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Power Source for Embedded Systems Kyung Kim 11/28/2004 Outline • • • • Introduction Wireless power transmission Solar panel Embedded system approach of charge controller for backup battery • Summary Introduction • Embedded systems get smaller. • Everything is wireless these days. • Many embedded systems are required to run in places where power grid cannot reach for long period of time. Wireless power transmission Tesla’s wireless power • The idea became possible in 1940 when Nicola Tesla got his patent for wireless radio and Dr. William Brown developed dipole rectifier with 90% eff. of converting microwave beam to DC electricity. Wireless power using microwave • High power microwave beam is used to transmit power • 2.45 GHz is used (lower freq. interferes communications and higher freq. may be dangerously intense) • Microwave beam transmission has 82% eff. due to its own interference • Dipole rectifier antenna has 90% receiving eff. Wireless power using laser • Uses infrared laser for power transmission • Photovoltaic cells (solar panel) converts the laser to electric power • Photovoltaic cells only uses thin slice of sun light spectrum wasting 80% of energy • Infrared laser increases efficiency Solar panel Generating 10kW per day Solar power density • = o COS (dt - wa) p • : solar power density on earth in kW/m2 • o: extraterrestrial power density (1.353 kW/m2) • : zenith angle (0 when sun light is perpendicular to ground) • dt: direct transmittance of gases except for water • p: the transmittance of aerosol • wa: water vapor absorptions of radiation. (dt=80% wa=2% p =95% at equator) Photovoltaic cells (PV) • PV cell is built like a diode out of semiconductor • Photons strike the semiconductor material in PV cell and break off electrons producing DC current Solar panels power rating • 1.5V 100mA • 1.5V 200mA • 6V 50mA Power needed to run a simple system • PIC16F877A: 5V 4mA • LCD(typ): 5V 4mA • OPAMP(typ): 10V 4mA Solar panel array • Connect solar panels in series to increase the voltage • Connect solar panels in parallel to increase the current Simple diagram for solar panel with embedded system Charge controller Embedded system approach of charge controller for backup battery • To power an embedded system, part of the embedded system is used to control the power State diagram NiCd and NiMH • NiMH: Charging terminated at voltage pick • NiCd: Charging terminated at a point past voltage pick Li-Ion • Charging terminated when charging current is at min 5% • Charging terminated when voltage reaches to float point. (timer may activate for topoff) Summary • Wireless power transmission is now an option for power source of an embedded system • Solar panel with a backup battery is effective power source for an embedded system • Battery charging controller can be designed using microcontrollers and software