I am designing a simple Arduino Nano-based circuit that will control a relay. The constraint is that it's going to run unattended, in a sealed box, for many days/weeks/months. So I'm worried about about choosing a power supply and how to power both Arduino Nano and the relay from it, with regards to longevity/heat.
Additionally, I want to use an external power transformer to keep the circuit at a low voltage, and I would like to use a relay module with all the necessary circuitry to be controlled via Arduino.
I can think of the following options:
1) Use 9V adapter, 5V relay, power the Arduino through its unregulated input pin with 9V, power relay through the Arduino 5V output pin.
2) Use 12V adapter, 12V relay, power relay directly from Vin, power Arduino with 12V through its unregulated power supply input pin.
3) Use 12V adapter, 12V relay, power relay directly from Vin, use a step-down converter to get 7V into Arduino's unregulated power supply input pin.
Option 1: I'm not sure though about the relay module's power requirements when powering through Arduino.
Option 2: 12V is at the upper limit of what the on-board linear regulator can handle.
Option 3: Extra components, but potentially more safe/stable?
As a sub-question, how can I source a decent power supply with a nice, regulated output? Do I care about "noise"? Should I get a switching supply or a transformer-based one? I need them to have "plugs" for easier installation and some semblance of electrical code compliance.