Technology is one thing that keeps on changing to make life simpler. But, let’s take the case of charging a device, for decades, we still confine to the old traditional style of wire system. Think of a device or a system that can charge up multiple systems without all those snake-look-alike wires. The solution is to go with wireless power transfer using inductive coupling.
Wireless Power Transmission using inductive coupling, is one of the effective ways to transfer power between points without the use of conventional wire system. Wireless power transmission is effective in areas where wire system is unreachable or impossible. The power is transferred using inductive coupling, resonant induction or electromagnetic wave transmission depending on whether its short range, mid-range or high range.
The goal of this project Wireless power transmission mobile charger circuit using inductive coupling is to charge a low power device using wireless power transmission. This is done using charging a resonant coil from AC and then transmitting subsequent power to the resistive load. The project is meant to charge a low power device quickly and efficiently by inductive coupling without the help of wires.
Fig1 : Block Diagram of Wireless Power Transmission Mobile Charger Circuit using Inductive Coupling
In this project, the wireless charger works mainly on the principle of inductive coupling. With this inductive coupling idea, we are trying to transfer power wirelessly to charge low power devices, such as mobile phones, cameras, wireless mouse etc.
From the block diagram, it is clear that for the overall functioning of wireless charger circuit, it required a wireless power transmitter & a wireless power receiver sections.
The transmitter coil in this wireless power transmitter section converts the DC power from an oscillator to a high frequency AC power signal. This high frequency alternating current, which is linked with the wireless power transmitting coil, would create an alternating magnetic field in the coil due to induction, to transmit energy.
In the wireless power receiver section, the receiver coils receives that energy as an induced alternating voltage (due to induction) in its coil and a rectifier in the wireless power receiver section converts that AC voltage to a DC voltage. Finally this rectified DC voltage would be feed to the load through a voltage controller section. That is, the wireless power receiver section’s main function is to charge a low power battery through inductive coupling.
This project has mainly two sections, wireless power transmitter & a wireless power receiver sections. The Transmitter section of wireless charger circuit consists of a DC power source, oscillator and a transmitter coil. A constant DC voltage is provided by a DC power source, and this DC signal is the input to the oscillator circuit. This oscillator converts this DC voltage to a high frequency AC power, and is supplied to the transmitting coil. Due to this high frequency AC current, the transmitter coil energizes, and generates an alternating magnetic field in the coil.
DC power Source: It consists of a step down transformer that step downs the supply voltage to a desired level, and a rectifier circuit to convert that AC voltage to DC signal.
Oscillator Circuit: A modified Royer Oscillator circuit is used in our project. With this circuit we can easily achieve a high oscillating current for the transmitter coil.
The oscillator circuit used in the Wireless power transmitter section is given below.
Fig 2 : Transmitter section circuit
Wireless power transmitter section :Here in the transmitter circuit section, we uses two N channel enhancement power MOSFET (IRF540 – Q1,Q2 ),Two chokes (L1 & L2) , capacitor C (works as a resonating capacitors), diode D1 & D2( provide cross coupled feedback) , the transmitter coil L(inductor), resistors R1,R2,R3 and R4 (works as a biasing network for Q1&Q2) etc. are used.
When power is given to the oscillator circuit, the DC current starts flowing through the two sides of the coil (L1&L2) and also to the Drain terminals of the MOSFET. During the same instant, voltage appears on gate terminal of both the transistors and tries to turn ON the transistors. Any one of the transistor will be faster than the other and it will turn ON first.
Assume that Q1 will be turning on first, and then Q1’s drain voltage will be clamped to near ground. At the same time, Q2 will be in less conductive state or in off state, then Q2’s drain voltage will rise to peak and starts to fall due to the tank circuit formed by the capacitor C and the primary coil of oscillator through one half cycle. The operating frequency of the oscillator is determined by the resonance formula given below
F = ½ × π × √ (LC)
Note: A Heat sink is provided with each of the MOSFET to protect them from overheating and make them cool.
Transmitter Coil: For this Wireless power transmission mobile charger circuit using inductive coupling project, we can use 6mm enameled wire (Magnet wire) for constructing the transmitter coils. Actually this enameled wire is a copper wire, which has a thin layer of insulation coatings on it. Here the transmitter coil is constructed with a diameter of 16.5cm or 6.5 inches and 8.5 cm of length.
The equation for finding the inductance of a single layer air core coil is given below.
L = 0.001 N2 (a/2)2 / (114a + 254l) H
Now we are applying the desired values for the coil,
L = 0.001×22× (0.165/2)2 / ((114×0.165) + (254×0.085)) H
L = 0.674 μH
Wireless power receiver section: The receiver section consists of receiver coil, rectifier circuit and a voltage regulator IC. The AC current flowing through the transmitter coil creates a magnetic field. When we place the receiver coil with in a specific distance from this transmitter coil, the magnetic field in the transmitter coil extends to this receiver coil, and it induces an AC voltage and generates a current flow in the receiver coil of the wireless charger. The rectifier circuit in the receiver section converts this AC voltage in to DC and the voltage regulator IC helps to provide a constant limited regulated output voltage to the load for charging the low power devices. Here we are using LM 7805 voltage regulator IC. It is used because the IC gives a regulated 5V as its output and it don’t allow more than 5V to the output.
The circuit diagram for Receiver section is given below.
Fig 3 : Receiver section circuit
The receiver coil in the wireless power receiver section is constructed using 18 AWG copper wire having diameter of 8cm.The equation for finding the inductance of a single layer air core coil is given below.
L = 0.001 N2 (a/2)2 / (114a + 254l) H
Now we are applying the desired values for the coil,
L = 0.001×32× (0.08/2)2 / ((114×0.08) + (254×0.01)) H
L = 1.235 μH
The whole circuit section of our project is shown below.
Fig 4 : Wireless Power Transmission Mobile Charger Circuit using Inductive Coupling
Transmitter Section
Resistors:
Diodes:
Transistors:
Receiver Section:
Capacitors:
From this Wireless power transmission mobile charger circuit using inductive coupling experiment, we conclude that wireless charging through inductive coupling is a better way for future energy transmission systems,that is witricity (wireless electricity) because with this technology we can transfer power wirelessly to charge electronic equipment, vehicles, etc.
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