I want to use an Mega 2560 Pro mini in my car. I will like to power it directly from the cars battery. That means that when the engine is off it will get 12.6v and when the engine is running it will get 14.2v. Also, because I want to use a power saving mode I will like to not use a step-down converter. I have read that the recommended voltage is 12v and the limit is 20v, but I was not able to find any information on what happens if I supply more then 12v.
The 5V voltage regulator is heating up.
If you only power the board without anything connected to it, it's probably fine. What else is powered from Mega in your setup?
I intend to add a relay that when the Mega wakes up it will power all the other sensors.
It's not just the higher voltages you need to worry about but noise and voltage spikes, especially the voltage spikes which can damage an Arduino.
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It would be better to post a drawing/scheme of your setup.
For now I have just the concept. For the power side I'm thinking somthing like this.
There are buck converters with enable pin, so no need for relay. Are these sensors connected to arduino?
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Just put a DC-DC converter between the car's (nominal) 12V net and the Mega and all of its sensors. Power the Mega with 5V from the DC-Dc converter by applying it to the Mega's 5V pin.
I don't see the need for a relay for the sensors. If you must use one, why not just use a PMOS high side switch instead?
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Years ago I used a simple LM7805 to power a small DC voltmeter in my automotive application. Yes, some wasted power. Today there are buck converters designed for automotive applications and they work just fine getting from as much as 40 volts or more down to 5 volts or whatever you want. I would not run standard automotive 12 volt system power into my uC.
Ron
I did not know that. Can you give me some examples? I'm connecting just the signal pin.
Why I'm hesitated on using a DC-DC convertor to power the Mega is because I don't want to pull to much power from the car battery when the engine is off. The thinking for the relay is the same, to turn it off when the engine is off.
Example of a well documented DC/DC converter (click), that draws less than 100uA idle and 2uA in shut-down.
Leo..
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Many of them have enable pin. But I suggest you to go with setup proposed by @rsmls and others after. If you don't make deep sleep cycling they probably perform better and you don't have to worry about heating up your arduino voltage regulator.
If you have sensors that draw significantly current, the answer would be different.
Just be very careful if you apply 5V on Mega's 5V pin. Connecting a PC with a standard USB cable at the same time might cause problems.
In my setup I use an external 5V power supply for LED strips and a small boost converter to generate 9V for Vin of the Mega from that 5V; that way I can use standard USB cables with peace of mind.
Determine the power draw of your circuit in idle/sleep mode. Then compare that to the capacity of your car battery. Use this to inform your decision to worry about it, or just let it slip.
Btw, in most cars, in-car entertainment and control systems are always on (in a low power mode) and likely draw more current than your Arduino.
Evidently, it's a good idea to keep an eye on power consumption, put sensors and the arduino itself to sleep whenever possible, select components with low idle current draw etc.
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