Hello!
I'm trying to power a 5v micro controler from both a solar panel and a 12v battery. The uC is responsable for charging the battery and if there isn't enough sun for a long time the battery may get depleted.
The idea is to take power both from the battery and the panel so if the battery is depleted, enough solar radiation will power the controller, starting the battery charge.
The problem is in the solar panel voltage ramp, it will start to climb while the sun goes up slowly 2, 3, 4, 5v and I dont want the uC to be in it's voltage limit.
Is there any way (i guess using a zener and some transistors) to make a 5.1v (or so) trigger to have the power instantly only when it is enough to power the uC?

Thank you!

 

Depending on the micro, some have a watchdog that included low voltage lockout. There are also a number of reset chips that will do the same.
For example....
http://www.ti.com/power-management/supervisor-reset-ic/products.html
and..
https://datasheets.maximintegrated.com/en/ds/DS1232.pdf

Have the micro read the solar volts when it wakes up and set a threshold higher than the reset voltage to allow for the volts to drop under load.
If the volts are not high enough, put the micro to sleep for a few seconds then wake and try again.

 

Here's a circuit using the common TL431 programmable zener and a P-MOSFET to turn the output on above the set voltage and off below that voltage.
The set voltage can be adjusted by pot U2.

For the U2 pot setting shown, the LTspice simulation shows it turning on at 5.1V and off at 5.076V (R4 provides some hysteresis so it doesn't chatter around the set point).

M1 can be just about any logic-level P-MOSFET (Vgs maximum threshold of 2V or less) that can readily handle the load current.