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Topic 1.7 – Switching Circuits. Learning Objectives: At the end of this topic you will be able to; 1.7.1 – Introduction. Understand that switching circuits are often used: o as an interface between analogue and digital sub-systems; o as a transducer driver to drive output devices; 1.7.2 – NPN Transistor. Draw a circuit diagram to show how npn transistors can be used as part of a switching circuit; Identify the base, collector and emitter leads on a transistor; State that a small base current can be used to control a much larger load current; Apply the following rules to a given transistor switching circuit: o For VIN < 0.7V, the transistor is off o For VIN > 0.7V, the transistor is on; 1.7.3 – N Channel MOSFET. Draw a circuit diagram to show how a MOSFET can be used as part of a switching circuit. Identify the gate, source and drain on a MOSFET; State that a gate voltage can be used to control a large load current; Explain why a MOSFET should be used to drive heavy loads due to the limited output capability of logic systems; 1.7.4 - Thyristor Identify the gate, anode and cathode on a thyristor; State that a small gate voltage can be used to latch a large load current; Draw a circuit diagram to show how a thyristor can be used as a latching transducer driver incorporating a reset switch. 1.7.5 – Voltage Comparators. Know that comparators have a greater sensitivity than transistor switches; Use data sheets to identify pin connections on a dedicated comparator IC; Predict the output voltage given the input voltages in a comparator circuit; Appreciate the current driving limitations of comparators; Design comparator circuits which cause output devices to respond to information from sensors; Design circuits to increase the output capabilities of a comparator circuit by the addition of a transistor switch. 1 GCSE Electronics. Unit E1 : Discovering Electronics Switching Circuits 1.7.1 Introduction In our previous topics we have mentioned the need for an output switching device, or transducer driver to interface a low power electronic circuit to output devices that often require a larger current than the electronic circuit can provide. This is the most common use for a switching circuit. However there are cases where switching circuits are required to act as an interface between analogue sensors and digital sub-systems. In this topic we will consider four different switching devices that have very different properties. It will be your task in an examination to select the most appropriate switching circuit from the ones discussed here, and in some cases you will have to give reasons for your choice so this is a very important topic in your understanding of electronic systems. 1.7.2 NPN Transistor The first switching device we will look at is the transistor. Transistors have three connecting leads. They are called the emitter (e), base (b), and collector (c). The following diagram shows the symbol used for the NPN transistor which is the only type we will look at in this course. 2 Topic 1.7 – Switching Circuits. APPEARANCE All transistors have three leads but their appearance can vary, depending upon their particular application. We shall now consider some types of cases used and how we can identify the emitter, base and collector leads. Case: E-line This type of package encapsulates the transistor in a plastic case. The ZTX300 is a popular choice in this series. It is a general purpose transistor which can be used for fast switching or as a voltage amplifier. The following diagrams illustrate the shape of the case and how to identify the leads. The ZTX300 can be used for switching devices which take current up to 100mA. It can withstand a voltage of 25V across its collector-emitter and can dissipate a power of 300mW. Case: T018 and T039 In such packages, the transistor is enclosed in a metal can. The small tag on the case serves as a guide for pin (lead) identification. The collector is usually connected to the metal case. 3 GCSE Electronics. Unit E1 : Discovering Electronics Another popular switching transistor, the BFY51, is enclosed in a type T039 can, as shown below. The larger can suggests that this transistor can switch higher currents and dissipate more power than the BC108. Check this out in a component catalogue. Power dissipation can be increased by fitting a heat sink onto the case. Look up heat sinks in the catalogue. Case: T03 This type of case is used for high power applications. Such transistors, mounted on a suitable heat sink, can switch very high currents and dissipate power in excess of 100W. 4 Topic 1.7 – Switching Circuits. WARNING: Transistors can be easily damaged if connected the wrong way in an electrical circuit. The above package descriptions are only a few of the many types available. Do not take these pin identifications for granted. Always look up your transistor type in the supplier’s catalogue. It will tell you the case style used and there will be a page showing the pinout for all of the different packages as shown below. Common transistor pinouts Pins NPN Pins NPN BC147 BC148 BC149 BC107 BC108 BC109 BC171 BC172 BC173 BC182 BC183 BC184 BC167 BC168 BC169 BC237 BC238 BC239 BC207 BC208 BC209 BC437 BC438 BC439 BC413 BC414 BC547 BC548 BC549 BC582 BC583 BC584 BC467 BC468 BC469 2N3903 2N3904 TIP3055 9013 9014 MJE 3055T BD267A TIP31A TIP41A BD131 BD139 BD263 2N3054 2N3055 5 GCSE Electronics. Unit E1 : Discovering Electronics Transistor Switching Action The arrow on the emitter shows the direction that conventional current can easily flow through the transistor. In our switching circuits the emitter is connected to the zero volt line. There are two routes that current can take through the transistor i.e from collector to emitter and from base to emitter. Current can only flow in the collector circuit if a small current flows in the base circuit. A small base current is used to control a much larger current in the collector circuit. The simplest circuit that we can set up with a transistor is shown below: +6V Lamp Flying Lead Collector Base Emitter 0V 6 Topic 1.7 – Switching Circuits. There are a couple of things to note about the way in which the transistor is connected, (i) the emitter terminal is connected directly to the 0V line, (ii) a resistor has been added to the base terminal, this is to limit the current flowing in the base circuit as only a small current is needed to switch the transistor on, (iii) the load (a lamp in this case) is connected into the collector circuit. The flying lead shown in the circuit diagram can now either be connected to 0V or to +6V to demonstrate the switching action of the transistor. Case 1 : Flying lead connected to 0V. +6V Lamp Collector Base Flying Lead VIN Emitter 0V In this case with the flying lead connected to 0V, there is no difference in voltage between the base and the emitter terminal, VIN = 0V, therefore no current flows, the transistor is switched off, and the lamp does not light. 7 GCSE Electronics. Unit E1 : Discovering Electronics Case 2: Flying lead connected to +6V. +6V Flying Lead Lamp Collector Base VIN Emitter 0V In this case with the flying lead connected to the +6V line, there is a voltage difference between the base and emitter terminals which causes current to flow from the base to the emitter, VIN = 6V. This switches the transistor on which allows a larger current to flow through the collector and emitter, and the lamp lights. Note: both base and collector currents come out of the emitter. This simple circuit provides a very good demonstration of the switching action of the transistor. The flying lead can be connected to the output of a digital processing system, e.g. a logic gates, and depending on whether the output of the digital system is at logic 0 or logic 1, this will determine the state of the lamp. There are two basic rules we have to remember about the transistor. i. if the voltage between the base and emitter, usually referred to as VIN < 0.7V then the transistor will be off; ii. if the voltage between the base and emitter, usually referred to as VIN > 0.7V then the transistor will be on. 8 Topic 1.7 – Switching Circuits. Practical Applications for Transistor Switching Circuits. (a) LIGHT ACTIVATED SWITCH (i) Lamp on in darkness. In bright light the resistance of the LDR (R2) will be low. Most of the current flowing down the voltage dividing chain flows through the LDR to the 0V line. Point A is near to 0V (VIN < 0.7V) and the transistor will be off as shown below. 9 GCSE Electronics. Unit E1 : Discovering Electronics In darkness the resistance of the LDR will be very high. The voltage at point A is now high (VIN > 0.7V), current flowing through the resistor R takes the easier path to 0V i.e through the base-emitter junction. The transistor is now turned on and the lamp lights up. (ii) 10 Lamp on in bright light. Topic 1.7 – Switching Circuits. In darkness the resistance of the LDR will be very high. The voltage at point A will be near to 0V, too low to turn the transistor on (VIN < 0.7V), so the lamp will be off, as shown below. In bright light the resistance of the LDR decreases. The voltage at point A increases and the transistor turns on (VIN > 0.7V), also turning on the lamp. In both cases; replacing R1 with a variable resistor would provide means of adjusting the sensitivity of the systems. 11 GCSE Electronics. Unit E1 : Discovering Electronics (b) TEMPERATURE ACTIVATED SWITCH (i) Low temperature indicator. At low temperature the resistance of the thermistor is high. Point A will be higher than 0.7 volt and current flows through the base circuit. The transistor is switched on and the indicator lights up, as shown opposite. At high temperature the resistance of the thermistor is low. Point A will be near to 0V (lower than 0.7 volt) and the transistor is off. The indicator will be off as shown opposite. 12 Topic 1.7 – Switching Circuits. (ii) High temperature indicator. At low temperature the resistance of the thermistor is high. Point A will be near to 0V (<0.7 volt). The transistor is off and the indicator will be off as shown opposite. At high temperature the resistance of the thermistor is low. Point A will be above 0.7 volt and the transistor is on. The indicator is on as shown opposite. 13 GCSE Electronics. Unit E1 : Discovering Electronics INVESTIGATING TRANSISTOR CIRCUITS. Activity 1: 1a. Set up the following circuit. The lamps are 6V,0.06A type. 1b. By connecting point A to the 0V line then the +6V line determine which condition switches the transistor on. Report on your findings and explain why this is so. ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ How do you know that a small base current is controlling a much larger current in the collector circuit ? ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ 14 Topic 1.7 – Switching Circuits. Activity 2: We shall now use some of the input signal sensors, covered in Topic 1.5 to switch the transistor. 2a. Set up the following arrangement. 2b. Cover the window (or move the slider if using a simulator) of the light dependent resistor and note what happens. Explain why the system behaves in this way. ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ 2c. Replace the 1kΩ resistor with a 10kΩ variable resistor. Experiment with the setting of the variable resistor and comment on your findings. ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ 15 GCSE Electronics. Unit E1 : Discovering Electronics 2d. Draw a circuit diagram showing a suitable arrangement for switching on the lamp when bright light falls on the LDR. 2e. Set up the system and try it out. DESIGN PROBLEM 1: Design a system, using a transistor switch that switches on an electric motor when the temperature is near 36°C (body temperature). Circuit Diagram: 16 Topic 1.7 – Switching Circuits. After designing the system, set it up and try it out. Give a full report on the performance of your system. ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ 17 GCSE Electronics. Unit E1 : Discovering Electronics Student Homework 1: Answer all questions in the spaces provided, continue on a separate piece of paper if required. 1. i) ii) What type of transistor is shown above ? ..................................... [1] The three leads of this device have special names, what are they ? ..................................... iii) 2. ..................................... On the diagram above, label the leads with their correct name. Study the following two circuits carefully. Circuit A 18 ..................................... Circuit B [3] [3] Topic 1.7 – Switching Circuits. Explain why the lamp in Circuit A does not light, but the one in Circuit B does. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ 3. ........................................................................................................................................ [3] Transistor circuits are sometimes referred to as switching circuits Why is this ? ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ 4. ........................................................................................................................................ [2] i) Draw a circuit which will switch on a light when darkness falls. [5] 19 GCSE Electronics. Unit E1 : Discovering Electronics ii) Describe how the circuit works. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ 5. ........................................................................................................................................ [3] Study the following circuit carefully. (Buzzer) i) What does the circuit do. ........................................................................................................................................ ........................................................................................................................................ [2] ii) Explain how the circuit works. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [3] 20 Topic 1.7 – Switching Circuits. 1.7.3 N-Channel MOSFET In this section we are going to investigate the operation of a different type of transistor which is called a MOSFET. This stands for Metal Oxide Semiconductor Field Effect Transistor, which is a bit of a mouthful, so we will simply refer to it as a MOSFET. There are many different types of MOSFET, available but we will be concentrating only on one type in this course, which is the n-channel enhancement MOSFET. You will not be asked about any other version in the examination. The symbol, and picture for an n-channel enhancement mode MOSFET is shown below. D, Drain G, Gate S, Source The leads for this type of transistor are labelled as Gate (G), Drain (D) and Source (S). The Enhancement-mode MOSFET has the property of being normally "OFF" when the gate bias voltage is equal to zero. A drain current will only flow when a gate voltage (VGS) is applied to the gate terminal. This voltage needs to be a positive voltage typically between 3V and 9V. The actual value of VGS depends on the type of MOSFET and the load current flowing through the MOSFET. The exact calculation of this is beyond the scope of this introductory course. 21 GCSE Electronics. Unit E1 : Discovering Electronics Enhancement-mode MOSFET's make excellent electronic switches due to their low "ON" resistance and extremely high "OFF" resistance and extremely high gate resistance. Enhancement-mode MOSFET's are used extensively to interface integrated circuit outputs to power high current output devices because they can handle large currents and can be driven directly by digital logic levels. So let us look at how the MOSFET is used in a circuit. Example : The following circuit shows a MOSFET being used to switch on a high powered lamp from a light sensing circuit. 12V 12V 48W Gate Voltage 0V Notice that there is no need for a resistor, to connect the sensor to the gate terminal, because virtually no current flows into the device due to its very high resistance. The gate voltage rises as it gets dark, and this is sensed by the MOSFET which switches on when it reaches the appropriate level. 22 Topic 1.7 – Switching Circuits. As far as the examination questions are concerned, these will be limited to: i. identifying the symbol for a MOSFET, ii. identifying the names of the three leads of the MOSFET, iii. completing a diagram to show how a MOSFET can be connected to a logic system to drive a high powered load, iv. explaining that the MOSFET is able to drive high current loads directly from logic gate because only a positive voltage is necessary at the gate terminal, no current is necessary, making it an ideal interface for logic gates that have a very low output current capability. Summary of NPN Transistor and MOSFET characteristics. Transistors are "Current Operated Devices" where a much smaller Base current causes a larger Collector to Emitter current. A transistor can also be used as an electronic switch to control devices such as lamps, motors and solenoids etc. The NPN transistor requires the Base to be more positive than the Emitter. MOSFET's are "Voltage Operated Devices" MOSFET's have very high input resistances so very little or no current (MOSFET types) flows into the input terminal making them ideal for use as electronic switches. The high input impedance makes the design of the sensing sub-system easier, since we do not have to worry about current being drawn from the sensing sub-system. The high input impedance of the MOSFET means that static electricity can easily damage MOSFET devices so care needs to be taken when handling them. 23 GCSE Electronics. Unit E1 : Discovering Electronics 1.7.4 Thyristors In the previous two sections we have discussed two types of output switching device. One was a current controlled device, i.e. the NPN transistor, and the other was a voltage controlled device, i.e. the MOSFET. Both of these relied on the continuous provision of a base current or gate voltage to keep the output on. The next device we are going to look at which is also a voltage controlled device is the thyristor. This device has the unique property that it is self latching, i.e. once triggered it remains on until it is reset. The circuit symbols with for and appearance of a thyristor is or The symbol on the left will be found in the ‘Livewire’ or ‘Circuit Wizard’ circuit simulation packages which you may use to simulate the operation of these devices. The symbol on the right is a more traditional symbol for a thyristor which you might find in text books, and other printed diagrams. For the purposes of the examination questions the symbol on the right will be used. 24 Topic 1.7 – Switching Circuits. The operation of the thyristor can be illustrated as follows. The voltage at the Gate of the thyristor is dependent on the amount of light falling on the LDR. The thyristor will switch on if the anode is more positive than the cathode, and a positive voltage > 1V is applied to the gate. The resistor R is to limit the current flowing in to the gate of the thyristor just like we used a resistor to limit the current flowing into the base of the NPN transistor. We will now consider a practical use for a Thyristor which is in a burglar alarm. One of the key properties of a burglar alarm is once it has been triggered it should remain on until it is reset. Many burglar alarms use a light sensor and a an infra red light beam which is invisible to the human eye. This is usually arranged around a doorway as shown below. Door Frame Light Source Light Beam LDR 25 GCSE Electronics. Unit E1 : Discovering Electronics When the light beam shines on the LDR, the voltage across the LDR is very small and this is not sufficient to switch on the thyristor, and the lamp is off as shown opposite. When a burglar walks through the doorway the beam of light is broken, and the LDR falls into darkness. In darkness the voltage across the LDR increases and this is now sufficient to switch the thyristor on, and so the lamp lights, as shown opposite. As the burglar moves through the doorway the light beam is restored to the LDR, and we can see that even though the light level increases again, as shown opposite, we can see the latching action of the thyristor because the lamp remains on, even though the voltage at the gate has fallen below the level needed to switch the thyristor on. The question now arises as to how to switch the thyristor off when it has ‘latched’ on. 26 Topic 1.7 – Switching Circuits. The thyristor can be switched off by using the switch shown in the circuit diagram. You should recognise this as a push to break switch from topic 1.4. The action of the switch is to stop the current flowing through the lamp and thyristor, which effectively resets the thyristor and switches the lamp off. Closing the switch again after it has been opened does not cause the lamp to come back on, the thyristor has been switched off. The lamp will only come on again if the gate voltage rises above the required value. This method works by stopping the current flowing through the thyristor. This method of resetting the thyristor is acceptable when the current through the load is less than about 5A. In industrial applications thyristors can handle very high powered output devices that require very large currents up to about 100A, and breaking this sort of current with a simple push switch can be dangerous because the current can jump between the contacts of the switch and cause sparks to jump across the gap. This is called arcing, and if repeated many times can destroy the contacts of the switch. So for industrial applications alternative methods of resetting the thyristor are used. As these are much more complex they will not be covered in this introductory syllabus. As far as the examination questions are concerned, these will be limited to: i. identifying the symbol for a thyristor, ii. identifying the names of the three leads of the thyristor, iii. completing a diagram to show how a thyristor can be used with sensing sub-systems in simple alarm circuits for example. iv. completing a diagram to show how a thyristor can be connected to a logic system to drive a high powered load, v. explaining that the thyristor is a latching device which will remain on until reset once triggered, by a suitable gate voltage. vi. explain that the thyristor can be reset by breaking the current flowing through the load. vii.the load current will be limited to applications requiring a maximum of 10A, which can be safely switched off using the method of breaking the current described in part vi above. 27 GCSE Electronics. Unit E1 : Discovering Electronics Student Homework 2: Answer all questions in the spaces provided, continue on a separate piece of paper if required. 1. i) ii) What device is shown above ? ..................................... [1] The three leads of this device have special names, what are they? .................................... ..................................... ..................................... iii) On the diagram above, label the leads with their correct name. i) What device is shown above? ..................................... 2. ii) [3] [1] The three leads of this device have special names, what are they? .................................... iii) [3] ..................................... ..................................... On the diagram above, label the leads with their correct name. [3] [3] 28 Topic 1.7 – Switching Circuits. 3. Study the following two circuits carefully. Circuit A Circuit B Circuit A and Circuit B perform similar functions. (a) What conditions are needed for the lamp to come on in Circuits A and B. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [1] (b) How does circuit A differ from circuit B ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [3] 29 GCSE Electronics. Unit E1 : Discovering Electronics 4. The output of a logic system is required to switch on a high power motor which requires a current of 10A. Complete the diagram below to show a suitable switching device and any necessary connections. 12V Motor 12V, 10A Logic System 0V [2] 30 Topic 1.7 – Switching Circuits. 5. A student made the following circuit to switch on a fan (driven by the motor) in an air conditioning system for a factory when it gets light, so that is not too hot for the workers when they arrive. a) Identify three faults with the students design. i) ................................................................................................................... ................................................................................................................... ii) ................................................................................................................... ................................................................................................................... iii) b) ................................................................................................................... ................................................................................................................... [3] Draw the circuit diagram for a solution to this problem that does not suffer from the faults identified. [3] 31 GCSE Electronics. Unit E1 : Discovering Electronics 6. The following circuit is used as a burglar alarm, with a pressure pad located under a doormat. The circuit suffers from the disadvantage that once switched on the buzzer will not go off. In the space below draw a circuit diagram that includes the modification(s) to the circuit so that the alarm can be turned off once it has been switched on. [3] 32 Topic 1.7 – Switching Circuits. 1.7.5 Voltage Comparators In the previous three sections when we have discussed the operation of the NPN transistor, N-channel MOSFET, and Thyristor. We have concentrated on their ability to enable large output currents to flow, from relatively small input currents or voltages. All three devices operate best when their input current or voltage change rapidly, i.e. like the output from a logic gate switching on from its off state. This rapid transition drives the output switch hard from its off state to the on state very quickly. Sometimes the output signal connected to these electronic switches takes much longer to increase and this can lead to problems with the transistor, MOSFET or Thyristor not fully switching on and then they become very hot. The reasons for this are beyond the scope of this introductory unit, but it is a situation that we need to avoid. The sort of signals that are slow changing like this are the ones that come from sensing circuits involving LDRs and thermistors primarily. These tend to have slowly changing outputs because light level and temperature don’t usually change very quickly. We need a device that can change these slow changing signals into fast changing signals in order to drive the output switching devices hard into their on or off states. The device that allows us to do this is the voltage comparator. The voltage comparator is an integrated circuit that has five connections. At first it may appear to be quite complex but it has very simple rules about how the output is determined. The circuit configuration (left) and pin-out (right) is shown below. Note: some comparators have a different pin-outs. For the purposes of this examination the pinout diagram on the right will be used. +ve Supply Voltage + _ Non-Inverting Input Voltage, V+ 0V 0V VOUT Inverting Input Voltage, V33 GCSE Electronics. Unit E1 : Discovering Electronics In most practical circuits the power supply connections to the comparator are not shown, which simplifies the diagram to the one below: + _ Non-Inverting Input Voltage, V+ Inverting Input Voltage, V- VOUT 0V Remember that if you are setting this circuit up in practice you must connect the power supply to the comparator otherwise it will not work. The operation of the comparator is such that it amplifies any difference between the two input voltages by an enormous amount, causing the output to be at one of the extremes of the power supply connected to it. This means that the output voltage will fall into one of the following categories. Case 1 : If V+ > V- then VOUT will be at the positive saturation voltage. Case 2 : If V+ < V- then VOUT will be 0V. A difference of just a few microvolts between the two inputs is enough to cause the output to swing rapidly from one state to another. The rapid transition makes the voltage comparator an ideal device to use with circuits employing slow response sensors like LDRs and thermistors. If a slow response sensor is to be used with digital circuits then the voltage comparator will be essential if these are to be connected to digital logic circuits, which do not react very well to input voltages which are not very close to either Logic 0 or Logic 1, or to switching circuits that require a rapid transition between voltage levels to enable clean switching of an output. 34 Topic 1.7 – Switching Circuits. As we have discovered in topic 1.5, LDRs and thermistors are used as part of a voltage divider, and depending on the orientation of the sensing component will provide either a rising or falling voltage at the output. To incorporate this type of sensing circuit with a comparator we actually use a second voltage divider to provide a reference voltage which controls the voltage at which the output of the comparator circuit changes. In GCSE examinations this reference voltage will be connected to the inverting input. The circuit diagram, which may appear complicated at first glance is shown below. +9V R1 = 5k + _ R3 = 10k R2 = 5k VOUT 0V Temperature Sensing Sub-system Reference Voltage Sub-system The first voltage divider shown in the ‘blue’ box you should recognise as the temperature sensing circuit, discussed at length in topic 1.5. You should be able to work out that the voltage at the non-inverting ‘+‘ input of the comparator will increase as the temperature rises. The second voltage divider shown in the ‘red’ box is a simple voltage divider containing two equal resistors. Again by this stage you should be able to work out that the voltage at the inverting ‘-’ input of the comparator will be 4.5V. 35 GCSE Electronics. Unit E1 : Discovering Electronics When the temperature is low, the resistance of the thermistor will be very high, the voltage at the output of the temperature sensing circuit will be low, and the output of the comparator will be at the minimum voltage of the power supply, because the voltage at the inverting ‘-’ input will be higher than the voltage at the non-inverting ‘+‘ input. As the temperature rises, the resistance of the thermistor begins to fall, this causes the voltage at the non-inverting ‘+‘ input to start to rise. When this voltage reaches just over 4.5V, the voltage at the non-inverting ‘+‘ input will be bigger than the voltage at the inverting input and the output will increase to 9V, since V+>V-. This circuit therefore performs the simple operation of providing a high output signal when the temperature is high, and could possibly be used as a simple fire alarm. As it stands the circuit has no adjustment of the temperature at which the output of the comparator switches from high to low. It turns out that it is quite easy to make this circuit adjustable, simply by making any one of the three resistors R1, R2 or R3 variable. Whichever one is chosen to be variable it will have the desired effect of either adjusting the voltage range of the temperature sensing circuit or changing the reference voltage at which the comparator switches. 36 Topic 1.7 – Switching Circuits. An alternative way of producing the reference voltage is to use a potentiometer as shown in the following diagram. This has the advantage that the reference voltage can be varied over the full voltage supply range, making the circuit extremely flexible, and most importantly very sensitive. +9V + _ VR1 = 20k VOUT R2 = 10k 0V Temperature Sub-system Reference Voltage Sub-system Just as it is quite straight forward to make this circuit adjustable it is also just as straight forward to switch the function of the circuit to provide the opposite behaviour, i.e. switch the output of the comparator on when the temperature decreases, as would be the case for an ice alarm. There are two ways in which this can be achieved, firstly by reversing the position of the thermistor in the sensing circuit as shown below. +9V R3 = 10k R1 = 5k + _ R2 = 5k VOUT 0V Temperature Sub-system Reference Voltage Sub-system 37 GCSE Electronics. Unit E1 : Discovering Electronics Secondly we can reverse the inputs to the comparator, which will also have the same effect, as shown below: +9V R1 = 5k + _ R3 = 10k R2 = 5k VOUT 0V Temperature sensing Sub-system Reference Voltage Sub-system The remarkable flexibility of this circuit means that in the design of any circuit there is usually more than one correct solution, sometimes in an examination, part of the circuit is drawn for you, so that the number of correct solutions are limited, but if you are designing your own circuit, then you are free to choose whatever solution you want provided that it satisfies your own design criteria. In all of the above circuits we have not considered possible output devices that could be connected to the comparator. The output current of many comparators is limited to a range of between 30mA and 300mA. This is of no use for driving high powered output devices like motors, or solenoids. However it is capable of driving LEDs with an appropriate series resistance, to limit the voltage across the LED to approximately 2V, buzzers, and some low power lamps. 38 Topic 1.7 – Switching Circuits. Consider the following circuit built from a comparator i.c. which saturates at the supply voltage, i.e. 9V: +9V R1 = 3k + _ RX R3 = 10k R2 = 6k 0V Light Sensing Sub-system Reference Voltage Sub-system This circuit uses an LDR as part of the light sensing circuit. From our work in previous topics you should be able to determine that the voltage at the noninverting ‘+’ input of the comparator will rise as the light intensity falling on the LDR increases. When this voltage reaches just over 6V, the output of the comparator will go high (9V) since the voltage from the sensor will be higher than that of the reference circuit, and the LED will light. The system therefore switches on the LED when it gets light. If we need to switch on higher power devices like motors and solenoids, then we can simply connect the transistor, MOSFET, or thyristor circuit from earlier in this topic to the output of the comparator. Whilst the comparator can be considered a switching device because it can switch on and off low power outputs, it’s main function is to act as an analogue to digital converter to provide fast changing signals to drive more traditional output switching devices like the transistor, MOSFET or thyristor into one state or another, as on the following page. 39 GCSE Electronics. Unit E1 : Discovering Electronics +9V M R1 = 3k + _ R3 = 10k R2 = 6k 0V Light Sensing Sub-system Reference Voltage Sub-system Now it’s time for you to have a go at a few circuits. 40 Topic 1.7 – Switching Circuits. Student Homework 3. 1. The following circuit shows a comparator. The resistance of the thermistor at 25°C is 50kΩ, and at 100°C is 5kΩ. The output voltage of the comparator will be either +6V or 0V. +6V R1 = 3k R3 = 10k P + _ Q RX R2 = 6k 0V a) Calculate the voltage at point P when the temperature is i) 25°C. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. ii) 100°C. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. 41 GCSE Electronics. Unit E1 : Discovering Electronics b) Calculate the voltage at point Q. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. c) The LED has a maximum forward voltage of 2V. Calculate the series resistance Rx required to limit the current through the LED to 20mA. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. d) Hence describe the function of the circuit given. ............................................................................................................................. ............................................................................................................................. ............................................................................................................................. e) 42 Show on the diagram how the reference voltage, can be made adjustable. Topic 1.7 – Switching Circuits. f) Draw two new arrangements for this circuit which provide the opposite function to the one given. Solution 1: Solution 2: 43 GCSE Electronics. Unit E1 : Discovering Electronics Solutions to Student Exercises Homework Questions 1. 1. i) NPN. ii) Collector, Base, & Emitter. iii) 2. In circuit A the base lead is connected to the 0V line. When this occurs there is no voltage difference between the base and emitter terminals of the transistor, and the transistor is off. No current can flow through the collector/emitter junction and so the lamp remains off. In circuit B the base is connected to the +6V line and a base-emitter voltage of 0.7V is achieved. The transistor is on and a collector-emitter current flows and hence the lamp lights. 3. Transistor circuits are sometimes referred to as switching circuits because a small current entering the base of the transistor produces a much larger current in the collector. If no base current flows then there is no collector current, so the transistor can be considered as a switch since current in the collector circuit can be switched on by a small base current. 44 Topic 1.7 – Switching Circuits. 4. 5. i) ii) In daylight the resistance of the LDR is low, compared to the 10kΩ resistor, therefore there will be a low voltage across the LDR. This ensures that the base-emitter voltage does not rise above 0.7V and so the transistor remains off, so the lamp is off. When darkness begins to fall the resistance of the LDR begins to increase and so therefore does the voltage across it. When this voltage increases above 0.7V the transistor starts to switch on, a collector current flows causing the lamp to come on. i) The circuit switches on a buzzer when the temperature being monitored by the thermistor goes above a certain value. ii) When it is cold the resistance of the thermistor is high, compared to the 10kΩ resistor, therefore there will be a low voltage across the 10kΩ. This ensures that the base-emitter voltage does not rise above 0.7V and so the transistor remains off, so the buzzer does not sound. When the temperature increases the resistance of the thermistor begins to fall and so therefore does the voltage across it. The voltage across the 10kΩ resistor rises and when this voltage increases above 0.7V the transistor starts to switch on, a collector current flows causing the buzzer to come on. 45 GCSE Electronics. Unit E1 : Discovering Electronics Homework Questions 2: 1. i) Thyristor ii) Gate, Anode and Cathode (any order) iii) Anode [1] [3] Gate Cathode [3] 2. i) MOSFET ii) Source, Gate and Drain (any order) [1] [3] iii) Source Gate Drain [3] 3. (a) (b) 46 The lamp will come on in Circuits A and B when the temperature rises above a particular value. [1] Circuit A differs from Circuit B because it contains a thyristor. Once the temperature goes over its set limit and the lamp switches on, it will remain on, even if the temperature falls again. The lamp has been latched on. In circuit B containing a MOSFET, if the temperature decreases again, then the lamp will go off. [3] Topic 1.7 – Switching Circuits. 4. The output of a logic system is required to switch on a high power motor which requires a current of 10A. Complete the diagram below to show a suitable switching device and any necessary connections. 12V Motor 12V, 10A Logic System 0V 5. a) b) i) The fan would switch on in the dark, not in the light. ii) Once switched on the fan could not be switched off. iii) The fan would switch on even if it was cold, as it is not dependent on temperature only light. [2] [3] [3] 47 GCSE Electronics. Unit E1 : Discovering Electronics 6. [3] 48 Topic 1.7 – Switching Circuits. Homework Questions 3. 1. a) i) 25°C. 50k 6V 50k 10k 50 6 5V 60 VP ii) 100°C. 5k 6V 5 k 10k 5 6 2V 15 VP b) 6k 6V 3k 6k 6 6 4V 9 VQ c) 62 20 10 3 4 200 20 10 3 RX d) At 25°C the output of the comparator is on, because the voltage from the temperature sensing circuit ‘P’ is higher than the reference voltage ‘Q’, and so the LED will be on. As the temperature rises the voltage at ‘P’ will be falling until it reaches 2V at 100°C. When the voltage falls below 4V, the voltage at ‘Q’ the comparator output will switch to 0V and the LED will switch off. The circuit is therefore behaving as a low temperature warning system. 49 GCSE Electronics. Unit E1 : Discovering Electronics e) Either +6V R1 = 3k R3 = 10k P + _ Q 200Ω R2 = 6k 0V or Or +6V R1 = 3k R3 = 10k P + _ Q 200Ω R2 = 6k 0V f) Option 1: +6V R1 = 3k + _ 200Ω R3 = 10k 0V 50 R2 = 6k Topic 1.7 – Switching Circuits. Option 2: +6V R3 = 10k R1 = 3k + _ 200Ω R2 = 6k 0V Now for some examination style questions. 51 GCSE Electronics. Unit E1 : Discovering Electronics Examination Style Questions 1. (a) The diagram shows a comparator IC shown from above. (i) Label pin 1 with the number 1. (ii) Label pin 7 with the number 7. [2] (b) A warning system is used in a shop to warn the assistant if the temperature in the freezer becomes too warm. The circuit diagram for this is shown below. (i) (ii) The circuit makes use of a thermistor. Draw the circuit symbol for a thermistor in the space below. [1] The output VOUT of the comparator saturates at +6V and 0V. Complete the table for the given values of the input voltages. Input 1 (V) Input 2 (V) 3.2 4.0 4.5 2.1 Output VOUT (V) [2] 52 Topic 1.7 – Switching Circuits. 2. Part of a transistor switching circuit is shown below. (a) On the diagram, label the base of the transistor with the letter ‘b’ [1] (b) At a certain time VIN is measured to be 0.4V. (i) Is the transistor on or off ? .................................. (ii) Is the buzzer on or off ? .................................. [2] (b) Some time later VIN is measured to be 2.5V. (i) Is the transistor on or off ? .................................. (ii) Is the buzzer on or off ? .................................. [2] 53 GCSE Electronics. Unit E1 : Discovering Electronics 3. Here is part of a comparator circuit. (a) What is the combined resistance of R1 and R2? ...................................................................................................................................... [1] (b) Use the equation given in the information sheet to calculate the value of voltage V1. ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... [2] (c) The output Vo of the comparator saturates at 9V and 0V. Complete the table below to show what the output voltage will be for the given conditions: V1 (V) V2 (V) 5.0 7.3 5.0 2.1 Output VOUT (V) Is LED on or off? [3] 4. The following symbol is for a switching device. (a) What is the name of this device? ............................................ (b) Label the three connecting leads by choosing from the following options. [1] base, drain, collector, anode, source, emitter, gate, cathode [3] 54 Topic 1.7 – Switching Circuits. 5. (a) The diagram shows a comparator IC shown from above. (i) Label pin 1 with the number 1. (ii) Label pin 5 with the number 5. [2] (b) A following diagram shows a circuit which indicates when the temperature in a greenhouse is too high. (i) The output VOUT of the comparator saturates at +9V and 0V. Complete the table for the given values of the input voltages. Input A (V) Input B (V) 4.5 2.3 4.5 4.8 Output VOUT (V) [2] (ii) The temperature at which the bulb comes on needs to be adjustable. Modify the circuit diagram above to allow this. [1] 55 GCSE Electronics. Unit E1 : Discovering Electronics 6. This is the symbol for a comparator. Choose the correct name for connections A and B from the following list. Ground Inverting Input Non-inverting input Output Positive Supply Write your answers in the spaces provided on the diagram. [2] 7. (a) Here is a voltage divider. (i) What is the combined resistance of R1 and R2? (ii) ...................................................................................................................................... [1] Use the equation given in the information sheet to calculate the value of voltage V1. ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... [2] 56 Topic 1.7 – Switching Circuits. (b) The voltage divider is now connected to a comparator circuit as follows: The output Vo of the comparator saturates at 10V and 0V. Complete the table below to show what the output voltage will be for the given conditions: Output Vo (V) V2 bigger than V1 V2 smaller than V1 [2] (c) A sensing subsystem is now connected to the comparator so that the voltage Vo is high when the temperature is high. Complete the diagram to show this circuit. Remember to use the correct symbols for the components. [3] 57 GCSE Electronics. Unit E1 : Discovering Electronics 8. Here is part of a comparator circuit. (a) What is the combined resistance of R1 and R2? ...................................................................................................................................... [1] (b) Use the equation given in the information sheet to calculate the value of voltage V1. ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... (c) [2] The output Vo of the comparator saturates at 9V and 0V. Complete the table below to show what the output voltage will be for the given conditions: V1 (V) V2 (V) 5.0 2.3 5.0 7.1 Output Vo (V) Is LED on or off? [3] 58 Topic 1.7 – Switching Circuits. (d) A sensing sub system is now connected to the comparator so that the LED is ON when it is light. Complete the diagram to show the circuit. [3] 9. The following symbol is for a switching device. (a) What is the name of this device? ............................................ [1] (b) Label the three connecting leads by choosing from the following options. base, drain, collector, anode, source, emitter, gate, cathode [3] 59 GCSE Electronics. Unit E1 : Discovering Electronics 10. Part of a transistor switching circuit is shown below. (a) On the diagram, label the collector of the transistor with the letter ‘c’ [1] (b) At a certain time VIN is measured to be 2.8V. (i) Is the transistor on or off ? (ii) Is the lamp on or off ? .................................. .................................. [2] (b) Some time later VIN is measured to be 0.5V. (i) Is the transistor on or off ? .................................. (ii) Is the buzzer on or off ? .................................. [2] 60 Topic 1.7 – Switching Circuits. 11. (a) The diagram shows a comparator IC shown from above. (i) Label pin 3 with the number 3. (ii) Label pin 6 with the number 6. [2] (b) The following incomplete circuit diagram shows a circuit which should indicate when the light level is too low. (i) The output VOUT of the comparator is either 12V or 0V. Complete the table for the given values of the input voltages. Input X (V) Input Y (V) 6.1 7.3 9.5 7.3 Output VOUT (V) Bulb On/Off [3] (ii) Complete the circuit diagram above to make: The voltage at X high when the light level is too low. The voltage at the input Y adjustable. [2] (iii) Name one use for this circuit. .............................................................................. [1] 61 GCSE Electronics. Unit E1 : Discovering Electronics 12. (a) The diagram shows a comparator IC seen from above. (i) What is the pin number of the non-inverting input of the comparator? .............................................................. (ii) What is the pin number of the output of the comparator? .............................................................. [2] (b) The following diagram shows part of a circuit which indicates when the temperature in a green house is too high. The output VOUT of the comparator saturates at 9V and 0V (i) What does the comparator do in this circuit ? ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. [1] 62 Topic 1.7 – Switching Circuits. (i) Complete the table for the given values of the input voltages Input A (V) Input B (V) 4.2 1.7 4.2 4.8 Output VOUT (V) [2] (ii) Complete the circuit diagram opposite to make: The voltage at input B high when it is warm, The voltage at input A adjustable. [3] (c) The gardener decides to improve his system by connecting the output to a heavy duty motor to open the windows of the greenhouse when the temperature gets too high. Complete the circuit diagram for the output of the new system, capable of switching on a motor requiring 8A to operate properly. [3] 63 GCSE Electronics. Unit E1 : Discovering Electronics 13. A tennis umpire uses a system to show when the light level is too low to play. An incomplete circuit diagram for this is shown below. (a) Which of the following graphs A, B C or D best shows the way in which the resistance of the LDR depends on the light level? .................................................................................................................................................. (b) What happens to the voltage V1 as the resistance of the LDR increases? .................................................................................................................................................. [1] 64 Topic 1.7 – Switching Circuits. (c) The output V2 of the comparator saturates at 9V and 0V. Complete the table to show what the output voltage will be for the given values of the input voltages. Input 1 (V) Input 2 (V) 6.1 7.3 9.5 7.3 Output V2 (V) Is LED On or Off? [2] (d) Complete the circuit diagram opposite by: (i) adding a voltage divider to provide a variable reference voltage for the comparator. (ii) (e) adding any other connections so that the LED lights when the light level is too low to play. [4] The current through the LED is 10mA and the forward voltage drop across it is 2V. (i) What is the voltage drop across resistor R when V2 = 9V? (ii) ...................................................................................................................................... [1] What is the current flowing through resistor R when R when V2 = 9V? (iii) ...................................................................................................................................... [1] Calculate a suitable resistance for resistor R. ...................................................................................................................................... (iv) ...................................................................................................................................... [2] Choose a suitable preferred value for resistor R from the E24 series in the information sheet. ...................................................................................................................................... [1] 65 GCSE Electronics. Unit E1 : Discovering Electronics 14. A warning system is used on a car to warn the driver if the temperature outside falls below freezing, and therefore there is a risk of finding ice on the roads. An incomplete circuit diagram for this is shown below. (a) The output VOUT of the comparator saturates at +6V and 0V Complete the table for the given values of input voltages. Input 1 (V) Input 2 (V) 3.2 4.0 4.5 2.1 Output VOUT (V) [2] (b) Complete the diagram above to show the sensing circuit used to ensure the voltage into input 1 of the comparator is high when it is cold. Remember to use the correct circuit symbols for the components. [3] 15. The following symbol is for a switching device. (a) What is the name of this device? ............................................ (b) Label the three connecting leads by choosing from the following options. [1] base, drain, collector, anode, source, emitter, gate, cathode [3] 66 Topic 1.7 – Switching Circuits. Self Evaluation Review My personal review of these objectives: Learning Objectives 1.7.1 – Introduction. Understand that switching circuits are often used: as an interface between analogue and digital sub-systems; as a transducer driver to drive output devices; 1.7.2 – NPN Transistor. Draw a circuit diagram to show how npn transistors can be used as part of a switching circuit; Identify the base, collector and emitter leads on a transistor; State that a small base current can be used to control a much larger load current; Apply the following rules to a given transistor switching circuit: For VIN < 0.7V, the transistor is off For VIN > 0.7V, the transistor is on; 1.7.3 – N Channel MOSFET. Draw a circuit diagram to show how a MOSFET can be used as part of a switching circuit. Identify the gate, source and drain on a MOSFET; State that a gate voltage can be used to control a large load current; Explain why a MOSFET should be used to drive heavy loads due to the limited output capability of logic systems; 1.7.4 - Thyristor Identify the gate, anode and cathode on a thyristor; State that a small gate voltage can be used to latch a large load current; Draw a circuit diagram to show how a thyristor can be used as a latching transducer driver incorporating a reset switch. 1.7.5 – Voltage Comparators. Know that comparators have a greater sensitivity than transistor switches; Use data sheets to identify pin connections on a dedicated comparator IC; Predict the output voltage given the input voltages in a comparator circuit; Appreciate the current driving limitations of comparators; Design comparator circuits which cause output devices to respond to information from sensors; Design circuits to increase the output capabilities of a comparator circuit by the addition of a transistor switch. Targets: 1. ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… 2. ……………………………………………………………………………………………………………… ……………………………………………………………………………………………………………… 67