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Design Process Analysis & Evaluation Part I Example Design: Solar Candle

This example design analysis and evaluation project focuses on the design of a solar candle, analyzing its product interfaces, evaluating its performance, and making necessary product changes based on customer requirements and technical research areas.

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Design Process Analysis & Evaluation Part I Example Design: Solar Candle

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  1. Design ProcessAnalysis & EvaluationPart IExample Design: Solar Candle by Prof. Bitar

  2. Analysis & EvaluationHomeworks #4 & 5 HW#4 Define Product Interfaces 1 Analyze 2 Design Analysis & Evaluation 3 Build HW#5 5 Evaluate 4 Test Define Product Changes

  3. The Customer Requirements • Explicit • Cat Safe • Look Nice • Different Colors • Automatic • Six Hour Minimum • Implicit • Low Cost • Reliable / Durable • Low Maintenance

  4. Product Requirements • Safety / Durability • Heavy Base (Batteries in Base) • Unbreakable Bulb (LED) • Secure to Window Sill, Sash or Window Pane • No Cords • Low Voltage (1.5V or 3V) • Operation & Aesthetics • Traditional Look • Interchangeable Color LED’s • Flickering Option • Low Operating Cost • Long Battery Life (Six Hour Minimum) • Rechargeable Battery • Solar Rechargeable • Photo Sensor or Timer

  5. Modified System Block Diagram Solar Cell Charge Controller Rechargeable Battery Efficient Drive Circuit LED Mode Selection Photo Sensor Switching Control Timer

  6. Technical Research Areas • Solar Cells – Availability, cost, size, shape, output voltage, output current, silicon types, spectrum sensitivity. • Charge Controllers – Voltage Regulation, Current Regulation, etc. • Rechargeable Batteries – NiCad, NiMH, Other?, Overcharging & Under-voltage Concerns, etc. • Drive Circuits– DC/DC Boost Converters, Charge Pumps, etc. • LED’s – Colors, Voltage Levels, Current Requirements, Intensities, Illumination Angles, etc. • Photo Sensors – Types, Interface Requirements

  7. What to Consider First? • Start at the OUTPUT. What does our design have to do? Drive an LED. • What current? What Voltage(s)? • Take a look at LED specification sheets…

  8. Agilent White LED Spec. SheetDissected

  9. LED SpecificationsAbsolute Maximum Ratings NOT TO BE EXCEEDED !!

  10. LED SpecificationsElectrical Characteristics Normal Operating Conditions

  11. LED Specifications – Curves !Relative Intensity vs. Wavelength

  12. LED Specifications – Curves !Max. Forward Current vs. Temperature Stay well under this curve !!

  13. LED Specifications – Curves !Max. Forward Current vs. Voltage Check: 20mA @ 3.2V (Typical)

  14. LED Specifications – Curves !Spatial Radiation Pattern Check Viewing Angle !!

  15. Mechanical Specifications

  16. LED Drive Requirements“First Stakes in the Ground” • Must supply a constant current of approximately 20mA. • Must supply a minimum of 3.2V for white LED’s. • Should work for other color LED’s, since they have lower voltage levels.

  17. Customer Requirement Check • NOTE: Each decision MUST map to a customer requirement !! • Question: Which customer requirements are met by designing for an LED current of 20mA at a minimum of 3.2V? • Answer: Multi-Color LEDS, Reliable, Durable, Low Maintenance

  18. Update System Block Diagram Solar Cell Charge Controller Rechargeable Battery Efficient Drive Circuit LED 20mA 3.2V(min) Mode Selection Photo Sensor Switching Control Timer

  19. What to Consider Next? • The Battery (or Batteries) • Battery Capacity: How long does the candle need to operate? (Six Hours Minimum) • Battery Voltage: What is the minimum battery voltage? ( 3.2V ) • Battery Availability: What rechargeable technologies will work in this application? (NiMH, NiCd, Lithium Ion, Sealed Lead Acid) • Battery Shape: Needs to fit in candle stem or base. • Battery Cost: I don’t want to spend a lot on batteries!

  20. Battery Selection - Two Viable OptionsMajor Factor? Price! Problem: Need at least 3 batteries in series to achieve 3.2V minimum.

  21. Energy Considerations… • Need: 20mA x 3.2V x 6hrs = 384 mW hrs (ie: 1382.4 Joules) • One NiCd: 700mAhrs x 1.2V = 840 mW hrs (ie: 3024 Joules) • Conclusion: A single NiCd has more than enough energy to do the job ! (But the voltage will have to be “boosted”).

  22. Battery Specifications

  23. Battery SpecificationDischarge Curve

  24. Battery SpecificationCharge Curve

  25. Update System Block Diagram Solar Cell Charge Controller Rechargeable Battery 1.2V NiCd Efficient Drive Circuit LED 20mA 3.2V(min) Mode Selection Photo Sensor Switching Control Timer

  26. What Next? LED Drive Circuit Solar Cell Charge Controller Rechargeable Battery 1.2V NiCd Efficient Drive Circuit LED 20mA 3.2V(min) Mode Selection Photo Sensor Switching Control Timer

  27. Searching LED Driver IC’s… • www.digikey.com • LED Drivers (1,319 IN STOCK!) • Narrowing Search (down to 47) • Step-Up (Boost) • Vmin ≤ 0.8V (why?) • Internal Switch • Number of Outputs (1) • Small Qty. Packaging (cut tape / tube) • Case…avoid DFN, QFN, LLP, BGA

  28. Possible LED Drivers

  29. Diodes/Zetex - ZXLD383ET5CT

  30. Typical Application Circuit

  31. Update System Block Diagram Rechargeable Battery 1.2V NiCd 700mAHr Zetex LED Driver 85% Eff. Solar Cell Charge Controller LED 20mA 3.2V(min) Mode Selection Photo Sensor Switching Control Timer

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