Need help figuring out how much power to use.

I am trying to power 2 small fans that each have an input of 12v DC 0.12A (120mA).  Do I need a 24v DC 0.24A (240mA) to power them both to max capacity?

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framistan6 years ago
You seem slightly confused about what kind of power source is needed. The item you are confused about is the AMPERAGE from the source and the AMPERAGE used by the fan. Here is the simple answer.... All power sources put out ZERO amperes until you connect something to it. If you attach ONE light bulb that uses only 1/2 amp... then only 1/2 amp will flow. Now attach a 2nd bulb (in parallel) and total amps is ONE amp. If the wall-wart or battery only can output 300 milliamperes, then even ONE bulb of that kind will OVERLOAD the supply. But if you buy a LARGER wall wart that puts out 3 amperes.... NOW you can run SIX of those kind of half-amp-bulbs. When using ohms law calculations it is easy to forget that in the REAL WORLD the voltage source must have the amperage capability. The ohms law calculations use a voltage source that is assumed to have UNLIMITED amperage capability.... which just does not happen in the real world.
zkemppel (author) 6 years ago
I plan on running them in parallel. So essentially what I'm getting from this is the fans would max out with both a 12v or 24v power source running them in parallel, it just depends on the amount of amps? And it wouldn't matter if my output was 1amp even tho the input on both fans is 0.12amp?
No. The amperage simply notes the maximum amperage that will be available by a given load or combination summation of loads.

A load will draw current from a voltage source by this equation

Iload = Vsource / Rload

Hmm. there is one caveat...failure mode.

It might be of some value at this point to explain how to protect your circuit (fans in this case) from faults, using either a fuse for each load, rated slightly above the expected load current, or circuit breakers for the same purpose.

for a load such as the ones you've indicated, I'd suggest a 200mA fuse, ie 2/10 A), so two 2/10ths Amp fuses, one for each fan.
If you put them in series, you need 24V @120mA, if you put them in parallel, you need 12V @240mA

zkemppel (author)  steveastrouk6 years ago
I decided to just go with seperate 12v power sources as i cannot find a 24v source. Would it be bad to use a source that outputs 12v @ 1A with the input of the fan being 12v @ 0.12A? Sorry, new to power sources.
Nope, no problem, put both fans in parallel with that one supply, and you'll be good to go.
Re-design6 years ago
YOu could use a 12 volt power supply as long as it is over about a quarter of an amp.
seandogue6 years ago
No, you need a 24V 0.12A supply (or actually, slightly over 120mA to keep from pushing the power supply into a hot zone (literally) ) and run the two fans in series, or a 12V 240mA supply (same idea as before...more is better to keep the heat down in the supply and reduce the thermal loading and so increase it's lifespan) and run them in parallel