Wireless "battery" energizes low-power devices
A simple 13.56-MHz oscillator broadcasts power to a receiver circuit.
Carlos Cossio, Santander, Spain; Edited by Charles H Small and Fran Granville -- EDN, November 22, 2007
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 1 shows the transmitter circuit, which incorporates a 13.56-MHz oscillator. The oscillator encompasses a CMOS 4069 inverter using power from a 9V battery to get a wide voltage swing. The oscillating signal then passes through a push-pull output stage comprising two small-signal MOSFETs to get enough current in the output coil. Finally, the output signal broadcasts to the outside by means of a serial-resonant-LC circuit incorporating a coil and a 60-pF variable capacitor tuned to 13.56 MHz.
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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How do you get a 4069 to oscillate at 13 MHz let alone drive the gate capacitanceof the BS170 and BS250 MOSFET's?
You are using 1N4001 diodes at 13 MHz???
You actually built this circuit with the components shown?
Please explain, thanks, Jay
Jay Philippbar - 2011-20-4 15:12:36 PDT -
i need a 12v dc output at the reciver... wat would be the changes made in the circuit??
Jatin - 2009-11-3 23:38:00 PDT -
For Ctick (AU/NZ), conducted emmissions are tested under 30Mhz, and radiated emissions are tested from 30Mhz - 300Mhz and then 300Mhz - 1Ghz, average and then quasi peak. The thresholds for average are generous. For this device, I doubt it would show up on conducted, but for emitted, I imagine there would be harmonics above 30Mhz (there is no low pass filtering. I would not call this device very low power as it is push pull and designed to transmit energy. You could always wabble the frequency with another oscillator, so the quasi peak measurements disapear.
Sam Crumpet - 2007-26-11 12:19:00 PST -
Since this is at 13Mhz, would someone building this need FCC approval. What about the specturm spread of this circuit? (The output looks like a square wave to me.) I understand that it is of such low wattage that the usage would not be detected. But if someone was to put this in a product, it would be a problem. Wouldn't it be better to redesign the circuit to work at an "un-regulated" area of the spectrum? But again, the interference caused would need to be considered.
Frank Lee - 2007-26-11 07:15:00 PST -
I am curious at how these circuits perform in reguards to EMC testing. I know EMC testing ignores the intended transmission frequency, but this circuit would have harmonics.
Sam Crumpet - 2007-25-11 13:49:00 PST
