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I have a power supply of 5v DC 5Ah and a 12v 2Ah lead battery. I was thinking to make a circuit to switch battery line when power is off. I made a little schema, what i'd have to put instead of switch that i draw?

Here's the circuit schematics I made. Thank you

enter image description here

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    \$\begingroup\$ A PNP transistor is not a MOSFET, and your schematic shows a NPN transistor. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 3 '14 at 0:53
  • \$\begingroup\$ I've modified the request, i was wrong on all! \$\endgroup\$ – user46689 Jul 3 '14 at 21:46
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Assuming you can live with a certain amount of slop in your output, you don't need a transistor.

schematic

simulate this circuit – Schematic created using CircuitLab

Consider if the battery is missing. Then depending on the load current, the output will be in the range of 12 to 12.5 volts (Schottky diodes have lower forward drop than standard diodes). Is this OK? If so,

Consider what happens when the battery is inserted. As long as the battery voltage is below 12.5 volts, it will not provide any current. If the battery is above 12.5 it will provide current at a higher voltage, but only until it has discharged to below 12.5.

Note that the diodes must be rated for the load currents you need. Also be aware that, if you short the load to ground accidentally, you're in trouble. The 12 volt supply will current limit at some (presumably) reasonable level, but batteries are not reasonable.

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A stated earlier you have drawn an NPN transistor.

I would actually recommend a FET especially if you are drawing over 50mA due to power dissipation in the transistor.

If you find a PFET, connect the source to the battery and the drain to your load. Keep in mind your FET must be rated for the current you wish to draw.

I would also recommend that R2 connects from the gate to the source (+Vbatt) and as you have drawn R1 driven by a 12V signal. This should result in ~6V on the gate which should be well over most FETs Vds(on) threshold.

R1 = R2 = 10k ohm would be a good start.

Note that your circuit will be on when R1 is pull low. It will be off is R1 floats or is pulled to +Vbatt.

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schematic

simulate this circuit – Schematic created using CircuitLab

Use Relay.

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