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I have an ac adapter that outputs 18 volts with a max current of 3 amps.

Is it possible that I can connect it with series with an atx power supply's 12 volt out in series to get 30 volts?

If so what will the current be limited to?

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Assuming the power supply outputs are DC, you can do that provided the two supplies are isolated - that is, there is no common ground between the two.

If you're talking about the typical wall-wart, chances are quite high that it's a flyback topology switching supply, which is isolated. That's probably also the case with the ATX supply, but I'm not positive. The ATX supply's ground wires are not isolated, so you should use it as the "lower" supply - with its ground being the actual ground for the target device. Connect the wall wart's negative output to the ATX supply's 12V output and the wall wart's positive output will be 12 volts higher relative to the ATX supply's ground.

The current capacity would be limited to the lower of the two capacities of the source supplies.

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  • \$\begingroup\$ As long as their current ratings of each are above the load current \$\endgroup\$ – JonRB Apr 7 '19 at 15:44
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    \$\begingroup\$ ATX supplies have their ground (black wires) connected to the metal chassis and mains PE is also connected to the chassis. So at least the ATX supply is referenced to earth. \$\endgroup\$ – Justme Apr 7 '19 at 15:52
  • \$\begingroup\$ Ah. Then it would work if the other supply was isolated, but it means that you should use the ATX supply as the "bottom" supply - with its ground wire being actual ground for the target device. \$\endgroup\$ – nsayer Apr 7 '19 at 17:28

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