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Question How do I make "POWER" and "ON" LEDs for PSU

s355

Junior Member
Jun 1, 2021
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I want to make a red "POWER" LED come on when plugged in, and a green "ON" LED come on if the PSU is in its fully on state. I have a switch that grounds PS_ON if turned on. How do I do so?
 

mindless1

Diamond Member
Aug 11, 2001
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Far easiest way is just run a series resistor for current limiting on the 5VSB which is always live, then for the green the main 5V rail which is only live when the PSU is fully on via the PS_On signal.

Resistor value depends on how bright you want the LED to be and its max rating, how much current to give it.

Red LEDs typically have around 2.1Vf forward drop, so say you want to use a typical 5mm diameter plastic encapsulated LED (or 3mm, either way the typical type found on case LED power indicators), they tend to have a max power of no more than 100mW but very bright and often short lived so I'd shoot for around 5mA current.

So doing the math, 5VSB - 2.1Vf LED= 2.9V to drop.
2.9/0.005A = 580 ohms (ohm's law, V=IR or V/I=R)

You'd put a 580 ohm resistor in series with the red LED on the 5VSB rail, and LED cathode to PSU ground. 5mA is a conservative enough drive current that if it's not convenient to find/use a 580ohm resistor, instead use the next higher or lower resistor value you have, within reason, I wouldn't go more than +/- 100% of that.

Similarly, the green LED might have a little higher forward voltage (check the LED spec sheets if you have them), let's say 3V. 3V could be significantly deviate from the true forward voltage but when giving only a conservative 5mA, like the resistor value it can be off quite a bit and still stay within reasonable margins.

5V -3Vf = 2V to drop
2V/0.005A = 400 ohms

400 ohm resistor on the 5V rail to ground instead of 5VSB. You could use the 12V rail instead and recalculate for that, but no real reason to, would then need 2 different resistor values for similar drive current and use a few thousandths of a watt more power in the resistor. In other words, if you were to get a common/popular resistor value like 470ohms, you could just get two of them and be close enough to use on both LEDs. Power dissipation is minimal, an 1/8W, (or 1/4W or higher) rated resistor would be plenty.

Now, if you want the red LED to turn off when the green LED comes on, you'd want the red LED running through a normally closed relay that gets energized by the main 5V rail to an open switch state when the PSU is turned on... probably more bother than it would be worth.

There are various online LED series resistor calculators to make this faster if you want different values. For example:

 
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