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I would like to power my DIY led aquarium light with dual power source and looking to protect the revers current in case both the power supply are switch on together. if i use simple diode, which specification should i use.

Both the power supply are 12v5amp, and the load is 15 led of 3 watt each hooked to 5 meannwell LDD700H (3led's to each driver) plus i have 2 12v PC fan's to keep the heatsink cool. i checked the current its around 3.18amp on full load.

As shown in the diagram, what diode should i be using. enter image description here

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    \$\begingroup\$ Use two 6A Schottky diodes. Which one depends more on where you want to buy. (Schottky diodes because you want to avoid too much forward voltage drop.) \$\endgroup\$ – Janka Dec 30 '16 at 13:53
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If you are concerned that any volt drop might be too much, consider using a dual supply hot-swap controller that has wire-OR functionality: -

enter image description here

The LT4236 arbitrates between one supply or the other and selects the appropriate MOSFET for the supply that is present or has the higher incoming voltage. As both are 12 volts it doesn't matter; one supply will be selected and the volt drop of an SiR158DP will be less than 10 mV for 12 volt incoming.

Linear Technology have a range of these devices.

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