# Power Supply negative voltage as ground and ground

I have a PC PSU and can get GND +5V +12V +3.3V and -3.3V -5V -12V out of it. I need to power a device that needs more than 12V. Am I able to take the -5V as GND and connect +12V to VCC and still use the usual GND as ground?

So I found a solution. I had an other Mosfet Driver laying around so I will use that. My PSU actually has the -5V wire but not the -3.3V so my bad sorry. Thanky you for your answers.

• Depends a lot on how his device is connected to other devices, and what the other devices think "ground" is. Mar 28, 2018 at 17:28
• I think the answer is no, but I'd need a schematic of what you are proposing. (Do you have two power supplies? I think you can make it work then.) Mar 28, 2018 at 17:29
• please post a picture of the label on the PSU Mar 28, 2018 at 20:28

## 3 Answers

No, you can't still use the GND as ground if you want to also connect -5V to the device's ground. That would mean shorting -5V and ground rails.

But yes, you can power your device from -5 and 12V rails, assuming that your device haven't and doesn't need another grounding (e.g. a device which has an usb connection or some sort of other connection with the outside world).

The GND from the Pc power supply will be grounded to the mains ground, so it's like it is an "absolute ground".

-5V rail from the ATX PSU will be at an "absolute -5V", if you then connect an usb cable which has it's grounding reffered to the mains ground you're basically shorting a -5V generator to ground. That's bad.

From now on, let's assume that you aren't going to connect anything else.

What you can do is take for example the -5V and make it your "new ground", now all the other voltages from that PSU will be shifted by those 5 volts. So your -3.3 becomes -3.3 + 5 = +1.7V, your GND becomes 0+5V = 5V, the 3.3 becomes 8.3V and so on.

In this configuration remeber that you can't exceed the -5V rated current

I have a PC PSU and can get GND +5V +12V +3.3V and -3.3V -5V -12V out of it.

You are mistaken.

First: There is no -3.3V rail on an ATX power supply. This has never existed.

Second: The -5V power rail was removed from the ATX standard in 2003. Unless you are working with a very old power supply, pin 20 is not connected, and there is no -5V supply available.

There is a -12V rail available on an ATX power supply, but its current capacity is minimal -- a typical 600W ATX power supply may have a current capacity of 300 mA on the -12V rail. It's primarily useful for supplying low currents to devices which need a split-rail supply, like operational amplifiers. Do not use it as a substitute for a 24V supply.

If you need to provide more than 12V, an ATX power supply is not appropriate for your needs. Use a standalone power supply designed to supply the voltage you need.

in terms of basic DC power supplies using +12V as VCC or your positive lead, and using -12V as your VEE or negative lead, that will give you 24 volts at your load... or whatever voltage depending on the combination of 5 ,12, and 3.3 you use for + and -. Use the +12 volt as positive and +5 volt as negative, that's 7 volts to your load.

However, on an ATX power supply check the current ratings or DC output specs of the different voltage rails, that's what will either make or break what you are doing.

Generally for an ATX PSU the +12v rail has a high amperage rating like 100+ amps to support high end graphics cards, much higher than any of the others rails. the -12v rail may be rated to do less than 1 amp, so for whatever two voltage rails you use to get a desired DC voltage, the minimum current rating from those two is what you have to stay under or blow the PSU.

only if you use the GND wire and 1 rail will the current rating of just that 1 rail apply.

and also check if the PSU has one 12v rail or multiple. If multiple, make sure you know which rail you are using and what it's current rating is if teh multiple 12v rails are different.

at this point if doing the above you use the PSU gnd as system ground on the device (i.e.) load that is fine, that's how every pc motherboard currently works, that DC ground is connected to PC chassis via mothererboard just like the AC earth ground is to the PSU shell connected to the pc chassis... that you handle while powered.