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I am new to power supplies, and I realize there are a lot of questions around already asking about using atx computer power supplies in series. My understanding is that they can indeed be used in series as long as they are identical models with matching transformers.

My question is about "hot swappable" power supplies used for computer servers. These 12v supplies are designed to be swapped in and out while a computer is still running. I am wondering if these can also be used in series, or if there is circuitry preventing over-voltage.

My application will be for tinkering and educational work - I plan on driving various dc motors as well as a making arcs for electrical discharge machining (up to 120V DC).

I have pasted an image below of one particular model, and although I couldnt find a datasheet for this particular model, I found this brochure here enter image description here

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ATX or other PC power supplies cannot be used in series.
Output ground is connected to Earth via a low impedance or direct short.

You must use laboratory power supplies rated for floating operation with higher common mode voltage.
Not all of them are! Read the manual.

You know they are capable of floating operation when it is written around the connectors.
For example on this Rohde and Schwarz HMC8043 maximum common mode is 250 Vdc:
enter image description here

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  • \$\begingroup\$ For experimental purpouses @user can as well disconnect Earth and connect them in series with no ill effects. \$\endgroup\$
    – fraxinus
    Mar 14, 2020 at 22:06
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    \$\begingroup\$ @fraxinus What would happen if you did that, have two ATX supplies in series with their grounds unconnected. All goes well but then one of the ATX supplies moves and its metal chassis touches the other supply. What would happen? Anyone who cannot answer this question should not be using ATX supplies in series. \$\endgroup\$ Mar 14, 2020 at 22:22
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    \$\begingroup\$ @fraxinus No ill effects? That would most likely make the metal case to capacitively float at half mains voltage due to input mains filter, and touching that can be a shocking experience. Do you think it is safe to suggest bypassing safety earth wire? Especially to people whose skills you don't know about. Especially if without further instructions they go modifying insides of the power supply. There is a reason why devices with earthed plugs need earth! \$\endgroup\$
    – Justme
    Mar 14, 2020 at 22:23
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Extremely unlikely that these can be connected in series. You can measure it though. These supplies seem to have 3-prong mains inlet, so they must be connected to earthed mains plug. If you use a multimeter between the metal cases, they should read continuity to each other via mains earthing. If the supply output return wire (0V) also has connectivity to metal case or mains earth, then these can't be connected in series. As these supplies are meant to be operated in a paralleled fashion of some sorts, where another board somehow combines the outputs together to share the load, there is a slight chance that the low voltage output is floating, i.e. not referenced to mains, but still unlikely.

Even ATX power supplies cannot be connected in series (normally and safely that is, there are unsafe and abnormal ways though which for everyone's safety I won't go into any further), so I don't understand why you have this understanding that they can be connected in series.

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