How to restore the circuit diagram according to the circuit board

When we get a product, we often don’t have a circuit diagram. So, in this case, how can we clearly explain the principle and working conditions of the circuit board? This is to restore the physical object to the circuit schematic diagram.

When encountering some small physical 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 physical object. Although it becomes very complicated in the case of a slightly larger scale, I believe we can still do it after mastering the following points. For simpler circuits, it is not a problem.

1.Select components with large volume, many pins and major functions in the circuit, such as integrated circuits, transformers, transistors, etc. as drawing reference parts, and then start drawing from the pins of the selected reference parts to reduce errors.

2.If the printed circuit board is marked with component numbers (such as VD870, R330, C466, etc.), since these numbers have specific rules, components with the same first Arabic numeral after the English letter belong to the same functional unit, so they should be used cleverly when drawing. Correctly distinguishing the components of the same functional unit is the basis for drawing layout.

3.If the serial number of the components is not marked on the printed board, it is best to number the components yourself for the convenience of analyzing and proofreading the circuit.

When designing the printed board to arrange the components, the manufacturer generally arranges the components of the same functional unit relatively concentratedly to make the copper foil routing as short as possible. After finding the device that plays a core role in a unit, you can find other components of the same functional unit by following the clues.

4.Correctly distinguish the ground wire, power line and signal line of the printed board. Taking the power supply circuit as an example, the negative end of the rectifier connected to the secondary of the power transformer is the positive pole of the power supply, and a large-capacity filter capacitor is generally connected between the ground wire and the ground wire, and the capacitor shell has a polarity mark. The power line and ground wire can also be found from the pins of the three-terminal regulator.

• When the factory is wiring the printed board, in order to prevent self-excitation and anti-interference, the ground wire copper foil is generally set to the widest (high-frequency circuits often have large-area grounding copper foil), followed by the power line copper foil, and the signal line copper foil is the narrowest.

• In addition, in electronic products with both analog and digital circuits, the ground wires on the printed circuit board are often separated to form an independent grounding grid, which can also be used as a basis for identification and judgment.

5.In order to avoid excessive connection of component pins, which makes the wiring of the circuit diagram cross-interlaced and causes the drawing to be messy, the power supply and ground wires can use a large number of terminal markings and grounding symbols. If there are many components, each unit circuit can be drawn separately and then combined together.

6.When drawing a sketch, it is recommended to use transparent tracing paper and use multi-color pens to draw the ground wire, power line, signal line, components, etc. by color classification. When modifying, gradually deepen the color to make the drawing intuitive and eye-catching, so as to analyze the circuit.

7.Master the basic composition forms and classic drawing methods of some unit circuits, such as rectifier bridges, voltage regulator circuits and op amps, digital integrated circuits, etc. Draw these unit circuits directly first to form the framework of the circuit diagram, which can improve the drawing efficiency.

8.When drawing a circuit diagram, you should try to find circuit diagrams of similar products for reference, which will help you get twice the result with half the effort.