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I want to power some old machines using an USB PD power supply connected via cables containing a USB PD sink configured for the proper voltage/current.

However, one of those requires three voltages: +5V, +12V and -12V (all with a common ground obviously).

Joining the grounds of the two positive voltages does not seem like it should be a problem, however I am wary of inverting one of the 12V cables in order to generate the -12V. Is there a risk of a short circuit inside the USB PD power supply or does the USB PD standard guarantees that each plug is isolated from the others?

More precisely: would it be safe, according to the USB PD standard, to do the following:

  • using three cables from the same power supply with respective voltages: A = 5V, B = 12V, C = 12V?
  • connect A & B ground together with C's +12V to generate a -12V rail?

I can easily test with a simple ohmmeter if a given power supply allows it or not but I would also like to know if the standard allows for it.

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  • \$\begingroup\$ You can just use a multimeter to figure out if the grounds are together or not. \$\endgroup\$
    – Justme
    Commented Dec 27, 2023 at 9:02
  • \$\begingroup\$ @Justme obviously that is something I can test myself but I would like to know if the USB-C PD standard allows for such a coupling. \$\endgroup\$
    – Laurent Giroud
    Commented Dec 29, 2023 at 21:39
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    \$\begingroup\$ It's not up to the Type-C PD standard to define if the configuration you suggest is allowed or not. So it is neither allowed or disallowed by that standard, because that has nothing to do with PD. \$\endgroup\$
    – Justme
    Commented Dec 29, 2023 at 22:24
  • \$\begingroup\$ @Justme well, it could define this, they chose not to but isolation of the outputs could be part of the standard, precisely to allow the configuration I am interested in. \$\endgroup\$
    – Laurent Giroud
    Commented Dec 30, 2023 at 21:11
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    \$\begingroup\$ You are correct, but standards have a scope, and defining that is definitely not in the scope of the PD standard. It might be in scope of some other standard related to USB, but it AFAIK isn't. Why would the PD standard would need to define how multiple outputs on the same power supply are allowed or not allowed to work in relation to sharing or disallowing common ground? I mean, you have a PC with multiple USB sockets, nothing defines if they must or must not share a ground either, and you could have fully isolated USB ports on PC if you wanted. Not a problem that needs solving or defining. \$\endgroup\$
    – Justme
    Commented Dec 30, 2023 at 21:14

3 Answers 3

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would it be safe, according to the USB PD standard, to do the following:

  • using three cables from the same power supply with respective voltages: A = 5V, B = 12V, C = 12V?
  • connect A & B ground together with C's +12V to generate a -12V rail?

The standard doesn't require the sources in one device to be mutually isolated (floating), and usually they are not. So no, it would not be safe using the same power supply for all three voltages.

But there is an easy solution for that: use two PD supplies. One for +5V and +12V. Those can share the ground reference. The second PD supply can be connected to generate -12V.

As long as the supplies are mains-powered and there is no internal connection between GND and protective earth ("ground" pin on the mains socket), this will work fine.

Another solution is to use a PD supply with mutually isolated outputs. Some "bricks" are designed that way. You'd need to qualify the particular brick for your application and test this isolation.

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  • \$\begingroup\$ > "PD supply with mutually isolated outputs" I have tried to find those bricks but was not able to get any useful results. Would you have pointers? \$\endgroup\$
    – Laurent Giroud
    Commented May 12 at 1:26
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You are correct to be cautious. It is most likely that the common (O V) of each of the outputs will be connected together internally. Any attempt to series connect the outputs will, most likely, short-circuit the the one whose positive is connected to the 0 V of the next.

You can confirm this with an ohmmeter on the GND (0 V) outputs. The V+ outputs are probably going to be supplied via current-limiting circuits so you may or may not get continuity between the various V+ outputs.

You can probably achieve what you want with a compact DC-DC converter module. Get one with a datasheet but check that there's enough information on the datasheet to give you confidence that the output is isolated from the input or that the output is negative with respect to the 0 V input / output.

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  • \$\begingroup\$ Thanks for the answer, I will indeed verify with a multimeter before trying but I was wondering if the USB C PD standard allowed for it. \$\endgroup\$
    – Laurent Giroud
    Commented Dec 29, 2023 at 21:41
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The standards do not require two connectors on same equipment to be isolated or non-isolated. So you can't assume anything if they are or are not isolated. Safe assumption of course is to assume they share grounds.

It might be easiest for your device to isolate with a DC/DC converter so it does not matter where you plug it and how.

But if you are going to use PD and an isolating DC/DC converter, why bother having two USB cables. Just make a power supply with DC/DC conversion that takes in one single USB cable and provides all three required supply voltages out.

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  • \$\begingroup\$ The idea is to use as simple a mechanism as possible so using an additional DC to DC converter would bring additional complexity. The ideal solution would be for either USB PD to support negative voltages (yeah, not gonna happen) or to use two distinct power supplies. \$\endgroup\$
    – Laurent Giroud
    Commented Dec 30, 2023 at 21:33
  • \$\begingroup\$ @LaurentGiroud Yes but USB PD is intended to be very simple and provide single voltage up to 20V and 3 to 5 amps. Your requirements are not simple and therefore require additional complexity. Have you ever seen a device that has the same requirement as you, meaning two isolated PD cables, for keeping the innards simple? Either PD is not a good solution for your problem if it requires weird use of it, or, you add power conversion and you can use any single supply DC power source for supply, including USB-C and maybe batteries. \$\endgroup\$
    – Justme
    Commented Dec 30, 2023 at 21:53

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