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I have a circuit which uses two 5 VDC PSUs to power a few addressable LED strips each and one is connected to an Arduino controlling them. Both PSUs are connected via the same 120 VAC input.

I've noticed that the LEDs only seem to respond correctly when the LED power is hooked up to the same PSU as the Arduino.

From what I gather I need to connect the -V terminals of the two power supplies together so they have the same base "reference" point but I have seen conflicting/confusing information about the safety of this, depending on the specific configuration of the PSUs. (See some of the comments on this question: Can I connect two power supplies' GNDs together?)

For reference: the PSUs in question are two Mean Well RSP-320-5 and from what I've read they aren't configured to be completely connected in parallel (+V and -V shared between supplies), but I wanted to double check that connecting the -V terminals is the right move before I fry something.

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  • \$\begingroup\$ Connecting the -V terminals together is mandatory. It is possible that something has damaged because they were not connected. I hope both supplies have the earth/ground terminals properly connected. \$\endgroup\$
    – Justme
    Oct 4, 2022 at 7:15

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You should ground FG (frame ground) to earth on both supplies. Block diagram:

enter image description here

You can connect the -V terminals together, connecting the +V terminals (without -V connected) is no problem for the supplies but could damage the other circuits so don't do it.

The normal approach would be to connect both -V terminals together and (most likely) connect that point to earth. Regarding the latter connection, Meanwell comments that it is okay but it may affect EMI.

Q59 The output ground (GND) and frame ground (FG) is the same point in my system, can MEAN WELL's power supplies be used in such system?

Ans Yes. Since our products are designed based on isolation concept, it will be no problem that the output ground (GND) and frame ground (FG) is the same point in your system. But, EMI may be affect by this connection.

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Yes, you can connect the ground terminals together on Mean Well RSP-320-5 power supplies. They are isolated AC-DC converters.

You cannot connect the positive terminals together.

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  • \$\begingroup\$ Can you elaborate why positive terminals can't be connected? Running 2 PSUs in parallel seems like OK thing to do. \$\endgroup\$
    – Petr Peller
    Oct 6 at 10:04
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    \$\begingroup\$ @PetrPeller You can only do that with supplies that have been specifically designed to operate in a parallel load sharing mode. Consider what happens if you connect two power supplies together and one is outputting 100mV higher than the other, since they won't have perfect regulation. If the cable between them has a resistance of 10mΩ, Ohm's law tells us that 10A of current will flow from one supply to the other! \$\endgroup\$
    – Polynomial
    Oct 8 at 8:40
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    \$\begingroup\$ @PetrPeller Some power supplies are designed to allow this kind of parallel operation. They typically have some sort of interconnect between supplies (either analog feedback or digital communications) so that each supply can adjust its output to closely track the voltage of the other, to achieve an event current balance. There are ICs to manage this, typically called N+1 load balancing controllers. Some supplies can also operate in parallel using a remote sensing technique where each supply has something akin to a 4-wire Kelvin connection near the load for precise current sensing. \$\endgroup\$
    – Polynomial
    Oct 8 at 8:47
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Your question relates to an electrical circuit's common reference 'C' or 'COM' and represented by the following symbol.

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Here's a schematic which shows an electrical circuit, with a negative common reference, wired to its power supply.

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The following relates to your example of two circuits with a negative common reference but with two independent power supplies. The power supplies are not be connected in parallel.

enter image description here

This one is similar to the previous one but with two different voltages.

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The last one is a dual-voltage circuit with a dual-voltage power supply.

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