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i got an old PC fan which according to its label runs at 12V drawing 0.14Amps.

I would like to run this with a 9V Battery so that it doesnt go too fast. Does this make sense?

How long would the fan run approximately with a typical 9V PP3 sized battery?

edit: another question is, how much amp will the fan draw when powered with 9V ?

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Yes, you can do it, but the battery won't last very long.

It will probably run at about 3/4 the speed and draw about 3/4 the current (rough estimate) so around 100mA.

Looking at a typical datasheet for an alkaline 9V battery, we can see the capacity will be perhaps 350mAh (down to 4.8V) so it might work for 3-5 hours, as the current will drop as the battery discharges.

If you were to step up the battery voltage so it was 9V continously, the operating time would probably be less than 3 hours.

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Unless the fan stalls due to Hall Effect position error in start position but starts with slight nudge, the fan current always decreases when V+ reduces to 3/4 voltage since less work is done and internal fan drivers are fairly efficient.

Since 9V Alkaline 6LR61 cell has 5Wh capacity at 20h and 3~4Wh in 5h
- and 12V 0.15A fan draws 1.8W with a fresh cell, power consumption will be less than 1.8 but more than 50%. I estimate 60% or 1.1W (from past fan DVT design validation tests.).

Therefore nominal run time will be expected to exceed 3~4Wh/1.1=~3~4 hours with declining RPM extending time at lower mA draw in exponential decay down to 4V then rapidly cutout and stop

RPM declines with Vbat and stall Voltage has hysteresis around 25% depending on bearing stiction from wear out.

If not using “top Brand quality” Alkaline, your results will be worse.

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The answer is that it will probably run, but ypu won't know until you try it. It is unlikely to cause it damage if it doesn't.

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  • \$\begingroup\$ ok, thanks it does run infact when connected. my question was more about how meaningful such a circuit would be. could it be that the battery is empty much faster then if i would run the circuit with 12V power source? \$\endgroup\$ – oleole Jul 1 '18 at 12:52

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