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I have some old devices like SMPS power supplies, and I would like to analyze their workings by first drawing a circuit diagram of them.

I'm looking for some tips/techniques for doing this. Should I first draw all components of the circuit, then find out which ones are directly connected to each other by using continuity setting on my multimeter, and then try to figure out the values of individual components? Is there a good general guide/procedure for this?

Assume that the diagram of the circuit is not available online or anywhere else, I want to be able to build it on my own. Also, I'm looking for techniques for doing this in general, without having assumptions of the workings of the circuit beforehand. I'm mentioning SMPS as just an example.

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  • \$\begingroup\$ Too short for an answer, but for the board depicted in the question you do not need a multimeter, it is a cheap single layer board, you look at it and see the connections. \$\endgroup\$ – Vladimir Cravero Jul 31 at 17:11
  • \$\begingroup\$ @VladimirCravero I removed the picture as I realized it might give a wrong impression of my question. I'm not looking to analyze just one particular circuit but rather know a general procedure for building the diagram of any circuit, even more complicated ones. \$\endgroup\$ – S. Rotos Jul 31 at 17:17
  • \$\begingroup\$ Not VTC, but this is far too broad of a question to answer. You're essentially asking about the entire topic of reverse-engineering a PCB, which can range from trivially easy for a single layer board, to near impossible for highly-integrated circuits on a several-layer PCB. \$\endgroup\$ – Shamtam Jul 31 at 17:28
  • \$\begingroup\$ I suggest that you have a look at some of BigClive's videos: youtube.com/channel/UCtM5z2gkrGRuWd0JQMx76qA He does a lot of reverse engineering. The way he does that is by taking photos of both sides of the PCB, flipping one of the photos so that the orientation is the same. Then drawing in the components and draw schematic. \$\endgroup\$ – Bimpelrekkie Aug 2 at 9:59
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I usually take a picture of the top and bottom of the board. I line the photos up in software and switch between the top and bottom pictures. Then draw the traces over it in a program that allows stacking of images and vector graphics (power point, visio).

If the program has layers, then traces can be created for both top and bottom layers. I then identify each component, either by visual inspection or by unsoldering and using a digital multi meter to find the functionality. It's also nice because it allows one to label all of the components. This would be difficult to do on a four layer board at which point, instead of taking pictures, an x-ray machine would need to be used.

The picture below shows this done for the top layer of a board, which all of the traces on the top layer have been traced out. If I couldn't see a trace, I used a microscope to find out where it went and sometimes a continuity check.

enter image description here

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I learned much about circuit design and layout from reverse engineering PCBs in my youth. Much of this was in the days before micro-controllers became widespread so pin function could be established from the printed catalogue diagrams. Life was a little simpler in that single-sided boards were generally used in audio circuits which I was most interested in. Double-sided makes it quite a bit harder and multi-layer almost impossible.

My general technique was to make a drawing of the components in the same relative positions and orientations as on the PCB. Then holding the board up to the light to start to draw the connections. When complete I would draw the schematic again but with a more conventional layout - positive, GND, negative from top to bottom and general signal flow from left to right. After several iterations I would have a decent schematic.

I have never tried it but I imagine that some of the schematic / PCB software should facilitate this by allowing placement of components in their relative positions, drawing in the nets and then rearranging.

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