Are there times you find that D1 is necessary? Or to put it another way, Do you find there are times when current tries to travel back to the main power source?
Is there any other load connected after the switch that may drain out the battery?
Is there the possibility that the switch is ON while the main power supply is OFF...and something is draining out the battery in this case?
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You could use schottky diodes if you want to reduce the voltage drop.
You may use "ideal diode" circuits to further reduce voltage drop.
There is no load connected after switch.
and yes there is the switch is ON while the main power supply is OFF. i have load in the resistor. but actually that's not a resistor.
i'm not problem with drop voltage about 0.5-0.7V. so this circuit is rigth?
Yes. Use the circuit with two diodes. Like in post#1.
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Do you know that most of the power is consumed by the resistor? More than three times than the blue LED takes.
You could easily safe 70% of your battery capacity (size, cost).
In detail it depends on the LED specifications.
Very late information. It really matters if the load is 3V/20mA (60mW) or 12V/3A (36.000mW).
Please give those informations with your first post next time. It saves time - for you and for us.
--> If you ask me - I don´t like this circuit for a 3A application. Just because of the generated heat.