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.

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/dp/B000292DO0/?tag=skimlinks_replacement-20

Generally, afaik, voltage dividers use two resistors, like the diagram here: http://en.wikipedia.org/wiki/Voltage_divider

afr0byte said:

Generally, afaik, voltage dividers use two resistors, like the diagram here: http://en.wikipedia.org/wiki/Voltage_divider

Click to expand...

I'm just trying to drop the voltage by x amount, so not sure how to use that diagram with no ground?

Perhaps use figure 3-64 as a reference: http://www.tpub.com/neets/book1/chapter3/1-35.htm

So is this saying by adding the resistor in, i'm unbalancing the circuit?

jflongo said:

So is this saying by adding the resistor in, i'm unbalancing the circuit?

Click to expand...

My knowledge of electronics is limited, but I think you'd be decreasing the current draw by simply adding additional resistive load.

jflongo said:

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

Click to expand...

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)

afr0byte said:

My knowledge of electronics is limited, but I think you'd be decreasing the current draw by simply adding additional resistive load.

Click to expand...

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.

...and that is IF the fan consumes 500mA. Your power brick's current rating has nothing to do with that.

IslandLizard said:

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)

Click to expand...

Ok, I wasn't paying close enough attention to that So basically it sounds like this resistor is way too big.

Sounds like I should try an 8 ohm resistor instead.

Isn't there a schematic floating around here using an LM317 voltage regular with pot meter? that will give you very precise control. You want to be able to control the speed depending on the volume you're stirring. The original post has all the details.

jflongo said:

Ok, I wasn't paying close enough attention to that So basically it sounds like this resistor is way too big.

Sounds like I should try an 8 ohm resistor instead.

Click to expand...

Guessing?

First measure the current as is. If you want to reduce it by say 20%, you can figure what value the serial resistor needs to be.

If you know the internal resistance of the fan (at a given speed), you can draw calculate it more precisely. LM317 + adj. pot is a better option.

IslandLizard said:

Isn't there a schematic floating around here using an LM317 voltage regular with pot meter? that will give you very precise control. You want to be able to control the speed depending on the volume you're stirring. The original post has all the details.

Click to expand...

I am just trying to use what I have at my house right now. I know I need a better solution, I was just testing this to slow the fan down.

Actually on my way out tonight, I think I'll swing by Radio Shack and pick these up, like I saw in one of the posts.

25 ohm 3 watt Rheostat (aka Potentiometer): Part number 271-265 - $3.99
12 Volt DC/30Amp Rocker Switch with LED - Part Number 275-018 - $3.99

Computer fans are usually rated 150ma @ 12V. 12/.15=80 ohms.

After that it’s proportional. 80 ohms in series with the fan will drop the voltage in half. That’s about where you want to be

Here’s the schematic with the LM317 adjustable regulator. The capacitor is totally optional.

http://www.stirstarters.com/instructions.html

I just bought one of these for about $8.

http://www.ebay.com/itm/12V-40V-10A-Pulse-Width-Modulation-PWM-DC-Motor-Speed-Control-Switch-Governor-/271143411686?pt=LH_DefaultDomain_0&hash=item3f21681fe6

I hooked up the Potentiometer and I was able to get this running again. I believe it cut the power down close to 50%. Unfortunately I'm out of town right now, so wasn't able to test it with my flask yet.