Originally Posted by jflongo View Post
So is this saying by adding the resistor in, i'm unbalancing the circuit?
My knowledge of electronics is limited, but I think you'd be decreasing the current draw by simply adding additional resistive load.
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Old 03-01-2013, 06:29 PM   #8
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Quote:
Originally Posted by jflongo View Post
I have a 330 ohm resistor, so i figure V = (0.0005)(330) = 0.165 voltage drop
you're 3 decimals off:
V = .5 (A) x 330 (Ohm) = 165V, if you had that much to drop.

500mA is .5 A !

A divider is easiest, although you could build it with a single resistor. Start out with 0.33 Ohms perhaps?

And calculate the needed wattage of such resistor: V*V/R or V * I (0.165 * 0.5)
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Old 03-01-2013, 06:32 PM   #9
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Quote:
Originally Posted by afr0byte

My knowledge of electronics is limited, but I think you'd be decreasing the current draw by simply adding additional resistive load.
That's correct. By only having one resistor in series with the fan, you're dropping pretty much the entire voltage across that resistor. A voltage divider would have to be used where the ration between the two resistors gives you the desired voltage drop across ONE of the resistors, which your fan would be hooked in parallel with.

The ground shown in the above diagram can be hooked up to the negative side of your power supply. It's a general diagram.
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Old 03-01-2013, 06:32 PM   #10
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...and that is IF the fan consumes 500mA. Your power brick's current rating has nothing to do with that.


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