I have found a relatively small 12V switching power supply that I want to build into my device, using it to power both the Arduino Pro Mini and a couple of 12V cpu fans.

Am I correct in assuming that I can have the Arduino RAW pin and Fan+ on the same power supply lead? Or do I need to protect the Raw pin in any way? Also, I need to power 2x cpu fans, can this be done with one transistor or do I need one for each fan?

If I stay within the Current specs of my supply (6A) could I keep adding other 12V components on the same +lead as in the diagram below? enter image description here


2 Answers 2


Sounds fine.

You can connect the two fans in parallel, as long as they don't use more than 5A combined, as that is the maximum of the TIP120. Though you definitely need to attach a heatsink to the TIP120, if you are using a largish amount of current. Though in that case, I'd suggest using a MOSFET instead of a Darlington Transistor (TIP120), as MOSFETs don't heat up that much.

The diode is not needed if you use the TIP120, as there is already a diode inside the package.

Since you are using 12V as a power supply, the voltage regulator has to convert 7v into heat. Depending on the amount of current you use, this could result in the voltage regulator getting very hot. If you are only powering the ATMega328 and maybe some LEDs from the 5v line, you'll be fine. Just don't connect something like a 5v fan.

  • I see, thank you. Out of curiosity, if I would want to connect some 5v fan, how would I go about protecting the built in regulator from overheating? Should I but some resistors on the RAW pin to make the 12v line go down to 5v before entering the arduino? I guess that would make the much cheaper resistors handle dissipation?
    – aelgn
    Commented Oct 2, 2015 at 9:10
  • Just bypass the onboard 5v regulator, and use an external voltage regulator in e.g. a TO-220. Depending on the load, you can easily add a heatsink to this TO-220 package. Connect the 5v output of the external regulator, to the 5v pin on the arduino board.
    – Gerben
    Commented Oct 2, 2015 at 13:30

The board will take 12V, but the regulator will have to dissipate (as in heat) the excess power.

However, how much do you trust your power supply to not exceed the 12V?

The same applies to the other loads.

About max current, you should do the math for both typical case and peak case. Ex: the fan will draw much more current if for any reason it is stopped.

If you use only 1 transistor, you will get both fans to behave in the same way, because you will be using only 1 control line. Is this what you want?

  • I think I trust the supply, I'm a trusty person :). I don't need to control the fans separately, in fact the PCR PID system I'm building would benefit from both fans being in sync. I guess that having only one transistor the fans would get max 6V and thus run at half capacity? Since the Arduino itself really doesnt need the 12V, should I put a resistor on the RAW pin to limit power supply voltage?
    – aelgn
    Commented Oct 1, 2015 at 11:30
  • Why would the fan get 6V? Commented Oct 1, 2015 at 12:03
  • Yes, you can put a resistor. You could also put a zener, to clip just the extra voltage. Commented Oct 1, 2015 at 12:04
  • I was thinking that I had to connect the fans in serial and that it would mean they got half the voltage each. but I guess I could just connect them both to the Collector of the transistor? But would I need to put a resistor/zener to protect the Arduino, or will it work without it, is it considered good practice?
    – aelgn
    Commented Oct 1, 2015 at 14:39
  • 1
    Why wouldn't you trust a 12v switching power supply? Maximum voltage of the mic5205 is 16Volt (absolute maximum 20Volt). Using a zener is a very crude way to make a voltage regulator. If anything, I'd just add one or two diodes. Those will drop the voltage by around 0.6Volt each. Added bonus is, that it will protect your board in case you accidentally connect the powersupply the wrong way around.
    – Gerben
    Commented Oct 1, 2015 at 14:45

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