user69173: I am building a small automated indoor greenhouse. I want to use an Arduino Nano to control some LEDs und a fan (both through relays) and a water Grove Water Atomization module. Everything works fine, as long as I power the LEDs and the fan with a 12v supply (3 watts) and the Nano through USB. But I want to power the Nano through the 12v supply, too.
So I built a voltage divider that delivers 7.25v. I measured it with a multimeter. But when I connect it to the VIN of the Nano, the voltage drops to 1.16v. Wich – of course – means that the Nano doesn't start. When I tried it, only the Nano and the relays were connected to the 12v power supply. The relays through voltage divider delivering 5v.
I tried it with an original Arduino and a chinese compatible and got the exact same result both times.
Does anyone have an idea, what I am missing?

user10794: You just can't use a couple of resistors to deliver a reliable voltage source like that. The voltage divider effectively works as advertised with zero current. Your Nano will take more than zero current. See here for one discussion.

69173: <p>I am building a small automated indoor greenhouse. I want to use an Arduino Nano to control some LEDs und a fan (both through relays) and a water Grove Water Atomization module. Everything works fine, as long as I power the LEDs and the fan with a 12v supply (3 watts) and the Nano through USB. But I want to power the Nano through the 12v supply, too.</p>
<p>So I built a voltage divider that delivers 7.25v. I measured it with a multimeter. But when I connect it to the VIN of the Nano, the voltage drops to 1.16v. Wich – of course – means that the Nano doesn't start. When I tried it, only the Nano and the relays were connected to the 12v power supply. The relays through voltage divider delivering 5v.</p>
<p>I tried it with an original Arduino and a chinese compatible and got the exact same result both times.</p>
<p>Does anyone have an idea, what I am missing?</p>
 10794: <p>You just can't use a couple of resistors to deliver a reliable voltage source like that. The voltage divider effectively works as advertised with zero current. Your Nano will take more than zero current. See <a href="https://electronics.stackexchange.com/questions/388450/voltage-divider-as-supply-voltage">here</a> for one discussion.</p>
