Here are some of the early words I wrote on the topic of the 3040's power supply. Scroll down to the bottom to find a comparison between the PSU area of the 5V and 12V models.
Unusually for modern thin clients the 3040 runs off 5V. The coax plug is 4mm x 1.7mm.
In May 2021 I heard from April who had got hold of a 3040 but hers ran off 12V. Investigating this led me to the Dell community pages where, in August 2020, somebody posted the as yet unanswered comment:
"...our latest 3040 clients come with 12v supply and 12v stated on the underside but our older 3040 clients say 5v on the underside and 5v on the supply.Both plugs fit in both clients so I would also like to know if they are interchangeable. Otherwise it is just an accident waiting to happen that someone plugs in the wrong adapter."
On a revisit to that page I see there have been several requests for an update including one in November 2021:
"Update would be nice.... One of my interns just killed a 5v model plugging it in to a 12v adapter....."...which I guess answers the question.
At least with DELL thin clients the required supply voltage is marked either by the power socket or on the product label. The pictured label above are off my 3040s.
In January 2024 I heard from James who pointed out that if you were after a 5V power supply for your 3040 then those PSUs advertised for the Orange Pi PC or the Sony PlayStation Portable were suitable. They are 5V PSUs fitted with the same 4mm x 1.7mm plug.
In July 2025 I heard from a correspondent whose 3040 had come with a 12V supply and it was only some time later he noticed that the 3040 was marked as running off 5V. By then his 3040 had died but he wasn't sure why tho' he did have a number of items powered via the USB ports.
In August 2025 I heard from David who was happily using a 12V version running from 5V.
The images below compare the power supply sections of a 5V and a 12V 3040. Basically I can't see any significant difference between them tho' there might be something in the values of the surface mount components. (Move the slider to switch between the two models).
[My apologies, I don't have the patience to ensure that the overlapped photos are in alignment to nearest fraction of a millimeter].
