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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?

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  • \$\begingroup\$ 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. \$\endgroup\$
    – Justme
    Sep 28, 2020 at 4:46
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    \$\begingroup\$ 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 \$\endgroup\$ Sep 28, 2020 at 4:51
  • \$\begingroup\$ old.pinouts.ru/Power/mac_lc_power_pinout.shtml \$\endgroup\$ Sep 28, 2020 at 4:53
  • \$\begingroup\$ 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? \$\endgroup\$ Sep 28, 2020 at 6:14
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    \$\begingroup\$ How much current do you need on the -5V? \$\endgroup\$
    – Jasen
    Sep 28, 2020 at 7:32

2 Answers 2

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"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?

The table that you linked doesn't say anything about "separate ground":

enter image description here

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.

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  • \$\begingroup\$ 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. \$\endgroup\$ Apr 28 at 6:18
  • \$\begingroup\$ @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? \$\endgroup\$
    – mguima
    May 7 at 1:55
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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.

enter image description here

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