# How to create an ATX -5 volt rail for vintage Macintosh

I am trying to build a simple adapter for a mini ATX power supply for a vintage mac, I need a 5 volt rail that has it’s own separate ground (+5 volts to -5 volts) how would I go about doing this?

• What options have you concidered? It depends if you really want to build one or buy a neat ready made adapter. Usually people just put a linear regulator to bring the negative 12V to negative 5V, or use charge pumps or switch mode converters to genrate the negative 5V from one of the positive supplies. Sep 28, 2020 at 4:46
• I’ve considered a linear regulator, I’m assuming this would also provide the +5 volts that the system needs, I can provide a pin out if you need to better understand the system, it is a Mac LC II Sep 28, 2020 at 4:51
• old.pinouts.ru/Power/mac_lc_power_pinout.shtml Sep 28, 2020 at 4:53
• So you want to make a complete power supply for the aforementioned PC? Or, you have already a power supply that doesn't have a -5V output and you want to add a -5V output to it? Sep 28, 2020 at 6:14
• How much current do you need on the -5V? Sep 28, 2020 at 7:32

"I need a 5 volt rail that has it’s own separate ground (+5 volts to -5 volts) how would I go about doing this"

"I have a tiny 12 volt ATX power supply that has a 12v and 5v rail"

I don't know what do you intend to say when you used the expression "own separate ground". Why do you need a separate ground?

According with those specs, the -5V is just a negative voltage, those -5V are measured in relation to the same "ground" of +5V and +12V.

If you connect a voltmeter between -5 V and +12 V pins of this PSU, you would find +17 V; accordingly, the reading between -5 V and +5 V pins of this PSU should be +10 V.

If you have an ATX PSU, your easiest option is using ATX's 12 V and 5 V, and, as you need -5 V too, using a negative linear regulator (the most common is 7905) to convert the -12 V from the ATX PSU (blue) to the -5 V that you need.

ATX PSU's doesn't have -5V output, but ATX standard requires a -12 V output, and you will convert those -12V to -5V using a negative voltage regulator as 7905 IC (79xx IC regulators are the negative cousins of 78xx positive regulators).

Usually the -5V current requirements are low, that's the reason why a linear regulator as 7905 can do the job.

Maybe you will need to do some googling in order to learn about the negative regulator; but it's not complicated.

• I think he needs a separate ground because he wants the ground to be the negative source, and positive is tied to the original PSU ground. Apr 28 at 6:18
• @JimmyFalcon ?! But didn't your answer said that OP could use a 7905 to convert ATX's -12V to -5V (an option described in my answer, too)? How could it be done without a common ground for ATX PSU and the 7905? May 7 at 1:55

You need to use a transformer-based 5v DC-DC converter. You power it from a +5V or +12v output, then you can tie its positive output to the PSU ground (since it is isolated with the transformer), and its negative becomes your -5v source.

Or (a simpler solution), if your ATX has a -12v source (which it should), use an LM7905 step-down to obtain the -5v source. But that will be 1A max.