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BTC108 2 Introduction To Electronics 2

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  • 1. Lecture 2 Introduction to Electronics 2 James Uren BTC108 Electronics
  • 2. Lecture 2 Introduction to Electronics 2 Resistor Networks Resistors can be combined in a network to create new resistor values. They can either be connected in Series or in Parallel. Series To find the equivalent resistance of resistors end to end simply add the resistances: R1 Rseries R2 (is equivalent to) Find the resistance of circuit that has 1kΩ, 4.7kΩ and 22kΩ resistors in series. Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 2
  • 3. Parallel When resistors are connected in parallel the inverse of the combined resistance is found by adding the inverse of the resistances: R1 R2 Rparallel (is equivalent to) Find the resistance of circuit that has three 4.7kΩ and 22kΩ resistors in parallel. Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 3
  • 4. Potential Dividers Resistors drop a voltage across them. This means more than one resistor connected in series will divide the input voltage. I R1 Vin R2 Vout Using Ohm’s Law, show that equation for the potential divider in the circuit above is: This method for voltage dividing is extremely useful in electronics as it provides a simple way of converting resistance to voltage. For example, a rotary control on a piece of equipment is a variable resistor (potentiometer or ‘pot’), and by using the control as one of the resistors in a potential divider it is possible for the rest of the circuit to read off the dial value as a voltage. The same principle applies for simple sensor circuits, e.g. a thermistor whose resistance changes with temperature. Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 4
  • 5. Resistivity All materials have a resistance, and this property is called the material’s resistivity. It is calculated in the following way: Where: ρ (pronounced ‘ro’) is the resistivity in m Ω R is the resistance in Ω A is the area in m2 l is the length in m Calculate the resistance of a circular piece of copper wire with diameter 1mm and length 0.2m. The resistivity of copper is 1.7 x 10-8. Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 5
  • 6. Power All electronic devices consume electric power when in operation. This power, measured in Watts (W) can be calculated using Joule’s Law: I + V R Where P is the electric power consumed in resistance R in Watts (W). Electronic devices and components will normally have a power rating. Ratings of resistors Joule’s Law and Ohm’s Law can be combined to give two more expressions for power: A piece of equipment with a 12V supply has a power rating of 40W. Is a 2A fuse sufficient? Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 6
  • 7. Practical: Potential Dividers and Power Read the Health and Safety Information on page 9. • Using only 4 identical resistors (of between 1kΩ and 100kΩ) and a 5V supply, design a potential divider that provides outputs of 2V and 3V. • Build the circuit and use a voltmeter to show that the voltage outputs are what you expect. • Calculate the current in your circuit and use an ammeter to confirm your answer. • The resistors are all rated at 0.25W. What power is being consumed in each resistor? • What is the lowest resistance you could use for all the resistors in your circuit to keep under the rating? Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 7
  • 8. Writing Numerical Answers Decimal Places Giving an answer to 3 decimal places means three digits after the decimal point. If the 4th decimal place is 5 or above, round up the 3rd decimal place. e.g. 65.342545 given to 3 DPs is 65.343 Significant Figures Giving an answer to 3 significant figures means showing only the 3 left-most digits (rounding in the same way. e.g. 65.342545 given to 3 SFs is 65.3 768593.25 given to 3 SFs is 769000 3 significant figures is normally enough information in electronics. Give all answers to this accuracy unless directed otherwise. Units For large and small numbers it is convenient to use an exponential notation with 10 - ‘to the power’ - X. e.g. 768593.25 expressed in this way and to 3 SFs is 7.69 x 105 For the following cases this is simplified: p pico x10-12 n nano x10-9 μ micro x10-6 m milli x10-3 k kilo x103 M mega x106 G giga x109 T tera x1012 e.g. 768593.25 can be expressed (to 3 SFs) as 769 k or 0.769 M. Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 8
  • 9. Black Brown Red Orange Yellow Green Blue Purple Grey White Sourced from showcase.netins.net Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 9
  • 10. Health & Safety Considerations Soldering and de-soldering: Solder melts at between 180 and 200°C. Soldering irons will heat up to between 250 and 400°C. Be extremely careful when soldering and take the following precautions: • Switch off the soldering iron at the mains when not in use • Always keep the iron in its stand • Make sure your workspace is clear, well lit and well ventilated • Never solder while your circuit is powered up • Never solder without tutor supervision • Only apply the soldering iron for the minimum amount of time • Keep your soldering tidy and use the minimum amount of solder • Avoid breathing in solder fumes • You must only use the lead-free solder provided • You must use tools e.g. pliers to support components that are being soldered and ensure the board is secure. Switching it on: Powering up a circuit that is incorrectly connected can cause components or equipment to get extremely hot or even ‘blow’. A short circuit (where unintended electrical connections are made) for example may damage equipment or blow components causing them to behave in an unpredictable way. • Before powering up your circuit you MUST have it checked by the tutor • Have your neighbour physically inspect your work before powering on • If your circuit does not behave as you expect, switch it off immediately • Use your nose! A faulty circuit with hot components will often smell or smoke Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 10
  • 11. If your circuit does not behave as you expect: • With the power off, confirm by eye that your circuit is connected correctly and that you are using all the correct components and mounted with the correct polarities • Inspect your circuit closely for short circuits, soldering faults and dry joints: • Do all the testing on your circuit that you can with it powered off. • Be extremely careful when probing your circuit live as the probe itself can cause short circuits • When probing with an oscilloscope ensure the earth connection is applied safely Lecture 2: Introduction to Electronics 2 BTC108: Electronics – James Uren 11