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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 enter image description here

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    \$\begingroup\$ It could be a dual-output supply, but you have provided zero information. Manufacturer / model number / web page / photos - ??? \$\endgroup\$ – AnalogKid Jun 13 at 19:32
  • \$\begingroup\$ A picture or link to the product would help. \$\endgroup\$ – ErikR Jun 13 at 19:33
  • \$\begingroup\$ A photo of the supply would help. In one lab I worked, we had a supply that had an adjustable output set with a knob, and a fixed +5V supply. This could plausibly be the same, but without further detail I cannot be sure. \$\endgroup\$ – nanofarad Jun 13 at 19:38
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    \$\begingroup\$ It does not have 5 V output. The components to make 5V output (most likely from VO+) are removed. \$\endgroup\$ – Justme Jun 13 at 19:45
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    \$\begingroup\$ The aliexpress link suggests that they use a single PCB for their 5V, 9V, 12V, 15V and 24V products. The higher voltage supplies likely have a 5V output in addition to the main voltage, but on the 5V version they omitted the parts for it. \$\endgroup\$ – ErikR Jun 13 at 19:52
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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.

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  • \$\begingroup\$ I am noting you as the best answer because you commented right away that the +5v didn't do anything ... which seems spot on. However 5v is getting pulled from VO+ like you mentioned. Thank you all so much for helping this newbie \$\endgroup\$ – KuraiFIN Jun 13 at 20:04
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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).

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  • \$\begingroup\$ You are correct! The capacitor has a 10v rating. The +5v hardly gets me voltage, but I discovered that using the VO+ and the VO- got me 5v DC. Thanks! \$\endgroup\$ – KuraiFIN Jun 13 at 20:02
  • \$\begingroup\$ @KuraiFIN - Hi, "You are correct! The capacitor has a 10v rating." Great, so that PSU was manufactured as a 5 V version. After I answered, I also noticed that you had then measured VO+ and it is indeed 5 V. Therefore the explanation for the lack of output on the separate "+5V" output pad is explained in the last paragraph of my answer - it makes no sense for them to add a low-current +5V output, when that version of the supply already has +5 V as the main output voltage on VO+. Any voltage seen on your DMM when you probe the +5 V pad, is just noise. There's no connection to a voltage source. \$\endgroup\$ – SamGibson Jun 13 at 20:13
  • \$\begingroup\$ Yes, thank you for the information. @ErikR mentioned that the manufacturer reused their PCB on higher voltage outputs which is what caused my confusion. \$\endgroup\$ – KuraiFIN Jun 13 at 20:22

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