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I recently got this "made in china" linear power supply which converts 220VAC/50Hz to 15VDC/1A.

Please see attached photo.

Would be possible to do some modifications to it so I can get 5VDC instead?

Ideally the changes should not introduce additional noise, since I'd like to use this psu in a low-noise application.

Efficiency reduction is acceptable, as far as I can get 500mA at the output.

Thanksenter image description here

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    \$\begingroup\$ If you could it would be very inefficient. If it's rated 15V 0.3A you would get 5V 0.3A. It would be better to use it as is, and add a 15V to 5V buck converter on its +15V output That could be expected to give you 5V 0.75A (0.9A minus buck conv efficiency) \$\endgroup\$
    – user16324
    Commented Jun 16, 2020 at 11:37
  • \$\begingroup\$ Thanks @BrianDrummond, it makes sense. \$\endgroup\$
    – Juan Cortines
    Commented Jun 16, 2020 at 11:44
  • \$\begingroup\$ Sorry, Just a question: The buck converter would introduce extra noise? It's for a low noise application. Thanks \$\endgroup\$
    – Juan Cortines
    Commented Jun 16, 2020 at 12:03
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    \$\begingroup\$ Yes it probably would. If current requirements are low enough I'd consider a linear regulator from 15 to 5V (LM317 etc) + additional LC filter if necessary \$\endgroup\$
    – user16324
    Commented Jun 16, 2020 at 12:07
  • \$\begingroup\$ Thanks @BrianDrummond \$\endgroup\$
    – Juan Cortines
    Commented Jun 16, 2020 at 12:11

2 Answers 2

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There is no "white transistor". There is a multi-turn trimpot near the upper right corner of the transformer. I don't see any other adjustments, so this probably is the output voltage adjust (as opposed to an output current-limit adjust).

Separate from that, my read of the transformer is that it is capable of supplying 15 Vac at 1 A. There should be plenty of headroom for a 0.5 A output at any voltage below 15 V.

If the trimpot does not have sufficient range, the next step is to post a photo of the underside of the board so we can determine which resistors to change to shift the trimpot adjustment range. That might be difficult, because it looks like the board is supported by grooves in two extrusions. You would have to dismount the main power transistor. Then the board (and the transistor) can slide out of the extrusions with the rear panel wires still attached.

Maybe, two screws release the rear panel from the extrusions, and they drop away. Dismounting the power transistor from the bottom cover lets that drop away, exposing the bottom of the PCB.

Or something like that.

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yes, it can be changed, you have to turn that screw head on the white transistor and measure the voltage at the output

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  • \$\begingroup\$ Maybe it can be changed to 5V if the adjustment range is large enough, but it means that at least 17V is required to make 15V at 1A, so at least 2W is dissipated. If you drop 17V to 5V at 0.5A, you will dissipate 6W inside the supply. Good luck making it heat up 3 times the intended amount. Even if it survives, it's not feasible. \$\endgroup\$
    – Justme
    Commented May 30 at 20:51
  • \$\begingroup\$ it will probably warm up a bit more. When you buy such sources on Aliexpress, the Chinese offer you several voltage options to adjust it for you. It's not like he has stocks for 9V/12V etc, no, he does it just adjusts it for you \$\endgroup\$
    – Mihalcea Adrian
    Commented May 31 at 1:05
  • \$\begingroup\$ Actually I looked at photos of these supplies yesterday and the models with different output voltages appear to have different transformers to match the output voltage. Do you know for a fact that they are simply same units adjusted for lower output and to waste more power, or are you just guessing? \$\endgroup\$
    – Justme
    Commented May 31 at 5:41
  • \$\begingroup\$ you can choose with bigzi or Talema transformer which is better quality but also more expensive. I'm not sure if it works for all of them to change their voltage, but for the ones that have that nut head, it certainly works. If they were on fixed voltage, the Chinese wouldn't ask you what specifications you wanted for the source, you simply bought the one you needed.image link \$\endgroup\$
    – Mihalcea Adrian
    Commented May 31 at 19:50

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