I am creating a LED lighting unit for an Aquarium and I have a 240vAC to 24v DC power supply.

I want to output 12V and 5V also in addition to the 24v DC.

I have 2 buck modules - like these http://www.ebay.com/itm/1Pcs-Lm2596-Power-Supply-Output-1-23V-30V-Dc-Dc-Buck-Converter-Step-Down-Modul-A-/301988858332?hash=item464ff049dc:g:5MgAAOSwYmZXEehH off ebay set to 5v and 12v from the 24v input.

Measuring the DC on the output side I get the correct voltage from the output + and - connections.

My question is in my project I have a need for a lot of 5v and GND connections.

Can I just join all the GNDs together?

Common GND

Will this work or is this a stupid thing to do? What would happen?

The 12v device is an Arduino and the 5v device will be connected to the arduino pins.

  • \$\begingroup\$ Those cheap converter boards often have negative terminals connected together. A fancy converter would use a transformer for the inductor to give an isolated output but those cheap Ebay boards usually do not. So you can just use one ground wire from the common gnd to each converter board. The extra wire is redundant and will likely just emit noise because it's makes a big loop with current going trough it. \$\endgroup\$
    – squarewav
    Jul 19, 2016 at 4:01

3 Answers 3


This is okay, except that your 240 vac negative terminal is NOT ground. The wiring diagram is a little ambiguous there, but I think you understand.

Best in this case would be a large bread board or something similar to give you a nice working surface, as well as plenty of access to your 5v rail.

Ideally, you want you ground path (current return path) to be reasonaby near your power source or power path.

  • \$\begingroup\$ "near your voltage path". I think you mean near your "power source". But I give a +1 anyway. (you can edit your answer if you want). \$\endgroup\$
    – Marla
    Jul 19, 2016 at 1:56

Although the link to the item is no longer working, in general most available DC-DC buck converter modules have a current limiting function based on a resistor on the output ground to measure current. This will not work if input and output ground are connected together outside the module. The difference between the input and output ground is at most e.g. 0.1V under full load, or zero under no load, so it is hard to measure. Yet is is essential for current limiting.

In addition, some converter designs create an "inverted" output, i.e. input ground is common to output positive. Such a converter will not work at all if connecting the input and output negatives. This is often the case for "boost" or "buck-boost" or "step-up" converters, but rarely or never for "buck" or "step-down" converters.

So if you have a common "buck" or "step-down" converter (not an "inverting" or "boost" design) and if you are not interested in current limiting then you are fine. And by your description this seems to be the case.


This is an old question, but here's a more concise answer. The circuit linked to uses an LM2596, these have a single ground pin and typical usage is with a common GND. You can see the typical usage by searching for the datasheet pdf.

  • 1
    \$\begingroup\$ This isn't really an answer to the question, although it adds a bit of useful info. Pretty much, for practical purposes, LM2596 must be operated with a common ground. \$\endgroup\$
    – HKOB
    May 13, 2018 at 8:59
  • \$\begingroup\$ For clarity, and this is what I implied with my answer. Typically, it's ok to assume GND is common in circuits that include these chips. I'm pretty sure someone out there can cook up a circuit without any respect for the common ground of other potentially connected devices. So, my answer is that it is 'Typically OK'. \$\endgroup\$
    – Neil Regan
    May 24, 2018 at 4:46

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