F R1 C1 C2 C3 10k 1000u 2200u 2200u 10k 1000u 1000u 4700u/n Overview: In this experiment, you will investigate several circuits related to switching. Your previous experiment (Exp Q) focused

on introducing the laboratory equipment and some key skills in building and testing circuits. This circuit is more focused on understanding one particular type of circuit – a basic switching circuit that is widely used in electrical devices. The experiment has three main parts: two experimental activities to be done in the lab and one analysis activity to be done at home. You should make sure that you have some results for both experimental activities before you leave the laboratory. You will carry out the laboratory activities individually. Staff and demonstrator advice is available for times when you need help. Experiment specifications: Module(s) Component Department of Electrical Engineering and Electronics ELEC171, ELEC172 Experiment S: Switching Coursework weight Lab location Experiment S Work Timetabled time Assessment method ELEC171/172 Experiment S: Switching ELEC171: 10%; ELEC172: 20% 3rd floor Electronics Laboratory Individual work 9 hours (lab time) + 6 hours (report writing) laboratory notebook (25%) + laboratory report (75%) Experiment S Schedule: Two lab days are allocated for this experiment. You will investigate several different circuits, following the instructions in Parts 2, 3 and 4. Part 5 is an analysis activity, which you should complete in the week between the first lab day and the second lab day. You will also have the opportunity to carry out a soldering practice activity, described in a separate instruction document that you can view and download from the module site on Canvas. You will carry out the laboratory activities individually. Staff and demonstrator advice is available for times when you need help. You should work at your own pace, and not worry if others in the laboratory seem to be ahead or behind you. There is plenty of time to complete the work. Assessment of your laboratory notebook and your report: Lab notebook (25%): You should submit your laboratory notebook when you finish the experiment. The deadline is 11:59 on the second day of Experiment S. The marking scheme for the lab notebook is the same as that used for Experiment Q. You can find the marking scheme on the Canvas site for this module, in the Experiment S section. Report (75%) You are required to submit a report about 10 days after the second day of Experiment S. The deadline is Friday December 8th, at 11:59 pm. Detailed guidance on the report content, structure, style and marking, is given in the following document: Experiment S report specification and marking guide General guidance on writing experimental reports and presenting your data clearly can be found in these documents: ● Yr1 Guides: Writing Reports Yr1 Guides: Plotting graphs using Excel, graphs with log axes 2 Experiment S Part 1: Introduction to electronic switches 1.1 Switches and electrical circuits Switches are basic components of many electrical and electronic circuits. The most obvious example might be the on/off switches on electrical appliances, but there are many more cases where switches play an important role. This might be a macro-scale switch that automatically turns on a fan in your PC when the hard-disk gets hot, or it could be a micro-scale switch that turns transistors on and off in a memory chip. Some switches are simple mechanical ones represented in circuits by the symbol shown in Figure 1.1. Other switches are formed by mini-circuits of electronic components that behave in a particular, desirable way. In this experiment, you will build and test two circuits. Using the first circuit, you will investigate one of the most important properties of switches: the time taken for them to open and close. Using the second circuit, you will see how the switching time influences the operation of a real switching circuit. S1 Figure 1.1. Symbol for a mechanical 1.2: Time needed for a switch to open or close: the time constant The time taken for a circuit property to change from one state to another is characterised by a quantity called a time constant. For a switch, this means the time taken for the switch to open or close. In theory, switches can open and close instantaneously: the switch is closed at one instant and then completely open. In a real circuit, this process always takes a finite amount of time. For some circuits, the time might be extremely short. In many microelectronic circuits, such as those in memory or processor chips, the time might be a few nanoseconds or even picoseconds. For some circuits, like the ones used in power distribution on the electricity grid, the time might be many seconds. The time it takes depends on the type of switch and the components in the circuits. One important point is that the time taken for a circuit to open or close is usually not a fixed time, because the circuit might change performance even when the switch is not 100% open or closed. One comparison of this could be the quality of vision in a room where the only light comes through an open door. A room might be so dark that you can't see anything, even if the door is not 100% closed and a small amount of light is present. And the room might have enough light coming through the door for normal vision, even if the door is not 100% open. A similar situation can arise in circuits, and we might consider a circuit to have started working even if a switch is only partially open, and the same circuit to have stopped working, even if the switch is not completely closed. 3 This process is shown in Figure 2.2, which shows the voltage measured at one particular point in a circuit. In parts (a) and (c) you can see two examples of the voltage changing: in once case changing from 0 V to 10 V, and in the other case from 10 V to 0 V. If we view the voltage change over the long time scales of the graphs in parts (a) and (c), then it might seem that the change happened almost instantaneously. However, if we view the change with more resolution, as shown in parts (b) and (d), we can see that a finite amount of time is taken for the change to occur. This behaviour is normal for all types of switches. Even mechanical switches take a finite amount of time to change from open to closed, with the switching time being determined by the amount of time needed for the two parts of the switch to form a good electrical contact. In electronics, we represent the time taken for the change to occur by a quantity called a time constant, which is represented by the Greek letter t. This can be defined in different ways, but one common definition is the time taken for the voltage to change by about 63% of the total change. (There is a reason for the value of 63% - you will learn about it in this experiment.) Figure 2.3 shows the same voltage changes in a much expanded scale. The change of ~63% means that t is given by the time required for the voltage to change from 0 V to about 6.3 V, for the change shown in Figure 2.3(a), and the time required for the voltage to change from 10 V to about 3.7 V for the change shown in Figure 2.3(b). voltage /V voltage /V 12 10 voltage /V 8 6 4 2 0 12 10 12 10 8 6 4 2 8 0 4 2 O 0 20 T (a) 20 40 time /s 40 time /s 60 1 1.2 1.4 1.6 1.8 Experiment S 60 =▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬C ▪▬▬▬▬ JOCO▬▬▬▬▬▬▬▬▬▬ _____________________________________ 80 80 voltage /V 2 2.2 2.4 2.6 2.8 3 time /s voltage /V 12 10 voltage /V + 2 0 12 10 8 t 2 0 (c) (d) Figure 2.2: Sudden voltage changes shown over different timescales. In (a) and (b) the voltage is changing from 0 V to 10 V. The reverse process is shown in (c) and (d), with the voltage changing from 10 V to 0 V. 12 10 8 6 4 2 0 0 1 2 T (b) 2 4 1.2 1.4 1.6 4 6 time /s time /s 6 8 8 10 10 ¯¯¯¯¯¯¯¯¯¯¯___ 1.8 2 2.2 2.4 2.6 2.8 time /s FOLL 3 (a) (b) Figure 2.3: Expanded view of the voltage changes shown in Figure 2.2. Using the definition of time constant to be the time for the voltage change to be 63% complete, the time constant in this case is about 0.3 s. Part 2: Capacitor Charging and RC Time Constants 2.1 Background A capacitor consists of two conductive electrodes, or plates, separated by a dielectric material which prevents charge from moving directly between the plates. The simplest practical capacitor consists of two wide, flat, parallel plates separated by a thin dielectric layer, which could even be just air. For electronic components, capacitors are usually formed of two plate electrodes separated by a dielectric material such as plastic, paper, mica or a liquid gel. Experiment S Capacitors are said to be charged when electric charge is added to one of the plates so that it is completely full and cannot accept any more. If the capacitor is attached to an external source of charge, such as a battery or voltage source, then charge can flow from the battery or voltage source to the plates of the capacitor. Over time, charge builds up on the plates and when this charge is equal to the power supply, no further charge can be added to the plate and the capacitor is said to be saturated. When the external charging connection is removed, the charge on the plates persists. Although charge cannot move directly from one plate of the capacitor to the other plate through the insulating material, charge can move from one plate to the other through an external circuit. Over time, the charge will flow out from the capacitor and the charge on the plates will decay to zero. The aim of this experiment to build a circuit that charges capacitors from zero to their maximum capacity, and to measure the circuit properties while the capacitors charge. This will give you an idea of the time required for this process. Figure 2.4 shows the circuit that you will use for this test. The time required for capacitors to charge and discharge is often one of the key factors that determines the time constant of a switch. Hol V1 10V O R1 www HH C1 HH 5 C2 31 Figure 2.4a: The capacitor charging circuit that will be built to investigate time constants C3 2.2 Setting up the circuit The test circuit uses three capacitors, and you will test two circuits in this experiment. To find the values for your group, check the Experiment S section of the Canvas home page: First, find the file called Experiment S Group List, and find which group you were assigned.