battery-operatedfan
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 ?
I hate to avoid the stated question, but you need to rethink your basic cooler idea.
Let's start with your solar cells. A 5W, 12V panel implies a short circuit current of ~0.4 amps. That is 0.4 x 12 = 4.8. Solar cells typically have an essentially constant current output, rather than a constant voltage output.
A standard 50W, 40mm x 40mm Peltier device will have a resistance of ~ 5 ohms.
So, when driving your cooler, the solar panel will produce a voltage of 0.4 amps x 5 ohms, or 2 volts at the cooler. Another way to look at it is the power delivered to the cooler will be 0.4 x 0.4 x 5, or .8 watts.
TECs typically run at about 10% efficiency, so your 5W solar panel will only produce about .8 watts at the cooler, and the cooler will only produce .08 watts of cooling.
I think you have much more pressing issues than your fans.
Do a Google search for "12 Volt DC PWM Speed Control". There are many products that you can buy (eBay will sell you something for less than $10) and even more that you can build.
Is this something that you want to build yourself or are you just trying to solve this particular problem?
A 555 timer driving a N-channel MOSFET is pretty easy to build.
simulate this circuit – Schematic created using CircuitLab
You will want to use a CMOS 555 timer such as the TLC555.
Adjust the value of C1 for your desired PWM frequency.
Ratio of R1 to R3 sets the minimum & maximum PWM duty cycle. The values shown allow the PWM to be varied from about 1% to 99%.
I would set the PWM frequency to anywhere from 100 Hz to 500 Hz. If you go above that, change D3 to a faster diode such as a UF4002 or similar.
Best Answer
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.