Wiktor Wise
Learning how to read electrical schematic drawings is a valuable skill for maintenance workers and managers, even if they are not licensed electricians. Electrical schematic drawings are used to identify faulty components, troubleshoot systems, and improve safety. To read an electrical schematic drawing, one must understand the different symbols used to represent system components. These symbols are often standard across the industry but may vary across companies. Common symbols include resistors, usually portrayed as zigzag lines with a terminal at each end, and capacitors, which are represented by two lines perpendicular to the terminal plates. Understanding the basics of electrical wiring and how to follow signals is also essential for reading electrical schematic drawings effectively.
| Characteristics | Values |
|---|---|
| Importance | Understanding electrical schematic drawings is an important skill for maintenance workers and managers, even if they are not licensed electricians. It helps identify faulty components, troubleshoot systems, and improve safety. |
| Symbols | Symbols are used to represent system components. Common symbols include resistors, variable resistors, potentiometers, non-polarized capacitors, and polarized capacitors. |
| Symbol Values | Each symbol is assigned a name and value. The value is expressed in ohms, farads, oscillating frequency, henries, or the name of the part chip. |
| Naming Conventions | Names are usually a combination of a letter and a number. The letter indicates the component type, and the number indicates multiple components of the same type. For example, three capacitors would be labeled C1, C2, and C3. |
| Voltage Sources | Constant voltage sources are commonly used in electronics, with symbols indicating direct current (DC) or alternating current (AC). |
| Batteries | Batteries are represented by a pair of disproportionate parallel lines, with the longer line indicating the positive terminal and the shorter line the negative terminal. |
| Node Voltages | Special symbols can be assigned to node voltages, which are tied directly to specific voltage values. Positive voltage nodes are indicated by an up-pointing arrow, while ground nodes use flat lines, downward-pointing arrows, or triangles. |
| Signal Direction | Schematics are typically read left-to-right, with signals following the same direction. However, this rule can be restrictive and may not always apply, especially in complex designs. |
| Reference Designators | Using reference designators, such as "R3" instead of "the third 10k resistor from the left," simplifies discussions about the circuit. |
| Conventions | Different companies may use slightly different symbols, naming conventions, and design structures, so context clues and familiarity with the specific schematic are important. |
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What You'll Learn
Understanding symbols and their meanings
Resistors
Resistors are usually depicted by a series of zig-zag lines with two terminals extending outward. However, international symbols may use a featureless rectangle instead of the squiggles.
Variable Resistors
Variable resistors are represented by adding a diagonal arrow across the standard resistor symbol. Since it remains a two-terminal device, the arrow is simply placed diagonally across the middle.
Potentiometers
Potentiometers are similar to variable resistors but are three-terminal devices. Hence, the arrow becomes the third terminal, also known as the wiper.
Capacitors
There are two commonly used capacitor symbols. One symbol represents a polarized capacitor, usually with a curved plate, indicating the cathode, which should be at a lower voltage than the anode pin. The other symbol is for non-polarized capacitors, with two perpendicular lines extending from the terminal plates.
Voltage Sources
When working with electronics, you'll commonly encounter constant voltage sources. Batteries, for example, are represented by a pair of parallel lines, with the longer line indicating the positive terminal and the shorter line the negative terminal. Positive voltage nodes are often indicated by an upward-pointing arrow, while ground nodes are shown as one to three flat lines or a downward-pointing arrow or triangle.
It is important to consult a symbol key or a cheat sheet to understand the various symbols and their assigned names and values. This knowledge is essential for maintenance workers and managers, even if they are not licensed electricians, as it helps in identifying faulty components, troubleshooting, and improving safety.
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Recognising resistors, capacitors and other components
Recognising the various components in an electrical schematic drawing is a crucial skill for anyone working with electronics. While the graphical symbols used to represent electrical components are often universally accepted, there are some variations to be aware of. For instance, the IEC (International Electrotechnical Commission) and IEEE (Institute of Electrical and Electronics Engineers) use different symbols for the same component.
Resistors
The standard symbol for a resistor in the United States is a zig-zag line, while the international standard is a rectangle. Resistors restrict the flow of current in a circuit. Variable resistors, such as photoresistors and thermistors, increase or decrease resistance according to external input. The symbol for a variable resistor is similar to that of a fixed resistor, with the addition of an arrow across the middle.
Capacitors
Capacitors are used to control rapidly changing signals and are usually represented by the letter 'C'. They can also be identified by the number of terminals they have: capacitors added to a crystal have two terminals, while resonators with capacitors have three. Polarized capacitors have a positive and negative side, indicated by a plus sign on the positive pin. Non-polarized capacitors have no polarity, so their orientation doesn't matter.
Other Components
Other common components in electrical schematic drawings include integrated circuits, represented by the letter 'U', and inductors, represented by the letter 'L'. Crystals may list their oscillating frequency as their value. Diodes, which allow current flow in only one direction, are polarized devices with positive and negative leads (anode and cathode). The flat edge of the triangle is positive, while the straight line is negative. Photodiodes absorb light and generate an electrical current.
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Reading signals
Reading a schematic drawing involves understanding the different symbols used to represent system components. Resistors, for example, are usually portrayed as zigzag lines with a terminal at each end, while international symbols may use a blank rectangle instead. Variable resistors are represented by a diagonal arrow intersecting the standard resistor zigzag symbol. Capacitors also have two terminals, but they run perpendicularly into plates. One symbol has a curved plate, indicating a polarized capacitor, usually the cathode, which should be at a lower voltage than the anode pin. Batteries are often shown as a pair of parallel lines, with the longer line representing the positive terminal.
When following signals on a schematic, it is generally recommended to go from left to right. This is a standard practice for all professional schematics and many amateur ones. However, there may be times when signals need to go from right to left, especially in bidirectional or power scenarios, or when dealing with bus signals like TX and RX. In such cases, it is good practice to set these signals apart to make them easily identifiable.
To further aid in reading signals, it is helpful to use reference designators. Instead of saying "the third 10k resistor from the left", using a designator like "R3" makes communication much simpler. Additionally, understanding the design structure and relationships between different components and their signals is crucial. Power and ground are also signals with their own origins and destinations, and they should be considered in the overall schematic interpretation.
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Using a cheat sheet or key
A cheat sheet will help you identify the different symbols used to represent system components. For example, resistors are usually portrayed as zigzag lines with a terminal at each end, while international symbols may represent resistors as a blank rectangle. Variable resistors are shown with a diagonal arrow intersecting the standard resistor zigzag symbol. You may also see symbols representing different types of voltage sources, such as batteries, which are usually shown as a pair of parallel lines. The longer line typically represents the positive terminal, while the shorter line is the negative terminal.
Some other common symbols you might come across include potentiometers, capacitors, and ground nodes. Potentiometers are represented by an arrow pointing to the zigzag resistor at a right angle, indicating the third terminal. There are two types of capacitor symbols: one for polarized capacitors (usually with a curved plate) and another for non-polarized capacitors, both with two terminals running perpendicular to the terminal plates. Ground nodes are often drawn as one to three flat lines or a downward-pointing triangle, and positive voltages appear as an arrow pointing up.
It's important to note that different companies may use slightly different symbols, naming conventions, and design structures, so context clues are essential. Once you understand the basics of electrical wiring, you can use a cheat sheet to identify the symbols and their meanings, and with experience, you'll be able to interpret schematic drawings with ease.
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Knowing wire numbers and naming conventions
When reading electrical schematic drawings, it is important to understand the different symbols used to represent system components. Each symbol is assigned a name and value, with the value expressed in ohms, farads, oscillating frequency, henries, or the name of the part chip. The name is usually a combination of a letter and a number, with the letter indicating the component type and the number indicating how many components of the same type are on the schematic. For example, three capacitors on a schematic would be labelled C1, C2, and C3.
Wires are represented by lines and are numbered to indicate their relative position on the page. If a wire spans multiple pages, it will include numeric designators for the page and line number where the other end of the wire can be found. These numbers are called "wire tags" and are a combination of letters and numbers that indicate which device or terminal block a wire or cable should be connected to.
In addition to wire tags, wires may also be identified by their colour. For example, black wires typically represent ground or negative connections, while white wires represent positive connections. Red wires often indicate a switch leg or secondary live wire. However, it is important to refer to the schematic or wiring diagram to confirm the specific wire colours and their meanings for a particular application.
Some electrical components, such as relays, may have different symbols or representations in schematic diagrams compared to electrical diagrams. For example, a relay in a schematic diagram may be represented by a coiled line, while in an electrical diagram, the coil may be represented by a circle denoted with R or CR and a number.
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Frequently asked questions
Understanding electrical schematic drawings is a valuable skill for maintenance workers and managers, even if they are not licensed electricians. It helps to identify faulty components, troubleshoot systems, and improve safety. The first step is to understand the different symbols used to represent system components. Resistors, for example, are usually depicted as zigzag lines with terminals at each end, while capacitors and batteries also have their own symbols.
Each symbol is assigned a name and a value. The value is the most important aspect, expressed in ohms, farads, oscillating frequency, or henries. The name is usually a combination of a letter and a number, with the letter indicating the component type, and the number indicating multiple components of the same type. For example, three capacitors would be labelled C1, C2, and C3.
Schematics are generally read left-to-right, and signals are followed in the same direction. However, this is not always the case, and some schematics may have signals going from right to left.
