What inline resistor would 7V a case fan?

[pelikan]

I want to solder a resistor to the red wire in some of my fans to run them at 7V. But I don't know how many watts or Ohms the resistor should be.
Thanks.

 

[JSSheridan]

If you want to make the modification this way, then it will depend on what the properties of the fan are. Could you give us the specs of the fans you mentioned? Peace.

 

[oldfart]

Just do a 7 volt mod

 

[Eug]

How about a Zalman Fanmate?

Or else oldfart's 7 V mod, or else trial and error with various 20-40 Ohm resistors I guess.

 

[thorin]

You could do an Inline Rheostat but if you just want a straight resistor then the quote from the start of this article gives you the info you want.

All we need to do is insert a rheostat or variable resistor in the 12V wire to the fan. The rheostat should be rated at least 3 Watt, (for fans up to 10Watt), and have a resistance of 20 ? 50 Ohms. If you are using this mod on your CPU fan then use the 20 ohm rheostat to prevent fan stall at the slowest speed setting.

Thorin

 

[Camion]

I run fans at 7v, similar to the way that oldfart does it, but with fewer steps and no risk of plugging the wrong thing into it. I accomplished the same thing by just switching one wire in the connector that is attached to the fan. Leave the red wire in the fan connector where it is, so that it mates with the yellow (12 volt) connector that comes from the power supply. Then move the black ground wire on the fan connector over so that it will mate with the red (5 volt) connector that is attached to the psu, when you plug it back in. Why this comes out as seven volts I don't understand, but it does. I write "7 V" on the fan connector with a marker to identify it as such. This way you don't have to worry about accidently plugging something else into an adaptor. The only tricky part is getting the wire to pull out of the connector. You have to use something skinny and pointy (I used a nut pick) to push the little barbs in. BTW Directron sells connectors like oldfart makes. You can accomplish the same thing in about two minutes for free using my method. Before you do this you should get out your voltage meter and measure the voltages across the different colors in a connector coming from the psu, so you can be convinced that putting the fan's hot wire to 12v and its ground wire to 5v makes it power at 7 volts. You could also run you fans at 5v if you wanted to. The quietest position would be running them at zero volts (you can't hear it at all!) but they don't cool very effectively.

Another thing about quieting down the fans -- I isolate mine with a strip of stick-on window insulation stuff from the hardware store. You would be amazed at how much noise the teeny amount of vibration puts out thru the resonance of the case. Try taking a fan out and first holding it in your hand. Then set in on top of the case. When I do this, even with a "silent fan" I hear a loud hum like if you set it on a drum. Try it.

 

[oldfart]

Yeah, that site (it's not mine) makes a simple operation WAY more complex than it needs to be. I do it the same way that Camion does it. Just switch a couple of wires. The reason it comes out to 7 volts is you are changing the reference point. +12 ref to ground (0 volts) is 12 volts (duh). +12 ref to +5 is 7 volts. 12 - 5 = 7. The 7 volts is the difference in potential between +12 and +5.

 

[Camion]

Thanks for the edification about why it works out to 7v. Now if I could only understand why the neutral wire in my house is bonded to ground.......( I probably wouldn't understand the explanation anyway) Thanks oldfart --- camion

 

[pelikan]

Thanks for all the help. These fans are in my power supply so I don't have room for large connectors. I'm going to solder in a 34 Ohm resistor. The fans are 1.5amp, so if I understand it correctly, a 34 Ohm resistor should give me 7V.

 

[oldfart]

If you want a quiet PS fan, you can replace it with a Panaflo L1A.

 

[pelikan]

Yeah, I was thinking of that exact fan as well. I havn't found a good deal on one though - anyone know of one?

 

[JSSheridan]

Originally posted by: pelikan
Thanks for all the help. These fans are in my power supply so I don't have room for large connectors. I'm going to solder in a 34 Ohm resistor. The fans are 1.5amp, so if I understand it correctly, a 34 Ohm resistor should give me 7V.

Did you mean 1.5 watts? I could believe that myself, but not 1.5 amps. 1.5 amps times 12 volts would give 18 watts, that's a screamer of a fan. Watts is a power measurement, amps is for current. Peace.

 

[pelikan]

I meant .15 amp. Sorry for the confusion and thanks for the math.

 

[JSSheridan]

Ok, 0.15 amp. That makes the fan 1.8 watts (12 * 0.15). Let me go throught the numbers again.

12V / 0.15A = 80 ohms. (80 / (80+x)) * 12 = 7V; --> X = 57.143;

So a resistor slightly less than 57.143 ohms. And now for the check.

12 * (80 / (80+57.143)) = 7V check.

And the power check. (5V)^2 / 57.143 = 0.438 watts. So get a .5 watt resistor.

Would anyone care to double check my numbers? Peace.

 

[pelikan]

I'm just learning this now, so I realize I could easily be wrong. I thought it went:

R=(V1-V2)/Current

(12-7)/.15=33.33 ohms

A 33 ohm 5 watt resistor should give me 7V? 5 watt because the fan is drawing at least a couple watts?

 

[JSSheridan]

Originally posted by: pelikan
I'm just learning this now, so I realize I could easily be wrong. I thought it went:

R=(V1-V2)/Current

(12-7)/.15=33.33 ohms

A 33 ohm 5 watt resistor should give me 7V? 5 watt because the fan is drawing at least a couple watts?

Here are a couple of equations you can follow when looking at resistive networks.

Ohm's Law: V = I*R;
Voltage division of 2 resistors in series: V * (R1 / (R1 + R2)) = V1; V2 = V - V1;
Electric Power: Power = V*I = V^2 / R = I^2 * R;

The circuit is 0.15A without any modification, but when you add a resistor, you will change the current in the circuit. So it follows.

Inew = 12 / (R1 + R2); Since you don't know either of the resistors at first, you need to find them both before you can find Inew. Do you see where I got the fan's power from also? Now, I'm not qualified to teach Circuits I, so I'll have to leave it at that and not give you a lesson. Peace.

 

[oldfart]

Panaflo L1A $5

 

[pelikan]

Originally posted by: JSSheridan

Originally posted by: pelikan
I'm just learning this now, so I realize I could easily be wrong. I thought it went:

R=(V1-V2)/Current

(12-7)/.15=33.33 ohms

A 33 ohm 5 watt resistor should give me 7V? 5 watt because the fan is drawing at least a couple watts?

Here are a couple of equations you can follow when looking at resistive networks.

Ohm's Law: V = I*R;
Voltage division of 2 resistors in series: V * (R1 / (R1 + R2)) = V1; V2 = V - V1;
Electric Power: Power = V*I = V^2 / R = I^2 / R;

The circuit is 0.15A without any modification, but when you add a resistor, you will change the current in the circuit. So it follows.

Inew = 12 / (R1 + R2); Since you don't know either of the resistors at first, you need to find them both before you can find Inew. Do you see where I got the fan's power from also? Now, I'm not qualified to teach Circuits I, so I'll have to leave it at that and not give you a lesson. Peace.

Okay. That makes sense. Thanks a lot.