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I have a condition where I need to power MCU with the second source when first is not working. MCU I am using is ATMEGA328p. The circuit is solar charge controller. I am using a battery to turn on the MCU. Battery voltage is going into LM7805 and its output (5v) is going into the VCC of MCU.

Now I also want to use the solar voltage (whenever available) to give power to MCU, so that if battery is faulty or is not working, then solar voltage will be used to power the MCU, thus at least load will on during day time.

I have the below schematic in my mind. enter image description here

Now I want to know if battery is healthy and is giving voltage ~12v and during sunlight solar panel is also giving ~20v, then what will happen in this case.? Is the above design safe to use. If yes, then what type of diodes should I use.

During day time both (solar + battery) voltage will be going to the regulator, will it create any trouble.? Please help. Thanks.

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    \$\begingroup\$ Because of the nature please consider using a buck converter, wasting energy in a solar power application doesn't seem the best idea (the 7805 will just turn the 15V drop into heat, wasting 75% of the power). \$\endgroup\$
    – Douwe66
    Commented Feb 9, 2017 at 13:51
  • \$\begingroup\$ Yes, let's not waste this finite energy source! \$\endgroup\$
    – user76844
    Commented Feb 9, 2017 at 13:55

4 Answers 4

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Your basic concept is OK, but there are some issues:

  1. You really need to add input and output caps close to the 7805. Read the datasheet.

  2. This circuit will work with 20 V in, but consider the power dissipation. Let's say the 5 V circuitry draws 100 mA. With 20 V into the 7805, it drops 15 V and dissipates 1.5 W. A 7805 can handle that, but will need a heat sink.

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Your circuit will work as expected.

Diode: You should use diodes with low forward voltage drop, to reduce power dissipation.

Also you need some caps around the 7805 to stabalize it's regulatoin. However in this case I would suggest the usage of an DC/DC converter like the TSR 1-2450. It's a plug and play replacement for the 7805.

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  • \$\begingroup\$ Exactly not a replacemet. Traco TSR 1-2450 is a switcmode device and generates some high frequency noise as a side product. An external LC low pass filter is needed to its input to block the noise. \$\endgroup\$
    – user136077
    Commented Feb 9, 2017 at 14:07
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The highest input voltage will win, and it won't create any trouble (basically, it means solar panel will have priority if its voltage is >12V and will be unused if <12V).

Given the input voltages seem much higher than what you need at the 7805 input, and current requirements are probably low, any diode will do. If you had voltages much closer to the limit of what is required for 7805 (5V + ~1.5V dropout at regulator), you would want to use schottky diodes to reduce the drop.

Now, this is all fine if the current requirements are not more than about a hundreds of mA. If it is more, the 7805 will heat too much and waste a lot of power. In this case, use a DC-DC converter.

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One thing i don't see they mentioned yet is that the power itself may not be a problem as long as you can actually supply it. So pay attention the PV panel is large enough and remember that this power will drop earlier in the evening and be available later in the morning.

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