user13718: 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?


user7856: 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?

user2881: 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.

13718: <p>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.</p>

<p>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?</p>

<p>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?
<a href="https://i.stack.imgur.com/st4Or.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/st4Or.png" alt="enter image description here"></a></p>
 7856: <p>The board will take 12V, but the regulator will have to dissipate (as in heat) the excess power.</p>

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

<p>The same applies to the other loads.</p>

<p>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.</p>

<p>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?</p>
 2881: <p>Sounds fine.</p>
<p>You can connect the two fans in <strong>parallel</strong>, 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.</p>
<p>The diode is not needed if you use the TIP120, as there is already a diode inside the package.</p>
<p>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.</p>
