PRESENTED BY-AKSHAY KASHYAP P

1RE08EC003VIII A

ECE

Portable electronic devices are very popular these days. As the usage of portable electronic devices is increasing, it demands for a longer battery life.

Batteries need to recharged or replaced periodically.

To overcome this, concept of wireless battery charging was developed.

So there is a need to make a proto-type device that converts microwave signal to DC power.

•A battery charger is a device used to put energy into rechargeable battery by forcing electric current through it.•The charging protocol depends on the size and type of the battery being charged•Some charges might be having temperature or voltage control sensors to adjust the charging current and cut off at the end of charge

Fig1:Overall wireless battery charger system

•This design includes a power transmitter.•Transmitter consists of an oscillator amplifier and an antenna.•At a pre-determined frequency i.e., 900MHz, oscillations are generated. These signals are amplified before transmission.

Power: 12V DC, 900 mA

Output Power: 3 Watts

Operating Frequency: 900 MHz

Connector Type: SMA – Female

Output Impedance: 50 Ω

Fig.2 900 MHz Video/Audio Transmitter

Table.1 The specification of the transmitter

To charge a battery, high power DC signal is required. To choose appropriate antenna, two factors are to be

taken into consideration:1. Impedance of antenna2. Gain of antenna

Impedance of the antenna should match with the output impedance of the power transmitter.

Higher antenna gains yields to a better gain result, but also increases size and cost of the antenna.

Taking the above design specification into consideration yagi antennas are best suited.

• It is a directional antenna consisting of a driven element (typically dipole) and additional parasitic elements(usually reflectors and one or more directors)

• Reflector element is slightly longer than the driven dipole whereas directors are little bit shorter. This design achieves a very substantial increase in antenna’s directionality and gain compared to simple dipole

• Highly directional antenna’s such as yagi uda antennas are commonly referred to as beam antennas due to there higher gain.

Figure 3: A picture of the 9 dBi gain Yagi antenna

•Block diagram of a receiver is as shown in the figure

•It consists of receiving antenna, rectifier circuit and load(AAA battery)

•Receiver main purpose is to charge AAA battery.

•Simple battery charging theory is to run current through the battery, and apply the voltage difference between the terminals to the battery to reverse the chemical process.

A full wave rectifier is used due to its simplicity and efficiency in converting the AC signals.

The full wave rectifier consists of four schottky diodes.

Schottky diodes are used because of their very low turn-on voltage and operating frequency of 900Mhz.

The final design the wireless battery charger consists of the following important components:1.Transmitter2.Yagi antenna3.Full-wave rectifiers circuit4.RC circuit5.Battery holder

picture of the transmitter with the Yagi antenna

picture of the final design circuit of the charging circuit

A. Multiple RectifiersBy using multiple rectifiers more power gets rectified. However this design will need a spiral antenna array which is difficult to design

B. High Gain Parabolic antennaParabolic antenna will be able to transmit power from transmitter with much higher gain and receive with greater power than yagi antenna. Disadvantage is parabolic antenna are size and fixed-direction antenna.

Advantages:1. Portable2. Relatively lesser cost3. Saves time

Disadvantage:1. Less efficient

Power loss and efficiency are the major problems for this design.

The characteristics of diodes must be such that maximum rectification is possible with minimum loss.

Time varying current and voltage relationship at the physical point of the diode in the cavity determines loss in the diode hence the dc efficiency. So, it is essential to limit this loss.