user52565: My current project will be sitting next to a 12VDC power line. It seems a waste no to tap it to power into the Arduino board too. Now I read everywhere that with 12V input quite  a lot power gets dissipated by the voltage regulator. Wouldn't it then a wise move to use the board 's own PWM capabilities to throttle the input voltage?
Nother point is whether it is sensible at all when the board is using little current (in this case less than 5mA over the pins for 0.1 sec every few seconds, plus the board in deep sleep mode).

user53522: How did you measure that it's going to be 5mA? Because if that's really the case, the regulator dropping 7V (I'm assuming it's a regular 5V arduino) will need to dissipate ~35mW of heat which is not much and it should not cause a lot of problems for your board.
Another option would be using a step-down power supply module, there are tons of available dc-dc converters, mostly based on LM2596 (search for LM2596 DC-DC module) which will handle your 12 power rail with ease. 

52565: <p>My current project will be sitting next to a 12VDC power line. It seems a waste no to tap it to power into the Arduino board too. Now I read everywhere that with 12V input quite  a lot power gets dissipated by the voltage regulator. Wouldn't it then a wise move to use the board 's own PWM capabilities to throttle the input voltage?</p>

<p>Nother point is whether it is sensible at all when the board is using little current (in this case less than 5mA over the pins for 0.1 sec every few seconds, plus the board in deep sleep mode).</p>
 53522: <p>How did you measure that it's going to be 5mA? Because if that's really the case, the regulator dropping 7V (I'm assuming it's a regular 5V arduino) will need to dissipate ~35mW of heat which is not much and it should not cause a lot of problems for your board.</p>

<p>Another option would be using a step-down power supply module, there are tons of available dc-dc converters, mostly based on LM2596 (search for <em>LM2596 DC-DC module</em>) which will handle your 12 power rail with ease. </p>
