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Engineering Design How to build stuff that works, and make it work better Dr. Tom Clarke, Second Year Electronics Laboratory Coordinator

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Engineering design in Second Year Electronics Laboratory 2 Why learn design? EEE courses teach the theory needed to do real engineering. Applying that theory happens later in projects, and some coursework. It helps you: Motivate theoretical work Become better at solving real problems Perform better in your year 3 & 4 project work Have fun on your EEE or ISE course

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Engineering design in Second Year Electronics Laboratory 3 What is design? 1. Specify the problem 2. Explore top-level decomposition 3. Discover relevant information from diverse sources 4. Perform system analysis (where possible) 5. Make assumptions, prioritise problems 6. Perform detailed prototype design 7. Evaluate prototype test assumptions identify problems & areas for further work 8. Improve design

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Engineering design in Second Year Electronics Laboratory 4 Design is not linear Specify Top-level InformationAnalysisAssumptions Prototype Evaluate Improve Top-down Bottom-up These activities are inter-dependent and concurrent

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Engineering design in Second Year Electronics Laboratory 5 Putting it all together Not all elements of design are “open”. Some problems are closed, with specific constraints that allow only one solution You will often find these problems in EEE coursework or exam problems Use analysis to solve closed problems, to simplify the design space No-one tells you which bits of analysis to do! Design requires both analysis and creative exploration

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Engineering design in Second Year Electronics Laboratory 6 Example Need to set bias voltage (Vb) input to alphanumeric LCD display module. From LCD datasheet: LCD module supply is 5v Vb >0.5v, Vb < 3v Circuit must adjust Vb to an unknown correct value in this range This sets LCD contrast Ib < 10uA I LCD = 2mA (typical) Specify

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Engineering design in Second Year Electronics Laboratory 7 Bias Adjust Circuit Bias Adjust Circuit LCD Module Vb +5V GND Top-down

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Engineering design in Second Year Electronics Laboratory 8 Design ideas Use voltage regulator IC? Need to read datasheets to see how to make adjustable over required range Use Zener diode (last year’s circuit) Does not help since not variable Use potential divider (P.D.) with variable resistor Simplest solution if feasible Could combine P.D. and Zener for slightly better stability (probably not worth it) Multiple viewpoints

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Engineering design in Second Year Electronics Laboratory 9 Variable resistors Preset resistors have three terminals, with a fixed resistance between the two ends and a slider which can move anywhere between the two ends. PR1 Information

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Engineering design in Second Year Electronics Laboratory 10 Detailed circuit design using variable resistor This circuit will allow Vb to be adjusted between 0.5v & 2v What values R1, R3, PR2? R1 PR2 R3 2v 0.5v Vb +5v GND Vx Vy Detailed design

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Engineering design in Second Year Electronics Laboratory 11 Know your resistors 22k resistor is not 22,000 ohms! 22k resistor has specified tolerance (1%,2%,5%) 2% tolerance: 0.98*22,000 < R < 1.02*22,000 Variable resistors typically have tolerance 10% Resistors have preferred values: Fixed resistors available in E24 series and multiples 1,1.1,1.2,1.3,1.5,1.6,1.8,2.0,2.2,2.4,2.7,3.0,3.3, 3.6,3.9,4.3,4.7,5.1,5.6,6.2,6.8,7.5,8.2,9.1 Variable resistors only available: 1, 2, 5 and multiples! Check catalogues and datasheets Design for available precision & values Information

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Engineering design in Second Year Electronics Laboratory 12 Analysis (ohms law) R3 = 0.5/(2-0.5)PR2 R1 = (5-2.5)/(2-0.5)PR2 Choose PR2 first, calculate R3,R1 How accurate do these ratios need to be? If Vx>2V, Vy<0.5V the adjustment range includes the required range of 0.5-2V Precision not required R3,R1 can be smaller then calculated Analysis

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Engineering design in Second Year Electronics Laboratory 13 Assumptions PR2 too low => more current used in circuit. Assume want current as small as possible PR2 too high => Vb will vary too much with LCD bias current change. Datasheet does not say how much bias current changes so assume 10uA is possible (worst case, since we know it is < 10uA) Datasheet does not say how accurate Vb must be: assume 10%. Assumptions

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Engineering design in Second Year Electronics Laboratory 14 Analysis Approximate analysis Assume Thevenin equivalent resistance at Vy = R3 (actually slightly smaller) Assume OK at all other voltages if OK at Vy 50mV > R3.I b = R3.10uA => R3 < 5k => total divider current = 1mA Not too bad, but significant compared with I LCD R1=50k, PR2=15k, R3=5k More analysis + approximation

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Engineering design in Second Year Electronics Laboratory 15 Are values realistic? Variable resistors are available 10k,20k,50k 15k not possible Could scale by 2/3 R1=33k, PR2=10k, R3=3k3 These resistor values are all available This is not good idea. Designing precisely to limits is dangerous. Reduce R1, R3 by 20% to ensure coverage of entire range even if resistor values vary R1=27k, PR2=10k, R3=2k7 (use E24 values) Note that precise values don’t matter R1=22k, PR2=10k,R3=2k2 would also be fine Detailed design

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Engineering design in Second Year Electronics Laboratory 16 Optimise circuit Why bother with R1, R3? Not really needed, but allows better adjustment Resolution = minimum change in resistance value that a variable resistor can be adjusted to. Typically 1%. 1.5V across PR2 =>15mV res R3 missing => 2V across PR2 => 20mV res R1 & R3 missing => 5V across PR2 => 50mV res R3 probably not needed (1.5V -> 2V) R1 maybe also not needed (1.5V -> 5V) Optimise

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Engineering design in Second Year Electronics Laboratory 17 Possible circuits R1 PR2 R3 2v 0.5v Vb +5v GND R1 PR2 Vb +5v GND PR2 Vb +5v GND 10k 30k 3k0 20k 27k 50k Guess which one is recommended in the LCD datasheets?

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Engineering design in Second Year Electronics Laboratory 18 Design in EE2 & ISE2 Laboratory Design activities during 1 st half of each Term Work through examples of design Learn skills useful in project work PCB design Embedded system design Prototyping Measurement Conducted in laboratory pairs Assessed individually by demo, interview, & logbook

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