Controlling 9v fan speeds

:(Rhodri:)

:(Rhodri:)

:(Rhodri:)

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Wasn't really sure where to stick this, but hoping someone on GD has some experience with electronics.

At the moment I have 2 20mm fans running off a 9v battery to stop my goggles steaming up whilst airsofting, but they are spinning much faster than I need them to most of the time and draining the batteries far too fast.

I could fit a fixed resistor to slow them down, but I'd ideally like to be able to adjust the speed to full if needed. Could someone point me towards what I'd need to do this please as I don't really know what I'm doing (am I better off trying to lower the voltage or the current?).
 
A few options come to mind.

Could just put them in series. You might be lucky and find 4.5V on each is about right. Can switch between both on 9V and each on 4.5V easily enough.

Otherwise a resistor (fixed or variable) is indeed an option. More resistance in the circuit will mean lower current and lower power consumption. Sizing the resistor is either trial and error or based on measuring / finding out the resistance of the DC fan (which may vary with voltage applied).

"Best answer" is pwm control, but that's probably overkill in that the control circuit may well draw more power than the fans did anyway. Or use multiple batteries.
 
JonJ678 said:
"Best answer" is pwm control, but that's probably overkill in that the control circuit may well draw more power than the fans did anyway. Or use multiple batteries.
Click to expand...

Really? Such overkill. Just needs a potentiometer/variable resistor to manually control the resistance.
 
If you unplug the fan, and blow it with a hair dryer, you should be able to control the speed the fans spin with the settings on the dryer.
 
Just use a 10k Pot? Wire it up in series and you can twiddle it to adjust the fan.

A stepped rotary switch would also do it. Speed options.
 
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