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first of all I am a real novice in electronics but according to my circuits and electronic classes I think this should be possible:

I want to use a 4-pin ATX cable as a power input in order to generate 24 outputs. Basically, my idea consists in using zener diodes.

Having 12Volts as input and then a 5Volts zener diode I should be able to have a +5V output, then connecting this output to a 3.3Volts zener diode I should be able to have a +3.3V output.

Here's a circuit I've barely drawn:

enter image description here

My question is how would I deal with the -12V output? And also once all voltages are tested and wrapped in the 24-pin connector, How safe would be connecting a motherboard to this result?

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  • \$\begingroup\$ There's a schematic button on the editor toolbar. It's very easy to use. Add in an in-line schematic. \$\endgroup\$ – Transistor Mar 9 '16 at 19:32
  • \$\begingroup\$ Thank you for the tip, but I've posted this question via mobile app since I don't have a desktop around me \$\endgroup\$ – Jorge Chayan Mar 9 '16 at 19:34
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    \$\begingroup\$ With the 10K resistors you show on your drawing, you will only be able to draw well under 1 mA from each zenered output. If you want to draw any significant power from the 5 volt and 3.3 volt outputs, you MUST use a voltage regulator - preferably a switch mode type. \$\endgroup\$ – Peter Bennett Mar 9 '16 at 19:50
  • \$\begingroup\$ Well what I'm actually trying to do is using a PSU's 4-pin 12V output, to actually feed the motherboard via the 24-pin input. If I understand properly what you are saying is that I have to design a switch mode voltage regulator on the zener ends? \$\endgroup\$ – Jorge Chayan Mar 9 '16 at 19:55
  • \$\begingroup\$ Don't use the Zeners. Instead, use two DC-DC converters, one 12 V to 5 V, and the other 12 V to 3.3 V (and watch the power/current requirements for each voltage, as well as the current availabe from your 12 volt source!) \$\endgroup\$ – Peter Bennett Mar 9 '16 at 20:09
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If you want to power a pc from a regulated 12 Volts, you should look into a compact SMPS setup. Or you know, just use a commercial product like a Pico PSU. Completely depends on how much power you need.

enter image description here

As far as using Zener Diodes, based on typical ATX current needs, you will need a huge Zener or heatsinking. It's not even close to ideal. Using a Zener Regulator calculator, a 500mA load at 12V in, 5V Out, will require the Zener to drop 2.55 Watts in heat.

Linear regulators would be much better. Switching regulators the best.

For -12 Volt, you will need a voltage inverter. But -12V is a legacy voltage on ATX supplies for normal PCI cards. Not really used. It depends on what you have connected. Many times it can be skipped on modern motherboards.

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  • \$\begingroup\$ Thanks for the idea. I thought that the discontinued pin was the -5V one, required for some ISA Cards. \$\endgroup\$ – Jorge Chayan Mar 9 '16 at 19:59
  • \$\begingroup\$ -5 was discontinued. -12 is required but unused 90% of the time. They will phase it out eventually. \$\endgroup\$ – Passerby Mar 9 '16 at 20:00

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