Using resistors to reduce voltage for fans question

Im planning on using 2x 120mm fans outside the case using a 12 volt 1500ma ac/dc converter. The fans are 12volts .52 amps. I want to be able to switch the fan from 12v to 9 to 7.5v using a 4 position switch. I want to use resistors for the power drops and not a rheobus. I know I will need an 8ohm and 6.8ohm resistor.

The question...... Im not sure what wattage I should get. Calculations said I will need something at least 5 watts but the ones at radioshack are only ¼ ½ and 10 watts. Would a resistor rated to 10 watts affect the resistance? If so where could I get one at 5 watts? I went to mouser.com and there were soooooooooooo many options it just confused me.

Also thinking about undervolting a D4 waterpump if its too loud for me at night so this would help with that as well.

Any help would be appreciated.. Thanks

JT

A 10 watt resistor would work just fine.

Jtanczos said:

Im planning on using 2x 120mm fans outside the case using a 12 volt 1500ma ac/dc converter. The fans are 12volts .52 amps. I want to be able to switch the fan from 12v to 9 to 7.5v using a 4 position switch. I want to use resistors for the power drops and not a rheobus. I know I will need an 8ohm and 6.8ohm resistor.

The question...... Im not sure what wattage I should get. Calculations said I will need something at least 5 watts but the ones at radioshack are only ¼ ½ and 10 watts. Would a resistor rated to 10 watts affect the resistance? If so where could I get one at 5 watts? I went to mouser.com and there were soooooooooooo many options it just confused me.

Also thinking about undervolting a D4 waterpump if its too loud for me at night so this would help with that as well.

Any help would be appreciated.. Thanks

JT

Click to expand...

You really only need a single pole double throw switch with a center position.

So the switch can be in center, up or down postion.

You then need two high wattage resistors.

When the switch is in the center position it will get 10 Ohms, and with my Tornado it gave me around 7 Volts. When the switch is up there is no resistance so the fan gets a full 12 volts. When the switch is in the down position you have two resistors in parrallel. In this case because they are both 10 Ohms, the value will be around 5 Ohms or half and with my Tornado that gave me around 9 Volts.

Radio shack has 10 watt fixed resistors in 10 Ohms, 8 Ohms and even 1 Ohm at least they did when I built this.

With the values you gave you could use a 10 ohm and an 8 ohm resistor instead, which will give you 0 Ohms for 12 Volts, 8 Ohms is connected to center of switch for 7.5 and with the 10 connected to bottom in parrallel will give you around 9.5 volts.

Sometimes with fans the calculations never work out exactly to the voltages you want using fixed resistors because the resistance and current of the fans may vary. So Before I solder everything I test the circuit using alligator test clips and I might pick up a few 1 Ohm resistors so I can raise or lower the resistance a little bit if I need to.

You might want to just use two 10 Ohms like I did because you will never get the voltages you want exactly. You also have to make sure the switch has a center postion, and it is connected to the center contact only when it is in the center position. I believe this switch is called a single pole double throw with a center off position.

This is the right switch, but the picture seems wrong because there isn't three connections on it and there should be.

http://www.radioshack.com/product.a..._name=CTLG_011_002_015_006&product_id=275-325

Don't forget to calculate how many AMP's the increased resistance will draw.

CrystalMethod said:

Don't forget to calculate how many AMP's the increased resistance will draw.

Click to expand...

What?

The current draw will not increase it stays the same. Your just using the resistors as voltage dividers in "series" with the fan they don't draw any extra current.

If you were lowering the resistance of the fan, by adding resistors in parrallel then yes that would increase the current draw of the fan. The circuit I posted is in series with the fan. I adjusted the picture to show this.

crull said:

What?

The current draw will not increase it stays the same. Your just using the resistors as voltage dividers in series with the fan they don't draw any extra current.

Click to expand...

Oops, sorry. Thinking about something else, you're right. Man do I need a vacation, I can't even get a simple formula straight.

if you add a resistor in series, the current drops.

Using resistors does decrease the current, but not in an ohmic way. The fan's "resistance" is not linear with respect to voltage. As the voltage drops across the fan you find it will draw MORE current than you think.

Instead of resistors use diodes. As you specified a voltage you wanted, just use diodes to drop the +12V to +9V. Forward voltage drop of a diode is ~0.7V so 4 diodes in series would give a 2.8V drop leaving you with 9.2V.

http://www.cpemma.co.uk/sdiodes.html

Diodes with a current rating of 1/2A should suffice.

Great thread all. ... just rated five stars.

The guide to Thermal Controlers* was exactly what I was looking for.

L337 M33P said:

Using resistors does decrease the current, but not in an ohmic way. The fan's "resistance" is not linear with respect to voltage. As the voltage drops across the fan you find it will draw MORE current than you think.

Instead of resistors use diodes. As you specified a voltage you wanted, just use diodes to drop the +12V to +9V. Forward voltage drop of a diode is ~0.7V so 4 diodes in series would give a 2.8V drop leaving you with 9.2V.

http://www.cpemma.co.uk/sdiodes.html

Diodes with a current rating of 1/2A should suffice.

Click to expand...

There is nothing wrong with using fixed resistors.
You can also use diodes, but why bother it's more work. You would need a lot of them because each one can only have a voltage drop of .7 volts. With the circuit I posted you only need two resistors....simple. There isn't any need to over complicate this. I have used the exact circuit I posted and it works great, but if the OP wants to get out his soldering iron and make a train of diodes then by all means be my guest.

Well.. Which would be better for a 1500ma converter? I want to power 2 ~520ma fans and 2 LEDs. Would seem they both cost me about the same.

Radio shack sells both components I would need.

For the switch I was thinking maybe something like http://www.mouser.com/index.cfm?han...uctid=308785&e_categoryid=144&e_pcodeid=01005
Think this would work? I would be using 1 for each fan.

JT

Jtanczos said:

Well.. Which would be better for a 1500ma converter? I want to power 2 ~520ma fans and 2 LEDs. Would seem they both cost me about the same.

Radio shack sells both components I would need.

For the switch I was thinking maybe something like http://www.mouser.com/index.cfm?han...uctid=308785&e_categoryid=144&e_pcodeid=01005
Think this would work? I would be using 1 for each fan.

JT

Click to expand...

First question. Why are you using a converter?


Second question. Why do you want to use a rotory switch?


Third question. Do you want the both fans to switch at the same time?

The micro switches at Radio Shack are good because they are small compared to rotory switches. They take less room if you mount them inside the computer.

crull said:

First question. Why are you using a converter?

Click to expand...

Going to be outside the case in an enclosed space.

Second question. Why do you want to use a rotory switch?

Click to expand...

Make it like an extra fan box w/ High, Med, Low and Off.

Third question. Do you want the both fans to switch at the same time?

Click to expand...

Wanted to control the 2 individually.

The micro switches at Radio Shack are good because they are small compared to rotory switches. They take less room if you mount them inside the computer.

Click to expand...

I am working on a venting project and I wanted to be able to control the fans seperately using a powersource outside the computer.

JT

Jtanczos said:

Going to be outside the case in an enclosed space.


Make it like an extra fan box w/ High, Med, Low and Off.



Wanted to control the 2 individually.



I am working on a venting project and I wanted to be able to control the fans seperately using a powersource outside the computer.

JT

Click to expand...

Is this fan box gonna be connected to the computer case is some way? Like maybe attached to the side?


Your still better off using the switch in the circuit I posted. You will need two of them because you want to control the fans individually. They take up a lot less space then rotory switches.

Radio Shack sells very small circuit boards like this

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=276-148

You don't even need to use the whole board you could cut it in half. You would solder the resistors to this board like on the circuit I posted.

Then you would attach the circuit board to the inside of your box with these or something similar.

http://www.radioshack.com/product.a...name=CTLG_011_002_008_000&product_id=276-1381.

Then drill some small holes for the switches in the box and run some wire to connect them to the circuit board.

If the box is metal it might be a good idea to ground it. You also want to make sure the circuit board does not make contact with the box or it might short out.

If you must, yes you can use a few resistors of varying ohms connected through a switch or you can use what everyone else seems to have forgotten. A couple of Zener Diodes of various voltages and accomplish the same thing.

I say rheostat. Near infinate control less work and money. KISS (Keep It Simple Stupid)

Here is something that will more then sufficently cover your needs. Just don't turn it all the way down or it may stall your fans.

15 Watts 15 Ohms Rheostat $1.65

Ok what I was going to do was route the air from the AC vent using this as a replacement cover. The fans would push the air into the case or have it blow directly over a radiator to cool it. My last option would be just to have it situated so it pulls air into the room faster just to get the ambient down rather than duct it in to the case and worry about condensation.

Electron Chaser said:

I say rheostat. Near infinate control less work and money. KISS (Keep It Simple Stupid)

Click to expand...

I wanted to make a neat looking control thing for 2 fans with different colored LEDs for each power setting. Ive seen the rheo before in a thread I found that was posted nearly a year ago and dismissed it because I wanted something more elaborate. After everything I will probably end up going with the rheo just because of cost but I wanted to expand on my wireing skills. I learned ALOT from this thread even if I dont use the info immediately. Thanks

JT

They make multi-colored LEDs that depending on the voltage it gets decides which color it will be. So if you use a rheostat and then using another resisitor in series connected to a LED. To step that voltage down further, there you go.

Jtanczos said:

Ok what I was going to do was route the air from the AC vent using this as a replacement cover. The fans would push the air into the case or have it blow directly over a radiator to cool it. My last option would be just to have it situated so it pulls air into the room faster just to get the ambient down rather than duct it in to the case and worry about condensation.



I wanted to make a neat looking control thing for 2 fans with different colored LEDs for each power setting. Ive seen the rheo before in a thread I found that was posted nearly a year ago and dismissed it because I wanted something more elaborate. After everything I will probably end up going with the rheo just because of cost but I wanted to expand on my wireing skills. I learned ALOT from this thread even if I dont use the info immediately. Thanks

JT

Click to expand...

I created that circuit with fixed resistors, but later switched to a circuit using that same exact rheostat Electron Chaser posted.
You do have to be careful that you don't set it too low or the fan will not start when you turn the power on like Electron Chaser posted, but other then that It works great though.