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If two 5V power sources exist, one being always on and the other not (such as an ATX powersupply's standby 5V and "on" 5V rails), how could I draw power only from the "on" rail when present, or else from the standby rail?

Additionally, how can it be indicated which rail the power comes from? For example, lighting a red LED when using the standby rail, or a green LED if the "on" rail is used.

I think I may need to use a MOSFET or perhaps an Arduino Nano, I would prefer a simpler solution.

Anas Malas
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John Schultz
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  • Is your purpose to take power from either of two sources, or is it to detect which one specifically you are drawing from? – Anas Malas Oct 20 '22 at 08:38
  • neither. an ATX PSU uses a 5V Stand by pin that is always on. I want to use this rail for the off-state and the regular 5vdc rail when the PSU is on. Basically, when off state, power a red led, on state, power a green led and not the red. – John Schultz Oct 24 '22 at 19:20
  • That doesnt sound like neither, it sounds like you want to do both. You want to take power from one rail or another based on some condition, and you want to detect which one the power is coming from. What circuit do you want to power with this? The answer is different whether you want low or high power. – Anas Malas Oct 25 '22 at 05:42
  • A simple LED ring on a switch. if the 5-volt rail has no power, then the LED needs to be RED and power drawn from the 5-volt standby rail (pin 9 or purple of the ATX connector), when there is power on the 5-volt rail (pins 4, 6, 21, 22, or 23 or basically red), then the 5vsb is disconnected from the circuit, and power is drawn from the 5-volt rail and then the led indicator is green. – John Schultz Oct 29 '22 at 00:13
  • Okay, I submitted an edit which is beeing reviewed. Hopefully with it someone can answer your question – Anas Malas Oct 29 '22 at 17:28
  • @AnasMalas - EXCELLENT revision!!! – John Schultz Oct 30 '22 at 09:08

1 Answers1

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I was hoping someone with more expertise in circuit design would answer, but as no one did I will attempt to do so. This circuit isnt tested, but you can simulate it.

Theoretically, this will work. When the 5V rail turn on, it will energize the relay coil and that will switch both internal "switches". This will switch from one source to the next, and will light up the correct LED. The capacitor at the relay output is placed so that the circuit wouldnt "feel" that the relay disconnected one power source and connected another, but im unsure whether it is a good idea practically. The diode next to the relay coil is necessary as otherwise the circuit can be damaged. Dont add the "ExampleLoad" resistor, that is only here to see the simulation result.

schematic

simulate this circuit – Schematic created using CircuitLab

There may be a method to do this with mosfets, and certainly there are ways with an Arduino, but I dont want to venture out too much.

Anas Malas
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  • "...I dont want to venture out too much..." That is funny when you think about it. Dude,.. you are my HERO!!! Thank you. I think I might try to figure out a MOSFET route. Just to see if it can be done. Thanks again, dude! – John Schultz Nov 11 '22 at 01:15