Unit 2 Electrical Circuit Diagrams

Size: px

Start display at page:

Download "Unit 2 Electrical Circuit Diagrams"

Baldwin Wiggins
5 years ago
Views:

1 2.1. Electrical Circuit Definition Unit 2 Electrical Circuit Diagrams In general terms, a circuit can be described as any group of electrical or electronic devices connected together by conductors. Conductors are most often metallic, and copper wires are used commonly. In the today s electrical circuits, it's more common to find metallic pathways, often called traces, on a board constructed of a mixture of fiberglass and epoxy. An electrical circuit is assembled normally on a printed circuit board, PCB, and the terms of board and card are often interchangeable. In addition to PCBs, wires and cables are used to make connection between parts of a power electrical circuit. To read an electrical or electronic circuit diagram, you need to get familiar with electrical and electronic symbols that are used for drawing a schematic diagram. The symbols are ranged from a simple form of a ground, GND, to a complex module of an IC. Electric circuits, whether simple or complex, are commonly described with minimum words. Saying something like "The anode of diode D1 is connected to upper tap of transformer T1" is a sufficient amount of words to describe a simple link between two devices. Fig. 1. Electronic circuit of an ac dc converter 2.2. How to read an Electrical Diagram A schematic allows a user or a service technician to understand how the circuit is functioning and become familiar with how the expected outputs of the electrical circuit are achieved. Reading an

Well, in the electrical diagram there is no words, instead, they use symbols to say the same thing so we have to understand the language of symbols.

2 electrical diagram can be pretty simple or very complex. The electrical diagrams have a language of their own to be read. Normally, we talk and use words as, for instance, the word of resistor which brings something like this into your mind, an image of the written word. Well, in the electrical diagram there is no words, instead, they use symbols to say the same thing so we have to understand the language of symbols. The language of diagrams always includes symbols, words, numbers, and lines. All this stuff is on there and it's all for the purpose of helping us follow the map. When you're looking at an electrical diagram remember that it is not a street map but a map of electrical circuit where the electrical current trying to find its path from positive to negative. Fig. 2 shows a map of Tehran and I happened to live there in the Jey District near the Ostad Moien St. and I had a friend who lived in the Tarasht Dorm of Sharif University. If we both wanted to meet in Azadi Tower, I should take the dashed line and he would take the straight line so we could meet in the Azadi Sq. Now if this were an electrical map, my path would be the ground side, my friend s path would be the power rail, and the Azadi Sq. would be the load. Fig. 2. Part of Tehran Map Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 12

3 It s always true on the map to find the destination and that's true also on our electrical diagrams the first thing to do is locate the load, a device that consumes the power, like a light bulb, blower motor, or a simple coil. The second thing on a map is to locate your location and to estimate how to reach the destination. This is also the same for an electrical diagram, in which where the power sources and ground are originated and how are fed into the load. The first thing to understanding the wide diagrams is to know what the symbols mean on it, so it's best if you get a page with all the symbols on it so you know what is going on and what about common ones you'll see on a wiring diagram. The symbols are as same as alphabets when you want to read a book. You need to put certain symbols together to translate a part of diagram into an electrical description. Some of the most common electrical schematic symbols have been given in Fig. 3, on next page. Here is the steps to read an electrical diagram. Step 1: Understand the electrical language. There will be a variety of schematic symbols on the schematic that represent real world devices and wires. A basic understanding of these symbols is required to read a schematic. Step 2: Read schematics in the pattern that you would read a text. With rare exceptions, schematics should be read left to right and top to bottom. The signal being generated or used by the circuit will flow in this direction. The user can follow the same path that the signal uses to understand what the signal does or how it is being modified. Step 3: Understand ground. Ground is represented by either a triangle pointing down or a set of parallel lines that become shorter as they appear below each other, in effect representing the inner area of the triangle pointing down. Ground is a common reference point that schematics use to show the overall unity of the various functions of the circuit. Step 4: Learn that a line represents a wire. Wires are used to connect the devices together. All points along the wire are identical and connected. Wires may cross each other on a schematic, but that does not necessarily mean that they connect. If they do not connect, one will be shown looping around the other in a semicircle. If they do connect, they will cross and a dot will be seen at the point where the lines cross. Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 13

4 Fig. 3. Electrical schematic symbols Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 14

5 Step 5: The most confusing devices and their tasks for an electrical engineer are active devices. Determine the diagram tasks performed by the active devices. To determine the tasks, acquire and read the manufacturer's data sheet for each individual device. Step 6: Evaluate what the diagram does. Based on the schematic, decide what parts of the diagram are performing what functions. This will help you determine the performance function of the entire circuit. If the circuit has a block diagram, it would be also better to use the system approach to understand the whole concept of the diagram first. Task 1: Study the Fig. 3 and write a short note on different components. See the example given in below and complete the text in your own words: Relays are switches that open and close circuits electromechanically or electronically. Relays control one electrical circuit by opening and closing contacts in another circuit. In an electrical diagram three different contacts of relays may be used: SPST, SPDT, and DPDT which are stand for. Furthermore, bobbins of different relays are presented differently in the diagram. For instance, bobbin of a thermal relay, that is used as a safety relay in 3-phase electromotor wiring circuits, is shown uniquely. Relays can be found in different,,. Moreover, a relay. Task 2: The difference between a schematic, a block diagram, and a wiring diagram has been given in the following text with some jumbled words bolded in the passage. Write the correct form of each word and then answer to the questions as they followed: A schematic shows connection in a cutciri in a way that is clear and standardized. It is a way of communicating to other engineers exactly what openstocmn are involved in a circuit as well as how they are connected. A good schematic will show component names and uselav, and provide labels for sections or components to help cutemonicam the intended purpose. Note how connections on wires are shown using dots and non-connections are shown without a dot. A block diagram shows a higher level or organizational youtal Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 15

6 of functional units in a circuit or a device, cinemah, or collection of these. It shows data flow or organization between separate units of function. A block diagram gives you an reviewov of the interconnected nature of circuit assemblies or components. A wiring diagram is sometimes helpful to illustrate how a mathsicec can be realized in a prototype or production environment. A proper wiring diagram will be labeled and show connections in a way that prevents confusion about how connections are made. Typically, they are designed for end-users or installers. They focus on connections rather than components. 1. What would a perfect wiring diagram be like? 2. What is an organization layout? 3. What information are provided by a good schematic? 4. How does a schematic communicate with an engineer? Task 3: Study the following block diagrams and describe them in a proper manner. Find the meaning of all abbreviations used in Fig. 5 according to their functions. Fig. 4: Block diagram of a feedback control system Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 16

7 Fig. 5: Block diagram of a Frequency Modulated Continuous Wave (FMCW) 2.3. Electrical Ladder Diagram Electrical ladder drawings are one of the common and reliable tools used to describe an electrical process and to troubleshoot equipment when it fails. As with any good troubleshooting tool, one must be familiar with its basic features to make the most of the diagram in the field. In other words, possessing a basic understanding of how the drawing is laid out as well as the meaning of numbers and symbols found on the schematic will make you that much more seasoned as a maintenance professional. There are typically two distinct parts of a ladder drawing: the power component and the control component. The power portion consists of items such as the motor, motor starter contacts and overloads, disconnect(s), and protective devices (fuses and circuit breakers). The control part encompasses items that make the power components do their work. In the following passage, we'll focus on the control portion of the drawing. Fig. 6 shows the most common components used in the ladder diagrams. Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 17

Fig. 6: A sampling of common symbols can be found on the majority of ladder drawings. Many times, the input devices are said to be either normally open (NO) or normally closed (NC).

8 Fig. 6: A sampling of common symbols can be found on the majority of ladder drawings. Many times, the input devices are said to be either normally open (NO) or normally closed (NC). The normally open or closed status refers to the shelf state of the device. If a device is normally open, a resistance check of the device with a digital multimeter will give a reading of O.L. If the device is normally closed, a resistance check will give a reading of 0.0. The normally open and normally closed state of the devices is not labeled on the ladder drawing. Rather, you must recognize the symbol. A helpful hint in trying to determine whether the contacts are open or closed is to think of them in terms of gravity. If gravity is acting on the device, its normal state is as shown in the drawing. An exception to this concept is found in devices that contain springs. For instance, in the drawing of a normally open pushbutton, it appears that the pushbutton should be falling down and closing. However, there is a spring in the pushbutton that holds the contacts in the open (up) position. The control voltage (AC or DC) for the system may come from a control transformer that is fed from the power portion of the drawing or a different source. For safety reasons, it's important to determine the source of the control voltage prior to working on the system because the power disconnect switch may not turn off the control voltage. Therefore, an electrically safe condition would not be established. The ladder drawing should clearly show the source of the control voltage, but always check for the absence/presence of voltage before making any off-line resistance checks. Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 18

9 The drawing is called a ladder drawing because it resembles a ladder in the way it is constructed and presented on the paper. The two vertical lines (wires) that serve as a boundary for the control system and deliver the control voltage to the devices are called the rails. The rails may have overcurrent devices in them (fuses and/or circuit breakers) and may have contacts from control devices. These control lines may be thicker than the others to help better identify them. Like a real ladder, the rails are the supports for the rungs. If the ladder drawing runs across several pages, the control voltage is carried from one page to the next along the rails. There are a number of ways that this may be represented on the drawing. One way this is done is with the use of continuation arrows at the bottom of the first page, which indicates that the rails continue on to another page. The page number on which the rails continue should be noted. On the page where the rails continue, there are also continuation arrows with the page number notation from which the control voltage lines originated. The rungs of the ladder are made up of wires and input devices that either allow current flow or interrupt current flow to the output devices. These lines may be thin lines when compared to the lines of the rails. From the placement of the input and output devices, you can determine the sequence of events that either energize or de-energize the outputs. The key to good troubleshooting is determining this sequence of events. Input devices are typically placed on the left side of the rungs, while the output devices are placed on the right (Fig. 7). Placement of input devices. The input devices are placed on the rungs in a way that indicates the current flow through the rung when there is a complete path to the outputs. There are several ways in which these input devices can be placed on the rungs, although as stated earlier, they are typically placed on the left side. The STOP input devices are typically normally closed and are placed in series on the rung. This means that they are placed end to end in the drawing. In order for the current to flow through them, they must be in the closed position. Typical STOP input devices include components such as normally closed pushbuttons and mushroom head buttons, limit switches, and contacts for light curtains, photocells, and proximity switches. Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 19

10 The START input devices are typically normally open and are usually placed in series with and after the STOP devices. If there is more than one START device, it is usually placed in parallel with the others. When the STOP and START devices are placed in order on the rung, the flow of current to the output devices can be seen. Understanding this flow is a great aide in troubleshooting. A key question to always ask yourself is: What does it take to energize the output? Fig. 7: A circuit arrangement of switches and output. Here's a simple example to analyze. In Fig. 7, what does it take to energize the output CR1? When either start pushbutton is depressed, the path is complete, and the coil of CR1 is energized. In following the path for the current, you can see the logic of the placement of the input devices. This logic determines the decision making process of the input devices and the path for current as it travels to energize the outputs. There are several logic statements that can be used in placing the input devices in the rungs. The most common of these are the AND, the OR, and the combination AND/OR function. In Fig. 7, all three are present. The end-to-end placement of the stop and start pushbuttons makes an AND logic statement. In other words, both the stop AND the start buttons must be used to energize the coil. The parallel placements of the start buttons make an OR logic statement. Either start pushbutton will complete the path and energize the coil. Placing the stop pushbutton in series with the parallel start pushbuttons supports the AND/OR logic statement. Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 20

11 As noted earlier, the output devices are placed on the right side of the ladder drawing. Unlike input devices, it's important that the output devices only be placed in parallel. If they are placed in series, electrical theory says that the voltage will drop across the resistance of each output. If this happens, they will not operate properly. Outputs include items such as lights, coils, solenoids, and heating elements. In addition to the commonly accepted symbols shown in Fig. 6, letters and numbers also help to identify the output devices. Coils will typically have contacts associated with them. These contacts will change state when the coil is energized. The changing contacts will either complete or open the path for current. When the pushbutton is pressed down, the path is completed, and current will flow to energize the coil. When the coil is energized, the contacts associated with the coil will change state. The red light will go on, and the green light will go off. In the ladder drawing, the contacts associated with the coil can be located using a cross-reference system. The rungs are usually numbered on the left side of the rail. A number on the right side of the rail references the contacts associated with the coil. These numbers are the rungs in which the contacts can be found. A number without a line under it references an open contact. Drawing a line under the number on the right rail references a closed contact. Task 4: Study the following ladder diagrams and write a short on how each diagram works, then translate each function into a logic expression Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 21

12 Word Study Nouns that are formed from verbs can name a person or a device through the suffixes or and er. They can also name the activity taking place, often by taking the suffix tion. Recognizing these noun endings can help with differentiating the two noun types and identifying them from the root verb. See the following table and complete the missing parts. Verb Noun Actor/Device Noun Activity/Concept amplify amplifier amplification attenuate attenuation communicate communication compress compressor conduct conduction/conductivity convert conversion detector detection generate generator identify identifier induct induction interrupt interrupter manipulate mediate modulate operate oscillate receive reflect regulate resist respond simulate subscribe transform transmit Writing Skill: Anatomy of a Scientific Paper Title, author, and contact information: Typically, research articles begin with a title. Next, the authors are identified along with their affiliation (i.e., who they work for, such as a university or Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 22

13 agency). Usually, one author - who can be contacted for further information or permission to use the article - is listed at the bottom of the first page of the research article. Abstract and keywords: This is a summary of the research article followed by keywords (sometimes also called Index Terms). It provides an overview of the research, which is useful to determine if the article is relevant to the reader s work. Abstracts typically follow a standard format. The authors briefly state why the research is important, the methodology used, the results, and a concluding statement based on the findings. Introduction or literature review: In this section, the authors describe the rationale for the study by outlining what research has already been done in this area. The literature review provides the reader with a summary of other research related to the topic. It also addresses questions that remain unanswered or require additional research. In general, this is also the section where the authors research question is introduced, and hypotheses or anticipated results are stated. Methods or methodology or Experimental: In this section, the authors outline how the research was conducted. Results: The authors present the research findings in this section. Any statistical analyses that the authors conducted are described as well. The results are often displayed using tables, charts, or figures along with a written explanation. Discussion: In this section, the authors interpret the results. The authors may provide possible explanations for what they found, including an interpretation of unexpected results. Conclusion and summary: In this section, the authors summarize what they found and link it back to the current literature in the area. Often, any limitations of the study are described in this section. For example, if the researchers used a convenience sample to recruit participants, the results may not apply to people that are different in some way from the study participants. The authors may also suggest directions for future research in this section. Acknowledgment, References or sources: The references section lists the publications that the authors cited in the article. The references may help the reader judge the quality of the article and can be used to learn more about the topic area. Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 23

14 Biography of Author(s) or Bio: Bio of authors is the part that explains the academic progress and achievements of the authors, their interesting research area, etc. Task 5: Browse the internet and find a related article to your field of study. Highlight the different parts of the article using different watermark colors. Faculty of Electrical &Computer Engineering, Hakim Sabzevari University Page 24

Take for instance this circuit:

Take for instance this circuit: Ladder diagrams Ladder diagrams are specialized schematics commonly used to document industrial control logic systems. They are called "ladder" diagrams because they resemble a ladder, with two vertical