Transcript Introductory Lecture on Electronic Design

Engineering Design
How to build stuff that works, and
make it work better
Dr. Tom Clarke, Second Year Electronics
Laboratory Coordinator
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:
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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
Engineering design in Second Year
Electronics Laboratory
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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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Electronics Laboratory
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Design is not linear
Top-down
Specify
Assumptions
Bottom-up
These activities are
inter-dependent and
concurrent
Top-level
Analysis
Information
Prototype
Evaluate
Improve
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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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Specify
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
 ILCD = 2mA (typical)
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Top-down
Bias Adjust Circuit
+5V
Bias
Adjust
Circuit
Vb
LCD
Module
GND
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Electronics Laboratory
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Design ideas
Multiple viewpoints
 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)
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Variable resistors
Information
 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
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Detailed circuit design using
variable resistor
+5v
R1
Vx
2v
PR2
Vy
Vb
0.5v
Detailed
design
 This circuit will
allow Vb to be
adjusted between
0.5v & 2v
 What values R1,
R3, PR2?
R3
GND
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Electronics Laboratory
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Know your resistors
Information
 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
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Analysis (ohms law)
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Analysis
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
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Electronics Laboratory
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Assumptions
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%.
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Analysis
More analysis +
approximation
 Approximate analysis
 Assume Thevenin equivalent resistance at Vy =
R3 (actually slightly smaller)
 Assume OK at all other voltages if OK at Vy
 50mV > R3.Ib = R3.10uA => R3 < 5k
 => total divider current = 1mA
 Not too bad, but significant compared with ILCD
 R1=50k, PR2=15k, R3=5k
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Are values realistic?
Detailed
design
 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
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Optimise circuit
Optimise
 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)
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Electronics Laboratory
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Guess which one is
recommended in the
LCD datasheets?
Possible circuits
+5v
30k
R1
R1
27k
2v
10k
PR2
3k0
R3
GND
+5v
+5v
Vb
50k
PR2
Vb
PR2
20k
Vb
0.5v
GND
GND
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Electronics Laboratory
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Design in EE2 & ISE2 Laboratory
 Design activities during 1st 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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