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This question already has an answer here:

I am currently using 1 12V 0.95A fan to circulate air in my humidor (coolerator) and using an AC adapter rated class 2 120V 60/60 with an output of 9V 300mA. Can I add 3 more fans with the same power source or do I need to increase power?

2 days later:

I have 4, of the pre stated fans connected to the aforementioned source. They surge but do not start without spinning them manually and then run very slow. What do I need to power them up?

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marked as duplicate by Passerby, Nick Alexeev Feb 25 '16 at 2:05

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • \$\begingroup\$ You need to understand what the voltage and current ratings on those fans mean. 12V means it requires 12 V to run (your supply is 9V, so that's an issue already). 0.95A means that that's the current it will (try to) draw when powered by 12V - you supply cannot provide this. As you are not supplying enough voltage, you might get lucky and the fan draws less current. However, you could just as well break your supply as it can only supply 300mA (0.3A) while the fan might draw much more than that. With three fans in parallel (I assume), they still need 12V but will draw three times the current... \$\endgroup\$ – RJR Feb 25 '16 at 1:55
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You have a fan that draws the following current:

12V 0.95A

and your power source supplies this:

9V 300mA

Can you see why this might be an issue? Not only are you undervolting the fan(s), which implies reduced torque/speed, you are only supplying a third of the required current.

Now you want to draw 4*0.95A = 3.8A, which is more than 10 times what your supply is rated to supply, even neglecting the voltage difference! Of course they are going to spin slowly. Your poor power supply is severely overloaded; it's surprising it hasn't gone into thermal shutdown.

You need a supply that provides 12V at 4A to power all four fans. Simple math.

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Well, you were providing one 12V fan that wanted 950 mA with 9V at 300 mA, so you started underpowered and out of spec. Then you added 3 more fans to the same undervoltage, under current power supply and they don't work. Not. Exactly. Shocking. News.

Try a supply rated to supply 12V (not more) at 3.8 (or more) amps.

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You already answered your own question:

I have 4, of the pre stated fans connected to the for mentioned source. They surge but do not start without spinning them manually and then run very slow.

Your fans require 4*950 mA = 3.8 A at 12V to run. They will run at a lower voltage (and draw less current doing so) as well, but your power supply at 9V and 300 mA is still grossly underpowered for even a single fan, let alone four.

You need a 12 V power supply capable of providing more than 4 A of current to run the four fans. Keep in mind that when the fans spool up they can draw even more current, so you should buy a power supply that can deliver about 6 A or more, depending on the fans.

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