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/m 2  o : extraterrestrial power density (1.353 kW/m 2 )  : 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 top- off)

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