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Wireless
"Battery" Energizes Low-Power Devices
Design Idea published in the
EDN magazine on November 22, 2007
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DESIGN IDEAS - CMOS IC Makes Low-Cost Digital Potentiometer - Power Management In The ST62 - Automatic Power-Off Circuit Saves Battery - Simple PC Smart Card Reader Enhancement - Digital Position Encoder Does Away With ADC - Single-Supply RS-232 Transmission Without Level-Translator ICs - µC Controls Charge Pump As Background Task - Clock Multiplier Circumvents PLL - Drive Smart Cards With A Low-Cost MCU's UART - Wireless “Battery” Energizes Low-Power Devices - Harvest Energy Using A Piezoelectric Buzzer - Drive A Single-Coil Latching Relay Without An H-Bridge Circuit DESIGN CONTESTS ARTICLES
CONTACT
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Wireless connectivity is a growing trend in portable consumer gadgets. Unfortunately, designs cannot achieve true mobility because of short battery life, so the power cord still must connect the device to the power grid to get the required energy or to recharge the batteries. However, thanks to the low-power requirements of today’s electronic devices, it is feasible to power them wirelessly. This Design Idea describes a simple approach to wirelessly transmitting energy to low-power devices at distances as great as 10 cm. This design uses the resonant-inductive-coupling principle working at 13.56 MHz. The system comprises the RF-power transmitter and the RF-power receiver.
Figure 2 shows the receiver circuit, which comprises an LC network tuned to a carrier frequency of 13.56 MHz. It includes a coil and a 60-pF variable capacitor in parallel with the coil. A full-bridge rectifier comprising four 1N4001 diodes rectifies RF power. Rectification efficiency is approximately 50%. Reaching 3.3V of output voltage requires a 9V p-p ac voltage across the coil’s pins. A shunt regulator incorporating a 3.3V zener diode provides voltage clamping beyond 3.3V to prevent power-level variations with distance. Finally, a 1-nF capacitor after the full-bridge rectifier decouples the power supply. The two coil antennas use five turns of 1-mm enameled-copper wire in a rectangular, 50×60-mm shape.
As an improvement, if your application requires greater distance, you can increase the power supply to 15V to get a greater voltage swing on the transmitter coil, thanks to the CMOS technology the oscillator design employs. In addition, designing a larger-coil antenna in the transmitter and the receiver sides helps to increase the distance of operation. |
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