I'm a filthy casual with electronics. I purchased a really simple step-down 5V DC power supply. It has 2 inputs (AC L/N) and 3 outputs (VO+, VO-, +5V). I assumed VO stood for Voltage Output but what then is the +5v out for? As a side question, there is no ground. Is that bad? Probably dumb question but thanks. Manufacturer: SANMIM SM-PLH12A
VO-, VO+ and 5V are the pins for output voltage.
It looks like the 5V output is made from VO+ with a linear regulator but it looks like the components to make 5V from VO+ are not present, so unless there is something going on under the PCB, this power supply does not have a 5V output on the 5V pin. However, it might have 5V output on the VO+ pin. Perhaps if it were a say a supply with 12V output, it would be possible to have the 5V output via the regulator.
So you can only use the VO+ and the VO- pins for output.
Many power supplies have only two-pin ungrounded input so they work without mains earth, this is such an ungrounded power supply.
It is not bad, but it depends if you need a power supply with grounded output or a power supply with a floating output.
The VO+ could already be a 5V output, so you need to measure it. It does say 5V output, and the same PCB can be used for higher output voltages, which will make the extra 5V output possible via linear regulator.
Based on its label of SM-009-5V-02 I suspect it does have a 5V output on "VO+" on that version. You can (carefully!) measure that to confirm.
However as commented, on other versions of the supply (just guessing, let's say with a 12 V main output), then a linear regulator (probably in SOT-223 or similar SMD package) could be fitted in position U2, as well as adding capacitor C6, to give an auxiliary low-current +5 V output.
We can see that it has a C-L-C filter on the main output. I think I can see that the two vertical blue capacitors in that filter (near to the VO+ pad) have a 10 V rating. [Later confirmed.] So this is consistent that VO+ on that version of the supply could be 5 V (as VO+ cannot be higher than the voltage rating on those two filter capacitors).
In that case, it makes no sense to "steal" any of the output current from a +5 V main VO+ output, to try to produce another +5 V auxiliary output (and due to linear regulator dropout voltage, it wouldn't be possible anyway - even an LDO regulator drops some voltage between input and output).