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

This example design, the Solar Candle, utilizes a photo sensor, solar cell, charge controller, rechargeable battery, DC-DC boost converter, and LED to create a flickering candle effect.

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

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

  2. Existing Window Candle Block Diagram Photo Sensor Solar Cell Charge Controller Rechargeable Battery 1.2V NiCd DC-DC Boost Converter LED 3.2V 20mA Mode Selection Flickering Control

  3. Diodes/Zetex - ZXLD383ET5CT

  4. Typical Application CircuitNote Control Pin!

  5. DC-DC ConverterEfficiency

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

  7. Changing Focus to Charging • How much energy is removed from the battery during a typical evening? • LED requires 20mA x 3.2V x 6hrs = 384 mW hrs (power x time = energy) • Converter is only 85% efficient, so energy taken from battery is 384 mW hrs / 0.85 ≈ 452 mW hrs • How much charge? • Dividing by the battery voltage gives the charge removed: 452 mW hrs / 1.2V ≈ 377 mA hrs

  8. The Prior Art Dissected

  9. On to the Solar Panel Requirements • After taking the Home Depot Landscape Light apart, I made the following measurements (in direct sun): ISC = 50mA , VOC = 4.3V

  10. Solar Panel V-I Characteristic

  11. Solar Panel Considerations • How much charge is restored if the panel is connected directly to the battery? What assumptions should we make? • How about • 10 Hours of Daylight • 50% Incident Light • This gives 50mA x 10hrs x 50% = 250mA hrs • Is this enough? We need 377 mA hrs. No. 

  12. Charge Options? • Use two solar panels in parallel to boost the current (but we’re throwing away voltage?) • Use the existing panel with some sort of Buck Converter (will need to look at efficiency). • Find a solar cell better suited for this application…

  13. Found something at Futurlec.com ! • Open Circuit Voltage (Voc): 2.2V • Short Circuit Current (Isc): 100mA • Dimensions: 61mm x 61mm • Price: $1.50 (100+ Qty)

  14. Modified Characteristic ISC = 100 mA , VOC = 2.2 V (VOCgreater than VBAT)

  15. A Possible Solution • Now we have: 100mA x 10 hrs x 50% = 500 mA hrs. • Is this enough? We need 377mA hrs. Yes! 

  16. Solar Panel Update to System Block Diagram Solar Panel ISC = 100mA VOC = 2.2V IAVE = 50mA Δt = 10hrs Q = 500mAHrs Charge Controller Rechargeable Battery 1.2V NiCd Zetex LED Driver LED 20mA 3.2V(min) Mode Selection Photo Sensor Switching Control Timer

  17. And now the Charge Controller… Solar Panel ISC = 100mA VOC = 2.15V IAVE = 50mA Δt = 10hrs Q = 500mAHrs Charge Controller Rechargeable Battery 1.2V NiCd Zetex LED Driver LED 20mA 3.2V(min) Mode Selection Photo Sensor Switching Control Timer

  18. NiCd Charge Control Methods (Panasonic)

  19. Which Charge Method to Choose? • Semi-Constant Current Charge • Most Typical Charge System • Simple and Economical • Typical Charge Time = 15 Hrs • Typical Charge Current = 0.1 It (0.1*700 mA Hrs = 70mA) • Time Controlled Charge • More reliable than Semi-Constant Current • Slightly more complicated. Requires timer. • Typical Charge Time = 6-8 Hrs • Typical Charge Current = 0.2 It (140mA)

  20. Semi-Constant Current Charge Seems Viable • With our low average current of 50mA, and charge time of 10 hrs, the Semi-Constant Current Charge method seems viable. • Also, if we are concerned about over charge, we can extend the on-time beyond 6 hrs. • This method is more economical and may not require a timer for this application.

  21. Charge Controller Update Solar Panel ISC = 100mA VOC = 2.15V IAVE = 50mA Δt = 10hrs Q = 500mAHrs Charge Controller Semi-Const. Current Method Rechargeable Battery 1.2V NiCd Zetex LED Driver LED 20mA 3.2V(min) Mode Selection Photo Sensor Switching Control Timer

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