This wireless battery charger is designed to operate at 900 MHz. In this project, a power transmitter acts as the power source. It will transmit power to the receiver side. And then, the rectifier circuit in the receiver will convert the RF/ microwave signal into DC signal. After the DC signal is produced, the charging circuit will store the power into the battery. Here is the block diagram of the overall design.

Fig2Wireless Charging At_decrypted
Figure 2 : Wireless Battery Charger Design

A. Transmitter

Since the group does not design the transmitter, therefore the design is mainly focus on the receiver side. A power transmitter is bought from a commercial website. It is a 900 MHz video/audio transmitter. Here’s the specification of the transmitter.

B. Antenna

The considerations of choosing the appropriate antenna are:
1. Impedance of the antenna
2. Gain of the antenna

C. Receiver

The receiver’s main purpose is to charge an AAA battery. A simple battery charging theory is to run current through the battery, and apply a voltage difference between the terminals of the battery to reverse the chemical process. By doing so, it recharges the battery. There are other efficient and faster ways to charge the battery, but it requires a large amount of energy which the wireless battery charger cannot obtain, yet. Therefore, in our design, we use a straight forward method to charge the battery.
Fig4Wireless Charging At_decrypted
Figure 4 : Full-Wave Rectifier Circuit

A full-wave rectifier is chosen for the project due to its simplicity and efficiency in converting the AC signal. The full-wave rectifier is consisted of four diodes. Since the power received by the receiver will be relatively low and the signal frequency is high, the diodes are required to have a very low turn on voltage and operating frequency at 900 MHz. For this reason, a Schottky diode by Skyworks is chosen for the design. (SMS3929-021 Bridge Quad Schottky Diode).

At the output of the rectifier, the signal is not a fully DC signal yet. Thus, by adding a capacitor and a resistor can smooth out the output to become DC signal. However, the time constant produced by the capacitor and the resistor should be calculated carefully to fit the desired time constant.

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