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    Employability Skills: Brush up Your Electronics
    Employability Skills: Brush up Your Electronics
    Employability Skills: Brush up Your Electronics
    Ebook525 pages1 hour

    Employability Skills: Brush up Your Electronics

    By Clive W. Humphris

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    About this ebook

    An enhanced eBook published in full colour. Now including extensive interactive content enabling exploration by inserting any values that would occur in a real situation whereby the graphics are redrawn to reflect those changes.

    Calculations can be also tested against any standard subject textbook to compare the results.

    Interactive Technology when used in the classroom can motivate passive students by encouraging their active participation where STEM subjects are ideally suited to Mobile Interactive Technology.

    Students are more likely to be comfortable with technology they understand i.e. their phone and can interact with, often preferring 'Learning-by-Doing' over traditional pencil and paper methods.

    Full colour graphics that are redrawn for every input change will make the learning experience more enjoyable and effective as it encourages experimentation of real world situations as almost any practical values are accepted.

    Students who struggle to be fully engaged in normal classroom activity can often achieve the unexpected once sat in front of a digital screen where they can learn without the embarrassment of full class exposure.

    Mobile Interactive Technology can bring any STEM textbook to life by inserting printed values from the book into their mobile device and comparing the results.

    Colourful visual presentation assists the learning process as students will more likely remember, thereby increasing their personal confidence as they believe they are learning more as a result. Knowing the content is on their phone encourages them to dip-in in a spare moment more than open a traditional textbook.

    Conclusion: Students will spend more time engaged with the Mobile Interactive Technology than with a traditional textbook.

    For each topic group students can test their understanding by considering an open question whereby their ease of answering will provide an indication of personal progress.

    LanguageEnglish
    PublisherClive W. Humphris
    Release dateOct 10, 2012
    ISBN9781301645381
    Employability Skills: Brush up Your Electronics
    Author

    Clive W. Humphris

    Clive W. Humphris M0DXJ: Ex Technology Teacher. Software Developer, Author and Director of eptsoft limited. Married with two children and four grandchildren.Apprentice Instrument Maker at Marconi’s with Senor Technical Management roles in Radio Rentals and Alcatel Business Systems before starting eptsoft providing educational software to schools colleges and universities worldwide since 1992.Interests outside of developing digital products for eptsoft, include Running, Walking and Reading.

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      Book preview

      Employability Skills - Clive W. Humphris

      SIMPLE DC CIRCUITS: Three Resistors in Series.

      Interactive Content!

      In a practical circuit consisting of just three resistors, connected in series across a battery, four circuit parameters can be measured using a simple multi-meter. Firstly the current I flowing which is determined by inserting an ammeter in series with the resistors and then the three voltage drops across the individual resistors.

      The current is a result of the applied voltage divided by the total series circuit resistance. Apply the formula for series resistance to determine the total resistance R.

      Individual resistor voltage drops are each found by applying Ohm's Law. Resistance R1, R2 or R3 multiplied by the series circuit current. Adding the individual voltage drops together will always equal the applied battery voltage.

      SIMPLE DC CIRCUITS: Two Parallel Resistors.

      Shown are three components connected together forming a circuit. A battery or source of electric current and two resistors. From this diagram a number of circuit parameters may be found. Some are known others need to be calculated.

      Considering the current as being 'conventional' where it flows from the battery positive terminal and divides into two branches. The amount of current in each branch will be inversely proportional to the resistor value, i.e. the larger resistor value the less current flows.

      Firstly we need to find the total current I, but before we can do this we require the equivalent circuit resistance from the formula. We know the applied battery voltage. However each individual branch current could just as easily be calculated by applying Ohm's Law and the two current values added together which will equal I. Current is never lost (Kirchhoff's Law) the sum of individual currents will always equal the total current.

      As you can see there are several ways of solving this problem. If you were given the total current and the resistor values, could you have found the battery voltage?

      SIMPLE DC CIRCUITS: Potential Divider.

      A simple potential divider is just two resistors connected in series across a battery. So long as we don't have large variations in load current the voltage at the resistor junction will be remain fixed.

      The voltage dropped across the lower resistor provides the output voltage, determined by the relationship between Ra and Rb. The calculations will demonstrate this in more detail. What is the output voltage if the lower resistor is made a quarter of the resistance of the upper value?

      The simple voltage divider circuit is commonly used to make a transistor base biasing network where the base current requirements are small.

      However, there are further considerations when the current requirements increase or are subject to large variations.

      SIMPLE DC CIRCUITS: Loading a Potential Divider.

      Loading the voltage divider connects the resistance of the load in parallel with the lower resistance Rb. We will call this parallel combination Rx. Note the shorthand for resistors in parallel. To determine the total current, first calculate the value of Rx, then divide the battery or supply voltage by the addition of Ra + Rx.

      Load current Iout is simply the resistor junction voltage over the load resistance RL.

      In practice the value of Rb is chosen to be about one tenth of the value of the load. This ensures that any fluctuations in load current have a limited effect on the divided output voltage.

      Experiment with the values of Rb relative to RL and note the changes on the Ra, Rb junction voltage.

      SIMPLE DC CIRCUITS: Pull Up, Down Resistors.

      The use of pull up and pull down resistors is a common feature in electronics. Closing S1 in the left hand diagram pulls down the voltage at the lower end of Ra by shorting it to the zero line. Current flowing in Ra will then depend solely upon the resistor value and the supply voltage.

      In the diagram to the right, the voltage at the top of Rb is pulled up to the supply voltage by closing S2. With equal value resistors will the current flowing be the same in both circuits when the switches are closed?

      S1 and S2 could be replaced by transistors acting as switches which effectively become short circuited between the collector and emitter terminals when made to conduct heavily. Resistor Ra is a collector load and Rb an emitter resistor.

      When a transistor is biased OFF, i.e. no base volts the transistor is open circuit. For T1 the collector voltage would be high, no collector current flowing and for T2 the emitter voltage would be zero, with no emitter current flowing. Biasing ON T1 and its collector output voltage is pulled down and for T2 the emitter voltage is pulled up.

      TYPES OF SWITCHING: Push Switch.

      Interactive Content!

      Switch contacts when open provide an interruption of the current flow within a circuit and when closed completes the conducting path. Shown is one of the simplest of schematic diagrams that consists of just three components, indicated by appropriate symbols. Clearly shown are the component connections and the effect of what happens when the switch button is pushed. One of the simplest types of switch has to be the push-to-make, i.e. for a doorbell, here is a push-to-break.

      Within the pages of a components catalogue you can find dozens of different combinations of switch types. When selecting a switch there are two main considerations, current rating and the maximum working voltage. Using a switch that is under-rated can be unreliable and dangerous because of arcing of the contacts or physically expose the user to an electric shock because of a voltage breakdown of the insulation.

      In this diagram the battery can represent any number of cells connected in series which increase the supply voltage (potential difference) as each cell is added. Battery cells are usually in multiples of 1.5V, and those of the rechargeable type are lower at 1.2V.

      To calculate the current I flowing in this simple circuit we can use Ohm's Law by applying the formula shown. Try changing the battery supply voltage and note the changing current. In a practical circuit the more current that flows the brighter the lamp would glow. Increasing the voltage and thereby the current, above that permitted by the bulb and the filament acts like a fuse.

      TYPES OF SWITCHING: Change-over Switch.

      Switches are available in many different types. Here is an example of a changeover switch that redirects the battery connection to either the lamp or the buzzer. This is a break-before-make switch.

      Others are make-before-break, where power is connected to both parts of the circuit during changeover. Switches are used as relay contacts, rotary selectors, slider contacts etc.

      Here we have given the two devices a different working resistance. This demonstrates that the current drawn from the battery changes as the switch is thrown. As the battery has what is called 'internal resistance' this can cause a reduction in the potential difference PD between the battery terminals as the load resistance increases.

      Electronic buzzers have no moving contacts and therefore do not generate RF interference, but produce a clear penetrating sound. Typical uses are in internal burglar alarm sounders. The output frequency is around 400Hz with impedance of a few hundred ohms, consuming approximately 35mA when a voltage between four to twenty volts is applied.

      TYPES OF SWITCHING: Stair-case Switch.

      The staircase switch arrangement derives its name from its purpose in domestic house wiring, where it is often necessary to be able to switch a single lamp from any number of different places. There are two types of switch used, A and D are normal changeover types. The others B and C are called intermediate switches (of which there can be any number) are crossover types.

      What happens is the intermediate switches changeover the central pair of conductors.

      There is no need for a clever logic diagram to explain or carry out this action, its simply the bulb lights or extinguishes for every switch action. Follow the lamp current path for yourself for all the available switch states.

      TYPES OF SWITCHING: Relay Switch.

      A relay is a device, which is an electrically operated switch. It works by energising an electromagnet that pulls or releases switch contacts that are make (shorted out) or break (open circuit) depending upon whether the relay coil is energised. Close S1 and a current is made to flow in the relay coil. This causes a magnetic field to develop which pulls the central switch contact [b] to the left, making the circuit with [a] for the bulb to light.

      Release S1 and the magnetic field collapses and the contact [b] returns to its rest position, against the right hand contact [c], causing the buzzer to sound. In use a reverse biased diode should be connected across the energising coil for protection as the high back-EMF that is generated when the current is switched off can easily damage any associated circuitry.

      Relays are available in both single-pole and double-pole switch actions. Typical switch contact resistance is measured as 50mOhms (milli-Ohms) with operating times of around a couple of milliseconds. Maximum switching currents can exceed 10Amps, but for most applications, a much smaller and less expensive device would suffice.

      A relay has a mechanical switching operation, not electronic, as is the case of a thyristor semiconductor device. The electromagnetic coil resistance is between 80 and 1kOhms depending on the type when designed for an operating voltage of around 5Volts to 30Volts. The life of the contacts can exceed 100,000 operations but this depends largely on the amount of current being switched and if it is AC or DC as the former tends to maintain cleaner switch contacts due to its uni-polar direction.

      TYPES OF SWITCHING: Three-level Switch.

      Most switches found in component catalogues are fairly standard, where only the method of mounting or presentation differs. However, there will be times when a specialist action is called for. Typical examples are found in electric cookers and washing machines. As the latter can be very complicated we will concentrate on the former to switch the heating elements in an oven or cooker hot plate.

      The example shown consists of three switch positions, plus an ON/OFF, which also forms part of the thermostat. The thermostat will be bi-metal strip, which bends and makes or breaks a pair of switch contacts to complete the circuit depending upon the temperature setting. But it's the switching action, which is of interest here.

      The two elements Re1 and Re2 are resistances which are designed to make use of their dissipated heat which is normally wasted. They could be considered as underrated wire wound types, which become hot due to excessive current owing to their low value. In the 'Low Heat' setting they are connected in series, as their combined resistance increases across the (L) - (N) terminals the current flow and therefore heat dissipated will be relatively low. 'Medium Heat' and just one element is connected and for 'High Heat' both are switched ON, which for purposes of calculations are in connected in parallel.

      Note the supply voltage is AC, but we use the RMS value, because it has the same energy content as an equivalent DC voltage. Select for more details of the calculations. As both Re1 and Re2 are always the same we can use a simplified method of finding the equivalent series and parallel values.

      3 4 2012-01-09T15:31:00Z 2012-03-29T09:22:00Z 2 463 2642 eptsoft 22 5 3244 9.3821

      VARIABLE VOLTAGES: DC Voltage.

      Interactive Content!

      DC voltages are constant over time. The voltage range is determined by the battery, which could equally be some form of regulated DC power supply.

      In practice the lamp would glow brighter as the battery voltage is increased, but it will always remain at the same level whilst the voltage is constant.

      3 4 2012-01-09T15:31:00Z 2012-03-29T09:22:00Z 2 463 2642 eptsoft 22 5 3244 9.3821

      VARIABLE VOLTAGES: Switching a DC Voltage.

      A switch causes the lamp to light when the contacts are closed, by placing a voltage across the bulb filament. Operating the switch makes the lamp to go ON and OFF as the applied voltage is first present and current flows and which then falls to zero as the voltage is removed.

      In a practical circuit the ON-OFF periods would not necessarily be constant as shown here, but would vary with the on-off time.

      The output has what is commonly called a square-wave-form. The ON period is referred to as the mark and the OFF as the space. We will explore this effect later when we look at timing circuits.

      3 4 2012-01-09T15:31:00Z 2012-03-29T09:22:00Z 2 463 2642 eptsoft 22 5 3244 9.3821

      VARIABLE VOLTAGES: Variable DC Voltage.

      By introducing a variable resistor across our battery, some interesting changes can be observed over time, i.e. the output voltage will vary. The output is changed slowly, not switched, so between slider positions the voltage will be made to increase or decrease gradually.

      You will probably by now see the variable resistor as a voltage divider circuit. The output voltage will depend upon the slider position, which effectively splits the variable resistor track into two quite separate resistor sections. Changing the variable resistor component value will increase or decrease the current drawn from the battery and can be calculated using Ohm's

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