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I have a new PC and want to build something in it, but I am wondering if I cut off a purple wire, +5VSB, from the power supply leading to the motherboard, will the PC still work?

Exactly I want to put a LED light to be on when the PC is off. Only one wire has a power under pc off. It's a purple one, and the green one with ground starts the PC, so I would like to connect the LED light to the purple one, but would the PC be damaged?

Cutting the purple wire will cause damage according to comments.

Is there a way to achieve having an LED light for when the computer is off and turn off when the computer turns on?

Peter Mortensen
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PooDy
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3 Answers3

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Don't cut off the purple wire that carries the +5VSB standby voltage. For the last 15-20 years the +5VSB rail is almost the most important rail in the entire PC. The standby (always ON) voltage keeps all mainboard service electronics alive, it powers all control buttons, and is inseparable part of PC south cluster (formerly South Bridge). It also supplies basic power to all USB ports, so the PC can wake up from keyboard or mouse. All power sequencing logic is usually powered from +5VSB, which turns all other power rails in proper sequence and proper delays. It also supplies the Intel AMT (Active Management Technology) if enabled and configured, to give access to PC updates and software maintenance by company IT professionals even when your PC is OFF and not used.

If you want a simple indicator that your PC is off (but still connected to mains AC), use the +5VSB as power to your LED, and put a transistor switch to ground (BJT or N-FET, doesn't matter much). Then use the green wire (PS_ON#) as the control signal. When PS_ON goes low (to power the PC), your transistor will turn OFF, and LED will turn OFF as well. If a n-p-n transistor is used, use a 10-20k resistor between PS_ON# and base. And don't forget a current-limiting resistor along the LED and transistor switch, about 680 Ohms (depending on the LED type).

Ale..chenski
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At the very least your RTC battery will drain much faster.

You wouldn't be able to use standby/sleep.

Depending on how the power switch and motherboard are wired, there's a good chance the computer won't be able to power on at all.

Why would you want to do such a thing?

If you want a 5 V power source at all times, you can splice the wire and attach something in parallel. But the wire itself must remain connected to the motherboard. Keep in mind that +5VSB is always on (as long as the PSU is powered), not just when the computer is switched off. Also keep in mind that there is a current limit, though it should be enough to power an indicator LED.

Also, be careful when fiddling with a PSU like this. If you short circuit something, you've probably just lost a PSU. The rest of the system might also be damaged. Don't test this with equipment you must use/can't afford to replace unless you know what you're doing.

See: https://superuser.com/a/821348/117590, http://www.pcguide.com/ref/power/sup/funcSoftPower-c.html

Bob
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+5SB is on whenever your PSU is not switched "hard off", not just when the PC is "soft off", so just wiring an LED to +5SB and ground won't give you the effect you want.

The conceptually easy and foolproof (but a bit clunky) way to have something (like your LED) turned off when something else (like your main power supply outputs) is turned on is with a relay with "normally-closed" contacts. A relay is just a switch that's flipped when current passes through a coil - an electromagnet. "Normally closed" means that the contacts conduct electricity when the relay coil is not energized. You can use one of the 12 volt "Bosch relays" for this. These are very easy to find in auto parts stores and are often used by car modders and stereo installers. (It is a convenient accident for us that 12 volts is available both in cars and in PCs.)

Just be sure that the relay you get has "normally closed" as well as "normally open" contacts, i.e. it's a SPDT (single pole double-throw) type.

The relay will have quarter-inch quick-disconnect terminals with small numbers next to each. Connect the relay coil (terminals 85 and 86) across the PSU's +12 (yellow) and ground (black) wires. (If you have a spare four-pin "Molex" drive power connector, that is a great place to connect to those.)

Wire the normally closed and common contacts (87A and 30, respectively) in series with +5SB, your LED, resistor (680 ohms is probably good), and ground. The anode of the LED goes to the +5SB side of that path, the cathode goes to the ground side. Easy.

Just, as someone else said, do not cut the +5SB wire (at least not permanently). Splice another wire into it instead. If you don't want to solder, the auto parts store will also have 3M "Scotchlock" insulation-displacement connectors that will let you splice into that wire easily.

Either way, be sure the PSU is completely off while you do the work. Either pull the AC power plug from the wall, or, if the PSU has a hard power switch on its back, turn that off.

There are other ways to do this. A single, very cheap IC chip like a 74AC00 could be used. Or a couple of transistors.

There is a trick involving using the PC's main +5 supply as ground for the LED, strange though that sounds. This lets you get away with nothing but the LED and resistor. 150 ohms would be about right in this case.

But the relay is the most straightforward and works in the most intuitive way.

Jamie Hanrahan
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