How to restore circuit diagram by ກະດານວົງຈອນ?

When you get a product, most of the time, we do not have a circuit diagram, so, we in this case, how to tell the principle of the PCB and the working situation, this is to reverse the actual circuit schematic diagram.
When encountering some small objects, or when there is a need, when encountering electronic products without drawings, it is necessary to draw the circuit schematic diagram according to the objects. Although in the case of a slightly larger scale, it becomes very complicated, but after mastering the following points, I believe we can still do it, for a simpler circuit, no problem.

1. Select large volume, many pins and play a major role in the circuit components such as integrated circuits, transformers, transistors and other drawing reference parts, and then from the selected reference parts of the pin start drawing, can reduce errors.
2. If the PCB board is marked with component serial numbers (such as VD870, R330, C466, etc.), as these serial numbers have specific rules, the components with the same alphanumeric prefix belong to the same functional unit, so they should be used wisely in drawing. Correctly distinguishing components of the same functional unit is the basis of drawing layout.
3. If the serial number of the component is not marked on the printed board, it is better to number the component for the convenience of analyzing and checking the circuit. In order to make the copper foil wiring shortest, the components of the same functional unit are generally arranged in a centralized manner when the manufacturer designs the components of printed board. Once you find the device that is central to a unit, you can trace it to other components of the same unit.
4. Correctly distinguish the ground cable, power cable, and signal cable of the PRINTED board. Take the power supply circuit as an example, the negative end of the rectifier tube connected to the secondary power transformer is the positive pole of the power supply, and the ground wire is generally connected with a large capacity filter capacitor, and the capacitor shell is marked with polarity. Can also find the power line and ground wire from the three-end regulator pin. When wiring printed boards, in order to prevent self-excitation and anti-interference, the factory generally sets the widest copper foil for ground wire (high-frequency circuit often has a large area of ground copper foil), followed by the copper foil for power line and the narrowest copper foil for signal line. In addition, in electronic products with both analog and digital circuits, printed boards often separate their ground wires to form independent grounding networks, which can also be used as the basis for identification and judgment.
5. In order to avoid too many connections of component pins to make the wiring of the circuit diagram cross and intersperse, which leads to the disorder of the drawing, the power supply and ground wire can use a large number of terminal marks and grounding symbols. If there are many components, each unit circuit can be drawn separately and then combined together.
6. You are advised to use transparent tracing paper to draw ground cables, power cables, signal cables, and components by color using a multicolor pen. When modifying, gradually deepen the color to make the drawing intuitive and eye-catching, so as to analyze the circuit.
7. Familiar with the basic composition and classical drawing of some unit circuits, such as rectifier bridge, voltage regulator circuit and operational amplifier, digital integrated circuit, etc. First of all, these unit circuits are drawn directly to form a circuit diagram frame, which can improve the drawing efficiency.
8. When drawing circuit diagrams, we should try our best to find circuit diagrams of similar products for reference, which will get twice the result with half the effort.
The above bold, are important summary, I hope you in the learning object to circuit diagram, can start from these points, to master this technology, because this is the basis of an electronic staff