Modifying a Chinese power supply for variable voltage


End result: 33A maximum output current, adjustable from 4.8V to 15V


I’m currently working on a product which uses a size 2430 “hobby” brushless motor and 25A electronic speed controller (ESC). In its “intended” use, the controller runs from two lithium-ion batteries with a total voltage of around 7.4V, but I want to run it from a mains-derived power supply instead. However, there are no off-the-shelf supplies available with that output voltage.

Fortunately, there is no shortage of inexpensive Chinese switchmode supplies with standard outputs of 5,12,24V etc. Most (all?) of these have the ability to adjust the output voltage slightly, by around ±10%. I reckoned that it should be possible to modify such a supply to provide a fully-adjustable output voltage which could be set to the desired 7.4V. This is by no means a novel idea – many people have modified supplies (a common mod is to increase the output to 13.8V, for ham radio use) – but I haven’t seen a good operational analysis of these supplies, so it’s a good excuse to do some detective work and figure out what makes them tick.

The supply

The model numbering scheme for these supplies appears to be S-AAA-BB, where AAA is the power rating in watts and BB is the output voltage. For this application, I’ve used an S-400-12 supply (400W, 12V, 33A). Here it is:s-400-12_original

Here is a copy of the original EBay listing. It was very cheap – actually less than some of the 360W supplies which are available! Because of the relatively high power rating, it has a cooling fan which turns on when the power supply’s temperature (measured with a thermal switch placed inside the output filter inductor) rises above a certain point.

Reverse-engineering the PCB

First task – get the main PCB out, scanned/photographed, traced, and a circuit drawn. My procedure was roughly the following (all processing done in Photoshop):

  1. Scan the bottom side (tracks) and stick it into Photoshop.
  2. In a new layer, make white dots over every one of the pads/vias/holes. This will help both for aligning things later, and for producing a nice image.
  3. Photograph the top side (components). I photographed the board in four quarters and reassembled these in Photoshop to try and get a “flat” view of the board. The white dots made in step 2 are a huge help for aligning the four images.
  4. Using the path tools, draw around every one of the bottom tracks.
  5. Use the paths as selection areas to fill in the tracks in a separate layer – use colors to identify the main tracks like DC ground, DC output, HV positive and negative, etc.
  6. Go through each component and track, see what they’re connected to, and start filling all this out in a schematic. Once fully done with each component or track, erase it in photoshop (or just draw over it in a separate layer with white) so you can focus on the stuff that hasn’t been traced yet.
  7. Use a lot of guesswork and artistry to lay out a nice circuit diagram!

Here’s high-res images of the PCB:

 And, what you’ve all been waiting for, the full schematic (click image to link to the PDF). An Eagle schematic is also available – s-400-12_supply.