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Questions about resistors

Eug

Eug

Lifer
I have a 12 V Panaflo fan that I'd like to run slower.

Options:
1) Run at 12 V and be annoyed.
2) Run at 5 V but not push that much air.
3) Run at 7 V, using the 5 V line as the ground.
4) Run at 12 V with a resistor.

I'll likely do number 2 (easiest) or number 3 (not too hard), but I am wondering about #4.

It's a 79 mA 12 V 0.95 Watt fan. Could I just stick a 0.25 Watt resistor on the red line? (It has a red and blue, and I believe blue is ground.) However, I walked into an electronics store today only to be confronted by a bunch of specs I didn't understand. (I'm an electronics n00b.)

Of the 0.25 Watt resistors there, there were 330K or 30K and 1% or 5%, and a bazillion other options. What do all those specs mean?

Or I could run a 0.5 Watt resistor, to give me about the same effect as running it at 5 V.
 
I wish I knew more about electronics, but I can tell you that you aren't going about it the right way. 😉

The wattage of the resistor is how much wattage it can tollerate.

The 330k and 30k is how much it actually resists, measured in Ohms. The percentage is the wattage tollerance.

 
if this was first or second term in my electronics course i could've told you right off the bat. 🙁

but i can tell you those percentages are tolerances. all resistors have colour bands, you can google what they mean.
 
Originally posted by: Eli
I wish I knew more about electronics, but I can tell you that you aren't going about it the right way. 😉

The wattage of the resistor is how much wattage it can tollerate.

The 330k and 30k is how much it actually resists, measured in Ohms. The percentage is the wattage tollerance.
Click to expand...

Actually no, the percentage is the resistance tolerance. In other words, a 100 Ohm resister with a 5% tolerance can actually be anywhere between 95 and 105 ohms.

 
If you're seeking to divide down the voltage, you need to double check your fan resistance.

Voltage (over fan) = 12V*(Resistance of Fan)/(Resistance of Fan+Resistor that you bought)

The best option if you don't know what to do is to at least get a ball park number and buy a potentiometer (sp?) aka variable resistor. Then you can tune to your heart's delight.
 
get a rheostat/potentiometer instead of a resistor because you don't know what value of resistance you should use and i dont know the fan's IR curve. i guess ~ 1 watt should be enough
 
Originally posted by: JoLLyRoGer
Originally posted by: Eli
I wish I knew more about electronics, but I can tell you that you aren't going about it the right way. 😉

The wattage of the resistor is how much wattage it can tollerate.

The 330k and 30k is how much it actually resists, measured in Ohms. The percentage is the wattage tollerance.
Click to expand...

Actually no, the percentage is the resistance tolerance. In other words, a 100 Ohm resister with a 5% tolerance can actually be anywhere between 95 and 105 ohms.
Click to expand...
Oops, duh, mybad. 😉


 
OK I see. So V = I x R. The fan is 79 mA.

R = V/I = 12/.079 = 152 Ohms.

So I want say a 100 Ohm resistor, right? 5% and 1 Watt would be fine, since the fan is only 0.95 Watt anyway, right?

I'm guessing that will be OK, since anywhere between 50% and 75% of the power going to the fan (which is a Panaflo case fan) should be fine. This is a Power Mac Cube, which runs fanless with a G4 450. A G4 1.7 GHz was just installed in it, and actually I was still able to run it fanless stably even with benchmarking, but it's kinda warm. (It works fanless because there is a vent at the top and the bottom. Air comes in the bottom, passes thru the fins of the CPU and hard drive heatsinks, and rises out the top.)

The case fan that came with the upgrade card is supposed to be a low flo fan but it's still too loud for my tastes.
 
you'd split the voltage using a voltage divider. since you probably don't know the resistance of the fan, you'd have to start guessing resistor values. as for the wattage, you really should worry about that you aren't pushing much voltage. as for a voltage divider, try google for some theory :|, assuming a fan has a resistance of 1 K, the R you should pick is

Vo = Vi ( R / (R+Rf) )

Vo = your fan output, say 7v? whatever you want
Vi = input voltage, 12v
R = the divider resistor (variable)
Rf = the resistance of the fan (guessing 1K)

7 = 12 ( R / (R+1) )

R = 7/6 K ?

edit: ah apparently you do know the Rf; Rf = 12 / 79E-3 = 152 ohm ..
 
Originally posted by: Eug
OK I see. So V = I x R.

R = V/I = 12/.079 = 152 Ohms.

So I want say a 100 Ohm resistor, right? 5% and 1 Watt would be fine, since the fan is only 0.95 Watt anyway, right?

I'm guessing that will be OK, since anywhere between 50% and 75% of the power going to the fan (which is a Panaflo case fan) should be fine. This is a Power Mac Cube, which runs fanless with a G4 450. A G4 1.7 GHz was just installed in it, and actually I was still able to run it fanless stably even with benchmarking, but it's kinda warm. The case fan that came with the upgrade card is supposed to be a low flo fan but it's still too loud for my tastes.
Click to expand...
How about you just get one of those ultra quiet Panaflos instead of ghettoizing it?
 
Originally posted by: Eli
How about you just get one of those ultra quiet Panaflos instead of ghettoizing it?
Click to expand...
Cuz it's a major PITA to remove the fan from a Cube. It's very cramped in there and requires disassembly of the entire machine just to remove the fan. Plus the fans aren't that easy to get. It's a 80 mm x 15 mm. 80x25 mm fans require a slight mod to the case, which I don't want to do.

It's supposed to be an ultra quiet fan anyway. 21 or 23 dBA or something like that. But it's still too loud for my tastes.

The resistor idea came up, because there's easy access to the fan wires. To run off 5 V might actually be harder, because I'd have to partially disassemble the unit to get access to once of the 4-pin molex connectors.
 
Thanks, but remember there's no room in that Cube. A resistor or some cables yes, but a circuit board no. I'm a n00b anyway so I don't want to be building circuit boards either.

So, would a 100 Ohm (0.1K) resistor be a good start? If it doesn't work I could just try another.
 
Originally posted by: Eug
Originally posted by: Eli
How about you just get one of those ultra quiet Panaflos instead of ghettoizing it?
Click to expand...
Cuz it's a major PITA to remove the fan from a Cube. It's very cramped in there and requires disassembly of the entire machine just to remove the fan. Plus the fans aren't that easy to get. It's a 80 mm x 15 mm. 80x25 mm fans require a slight mod to the case, which I don't want to do.

It's supposed to be an ultra quiet fan anyway. 21 or 23 dBA or something like that. But it's still too loud for my tastes.

The resistor idea came up, because there's easy access to the fan wires. To run off 5 V might actually be harder, because I'd have to partially disassemble the unit to get access to once of the 4-pin molex connectors.
Click to expand...
23dB is too loud?!

LOL

I'm using the same fan as a CPU fan and my HDDs are louder.....

I see, well, goodluck. 😉

You could get a bunch of resistors in 25ohm increments(100, 125, 150, 175, etc), and try them all until you've got the sound level you want... since we don't know the resistance of the fan.
 
Originally posted by: Eli
23dB is too loud?!

LOL

I'm using the same fan as a CPU fan and my HDDs are louder.....

I see, well, goodluck. 😉

You could get a bunch of resistors in 25ohm increments(100, 125, 150, 175, etc), and try them all until you've got the sound level you want... since we don't know the resistance of the fan.
Click to expand...
Good idea. I will get a 100 and a 125.

And yeah, 23 dB is too loud since the machine was completely fanless before the CPU upgrade, and my hard drive is a Seagate Barracuda V in the quiet setting. Also, the Cube is sitting on my desk right next to the monitor.

P.S. This is what the Cube looks like.

Pic1 (Front view - Vent and optical drive slot on top.)
Pic2 (Ports on bottom)
Pic3 (Inside)
 
Get a potentiometer. Hook it up in series through them middle pin and either of the outer pins- it doesn't matter which one. If there are only two pins, just hook it up straight through. Turn the knob till you get what you want.
 
I have a multimeter, but I'm embarrassed to say I have never used it, or even read the instructions. 😛 Mind you I'm not even sure if the Cube works with the cover removed anyway.

So I'm thinking this part should be OK:

1-WATT, 5% METAL OXIDE RESISTORS
Metal oxide resistors for all of your do-it-yourself electronic repairs. Includes two pieces of each: 100 and 330 Ohms. $2.99

P.S. Anyone know what colours that pic is supposed to be showing?
 
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