# 5v fan, on 6v, too loud

1. May 2, 2012

### jiggleswiggly

amazon.com/gp/product/B000XTFYSQ/ref=oh_details_o01_s00_i00

So I bought this fan. My modem zoom 5341 kept overheating. So I stuck this in there, and soldered it to the 6v incoming.

Now, it's a bit too loud. I've never run it off 5v to see how loud it would be, but I imagine it would still be a bit loud. I want to make it near silent.

So what value resistor should I use to make it silientish?
Thx guys

Something like dis?
(6-4)/.15 (.15 is the amps the fan uses)
thats 13 ohms.
sound reasonable?

Last edited: May 2, 2012
2. May 2, 2012

### vk6kro

In normal use it draws 150 mA at 5 volts.

So, if you had 6 volts and wanted to run the fan off 5 volts, you would drop 1 volt in a series resistor.

The value of this resistor would be (1 volt / 0.15 amps) or 6.66 ohms. It would dissipate 150 mW, so a half or quarter watt resistor would be OK. The nearest standard value would be 6.8 ohms.

15 ohms would drop the voltage to about 4.125 volts which should be quieter, but may stall the fan.

EDIT, just saw your calculation. The fan would draw less than 150 mA on 4 volts. To do the calculation, you work out the resistance of the fan (5 volts / 0.15 amps) or 33 ohms.
Then you work out the voltage from this.
It is still a bit of a guess because the fan may be generating back EMF.

3. May 3, 2012

### jiggleswiggly

Thx
what do you mean by generating back emf though?

4. May 3, 2012

### Bignose

A novel idea would be to make a mini duct muffler. Its hard to link you because its highly associated with an illegal activity.

5. May 3, 2012

### vk6kro

I mentioned that because a motor does not behave exactly like a resistor, so its current cannot be accurately predicted if the supply voltage is not the rated voltage. 5 volts in your case.

What happens is that the battery makes the motor turn, but a turning coil in a magnetic field is also a generator, so the motor generates a voltage which opposes the supply voltage.

This means the current is less than you might expect from just measuring the resistance of the motor.

So, you can find that a stalled motor draws a lot more current than a turning one, even if it has a lower supply voltage on it.

If you study motors you will find this is a very important effect. So, you learned something you didn't ask for.