The Powers of Induction

Team Members

Blake Bordelon, Elizabeth Onder, and Andrew O'Sullivan (TA)

Overview

The goal of our project is to create a wireless charging pad capable of transferring power from a base station powered by a Lithium Ion battery to a receiver system that outputs power to a standard port at 5 V and .5 A. The device will consist of a transmitter base and a receiver, each of which will house an inductive coil capable of coupling at close distances. Unlike many wireless chargers on the market, our transmitter base will be powered by a 7.4 V lithium ion battery, rendering our transmitter pad portable. Switching and power delivery will be handled by an Arduino in the transmitter pad. The Arduino will switch a MOSFET transistor at a frequency between 10-20 kHz to allow for the delivery of alternating current to the primary coil. A battery management fail safe circuit will be implemented in the transmitter base to prevent excessive spikes in current or voltage across our lithium ion battery. Information from this component(s) will feed into the Arduino allowing for it to switch open the MOSFET permanently or to keep switching but at a different frequency. The alternating current in the primary coil should in turn produce a varying magnetic flux through the receiver coil, driving an oscillating current through the receiver. Both coils will be in parallel with capacitors in order to reduce some losses to parasitic impedance. Additional capacitors may be placed to filter the signals and to snub the inductive kickback on the transistor. The current passing through the receiver will then pass through a full bridge rectifier constructed from a diode bridge in parallel with a capacitor. This unregulated DC voltage (hovering around 7-10 V) will then pass through a 7805 Voltage Regulator to produce the desired 5 V output. Because we can control the switching frequency directly with the Arduino, we aim to experiment with our code in order to improve power delivery.

Proposed Design Diagram

Objectives

• The objective of this project is to create a working wireless power delivery system as described above. The device should be powered by the Li Ion battery and should be capable of transferring power over the coupled coils from several millimeters distance.
• The transmitter and receiver should both be housed in a 3D printed body.
• The transmitter should be less than 7 lbs to be portable.
• The receiver should be less than 5 lbs to be portable.
• For the demo, we intend to demonstrate power transfer by lighting an LED with our regulated power on the receiver end. Starting with the receiver module far away from the transmitter and the LED clearly off, we can move the receiver coil closer to the device. The LED should power on when the distance between the two coils becomes sufficiently small.

Challenges

• Lithium ion batteries can be dangerous if exposed to high temperatures or large fluctuations in current or voltage. Thus maintaining appropriate and safe conditions for the battery presents a significant challenge. To overcome this challenge, we aim to include a temperature fail safe from our temperature sensing IC as well as a battery management regulator IC. Should the temperature get too high, a switch should open to disconnect the battery from the circuit.
• Another significant challenge is the magnitude of the alternating voltage required in the receiver. The 7805 voltage regulator requires a voltage from ~7-18 V in order to produce the 5 V output. Thus the voltage across the coupled receiver coil must be around this magnitude. If we only use a 7.4 V DC battery on the transmitter side of the system, then we may not produce a high enough voltage on the receiver end to successfully produce a 5 V signal on the other side of the rectifier and 7805 voltage regulator. We may have to choose or design a receiver coil with more turns or a transmitter coil with less turns. Alternatively, we could use a Lithium Ion battery of a higher voltage (12 V) but this could potentially exacerbate our safety concerns.
• We must learn:
• How to use PSpice/MutliSim or some other circuit simulation software to create a schematic of our design and simulate its behavior under different parameters.
• How to use AutoCad or equivalent 3D design software to design a case for the
• How to program on Arduino to implement the safety features described above as well as switching a transistor at a frequency on the order of 10 kHz.

Gantt Chart

Budget

• Two wireless charging coils: 2 x $8.22 = $16.44 (Link)
• Battery Management IC: $4.50 (Link)
• 7805 Voltage Regulator: $8.51 (Link) (Link)
• Temperature Sensor: $0.69 (Link)
• 7.4 V Lithium Ion Battery: $19.99 (Link)
• Lithium Ion Battery Charger: $14.99 (Link)
• Bridge Rectifier: $8.81 (Link) (Link)
• Breadboard: Supplied by Class
• Arduino: supplied by class
• 3D-printed parts: supplied by class
• MultiSim Circuit simulation software: supplied by school
• N Channel MOSFET 60V 30A: $0.95 (Link)
• Resistors: $0.10+$0.10+$0.1+$0.56+$1.55= $2.41 (Link 1) (Link 2) (Link 3) (Link 4) (Link 5)
• Spark Fun Capacitor Kit: $6.95 (Link)
• Diodes (for Rectifier Bridge): $5.38 (Link)
• LED's: Supplied by Class
• Tenergy 3.7 V Lithium Ion Battery: $9.99 (Link)
• Round Magnets: $5.98 (Link)
• Prototype Boards: $10.59 (Link)

Total before tax and shipping: $120.68

Design and Solution

Transmitter

The problem we initially faced was figuring out how to generate an alternating current from a DC power supply on the transmitter side. To accomplish this, we chose to implement a switching MOSFET. The transistor acts like a gate that opens and closes as the gate-to-source voltage passes its threshold value. To change the gate voltage, we decided to have an Arduino supply a square pulsing signal. After learning about different switching techniques, Humberto helped us realize that for our purposes we needed to switch on the low side of the circuit to ensure that the voltage difference between the gate and source varies between 0 and 5 V (the output of the Arduino). To make sure that the transistor worked, we first constructed a simple circuit with a 5 V DC source, a 10k-ohm resistor and the MOSFET (switched by the Arduino) on the low side. We measured the voltage across the resistor with an oscilloscope to ensure that it varied from 0 to the DC source voltage. After proving the MOSFET worked satisfactorily, we then set out to validate our other components. The transmitter and receiver coils, for instance, needed to be tested and measured to make sure that the data sheets provided accurate information. To get a good estimation for the inductance of the coil, we constructed and probed the behavior of an RL series circuit with an oscilloscope. By varying the frequency until the voltage across the inductor equaled exactly half of the input voltage and the signals were out of phase by 45 degrees, we found the point where the magnitude of the imaginary impedance of the inductor (wL) was equal to the resistance value chosen (R). We repeated this process for all 4 of our coils, finding values that varied from 17-22 uH. Once these components were tested, we then constructed the transmitter. We were, however, reticent to test the circuit with the lithium ion battery without first illustrating that the switching circuit worked as specified and the behavior of the coil Instead of using the lithium ion battery as a DC source, we initially (naively believing we were clever for saving space/time/energy) used another Arduino pin as the DC source. While this offered a safe means to make sure that the circuit wasn't inappropriately assembled, we later discovered that the Arduino has an absolute power limit, meaning that it would not draw more current (at the 5 V DC voltage) than some absolute threshold. Any flaw in our circuit when drawing power from the battery would not have been detected. After Humberto pointed this out, we tested the circuit again with a DC power supply (set with absolute current limit 3 A) from lab with identical results. We were then poised to introduce the 7.4 V Li ion battery. Measuring the voltage between high and ground, we got a simple 0 to 7.4 V square wave. We added a 1.5 ohm resistor between the source gate of the transistor and ground. Our hope was that while only drawing a small amount of power, we would be able to measure the current in the transmitter circuit by reading in the analog voltage signal across the resistor. This idea was later abandoned as will be explained shortly.

Receiver

The next step was testing and constructing the receiver circuit. We first wanted to get a sense of how well the two coils would couple in operation. For our transmitter coil, we chose to use the coil with the smallest inductance value (17 uH) and the receiver coil with the largest value (22.7 uH). This way the voltage should be boosted somewhat on the receiver side. The motivation for this decision followed from the requirement of the 7805 Voltage Regulator's input specifications. We were required to input a DC voltage greater than 7 V. We connected our receiver coil in parallel to a resistor and measured the voltage across the resistor with an oscilloscope. This signal also consisted of spikes with somewhat sharper peaks (this was consistent with the basic observation that the receiver signal should be the first time derivative of the transmitter signal). We then rectified the receiver signal with a diode bridge and a capacitor. When measuring this now DC output voltage we typically got around 8 V. We then connected this to the 7805 voltage regulator and a 1k-ohm load. We used the oscilloscope to measure the output voltage and noticed very irregular signal around 3V rather than the expected 5 VDC. The power supplied to the receiver was insufficient to supply the desired output. We noticed that if we removed the resistor from the transmitter side that the voltage across the receiver's load would increase, but was still "unstable." Thus we decided to increase the DC source on the transmitter side. After adding an additional 3.7 V (for a total voltage of 11.1 V), the receiver signal was steady at 5 VDC. We connected this to a standard USB port.

Housing

We transferred the complete circuits to prototype boards (a valuable skill that we documented in our how-to article) and trimmed the prototype boards to eliminate unnecessary material. We used AutoDesk Inventor to design a transmitter housing and a receiver case to be 3D printed. The design was uploaded to PinShape here: (Link). The transmitter housing was designed to hold the transmitter coil and the prototype board containing the transmitter circuit, as well as a couple of magnets to hold the phone in place. The receiver case was designed to be universal and simplistic, without specific slots for the circuit components. After the parts were printed, the two halves of the receiver case were rubber-banded together and attached to the receiver coil and circuit using hot glue.

Results

Consistent with our original objectives, we did create a working wireless charger capable of charging phones, watches, and other devices that charges with a standard USB connection. Because we output to a standard port, our design was even more general than the phone charger we initially conceived it to be.

During our initial demonstration of the circuit, the transistor began to inexplicably heat up. We soon discovered that the computer running the Arduino switching had powered off. After turning it back on, the demo ran smoothly.

While our project was ultimately a success, we were unable to implement some of the more sophisticated power transfer and "smart switching" techniques that we had initially hoped to devise. For instance, the resistor on the transmitter side of the circuit that could have been used to measure the current could have fed into an Arduino analog input. The switching code could also have taken this measurement into account and switched faster or perhaps left the gate open should the current get too high so that the coils would not be damaged. We were not able to implement this technique because the resistor was drawing too much power from the transmitting side of the circuit.

Future

Some future possibilities for this project could be to have the microcontroller that switches the MOSFET be powered by the batteries that power the receiver. This would make the apparatus truly portable. In addition, the transmitter could be capable of trickle charging so that the lithium ions do not need to be charged independently before being used in the circuit. Both of these issues are dangerous and would require much more electrical engineering expertise than we currently possess.

References

1. An Article from Nuts and Volts magazine about how to build an inductive charger. (Link) 2. Blog Post about a DIY wireless phone charger (Link)