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Using a relay, if I found a 12v source on the ps3 motherboard that was on only when the ps3 was on. Could I attach a 12v relay switch to it and the connect some LEDs that run off another power supply so that when the ps3 turns on the LEDs do too?

Thanks

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You could do that, but instead of using the PS3's power to control the relay it's maybe more efficient to drive the LEDs directly with that. Standard red LEDs are about 2 V, then you can place 5 of them in series and add a current control resistor in series to fix the current at 20 mA. A relay would probably use double of that.

That being said, and though it's not likely that the PS3's power supply wouldn't have 20 mA to spare, you better use a transistor controlled by the PS3's power (doesn't have to be 12 V) to switch the externally powered LEDs. In that case you hardly load the PS3's power supply.

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The 5 V comes from the PS3, the 12 V is the external power supply. You'll have to tie the grounds of both power supplies together.

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Assuming the 12V source is capable of providing the required current, (typically ~5-20mA) then yes, this would be possible.

A better solution might be to use a standard NPN or darlington transistor, which would not load the supply as much. If isolation is required then a SSR (solid state relay) would be good.

If you are only driving a few LEDs then as Steven says, putting them in series (number depends on Vf of LED) with resistor is an idea.

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