University of California
Type of paper: Thesis/Dissertation Chapter
Piezoelectric Charger: Pcb Design Project Report
Alternative energy sources are one of the main focuses in research and development in many aspects in engineering, especially electronic devices. By targeting renewable energy sources, not only will the amount of pollution decrease but also help in preserving the rapidly depleting resources available on Earth. Compared to all the other energy sources, electrical energy conservation has the greatest potential and most opportunity. The objective of our circuit design project was to create a portable mobile device charger that will operate using vibration energy sources other than the standard charging mechanisms. As most of latest mobile phones are capable of charging from Universal Serial Bus (USB) we decided to design our circuit capable of supplying standard USB voltage and current rating. The circuit consists of a piezoelectric buzzer, a rectifier with a storage capacitor, an amplifier section, regulator and a switching mechanism that controls the energy flow into the battery. Experimental results showed that the method is feasible for practical use.
The number of mobile phones sold yearly is growing tremendously. These devices rely on battery power for their operation which needs frequent recharging. Situations where external power sources are not accessible and the battery getting drained at the most inconvenient time are common nowadays. The feasibility of providing power to these portable devices on the go is gaining much research interest due to this fact. Advances in semiconductor technology have brought down the power consumption modern portable electronics considerably. So energy harvesting is a feasible approach for operating these low power devices. Energy harvesting or scavenging is the processes of capturing the energy from environmental sources and convert it into usable electrical energy As these devices are meant to be carried around, one suitable way of power scavenging is to harvest the usually wasted kinetic energy due to human physical activities. Piezoelectric energy harvesting can be used in order to do so. A transducer is a device capable of converting one form of energy into another. A piezoelectric transducer is capable of converting energy due to mechanical vibrations into electrical energy and vice versa.
2 BLOCK DIAGRAM AND DESCRIPTION
The energy produced by piezoelectric transducer is continuously varying depending on the vibration level and also does not possess enough power for
directly act as a power supply. So the generated electrical energy thus has to be conditioned to a constant sufficient power level for practical use. The piezoelectric harvesting system includes two stages namely harvesting structure and the harvesting circuit. The harvesting structure consists of piezoelectric buzzer, typically placed such that it is susceptible to vibrations. The energy produced by the harvesting structure is collected by the harvesting circuit where it is conditioned for practical use.
Rectifier & Storage Capacitor
Voltage Buffer and Amplifier
Figure 1: Block Diagram
The first stage of harvesting circuit is to convert the alternating current (ac) produced by the harvesting structure into direct current (dc) by rectification. The output from a piezoelectric buzzer is only in the range of a few millivolts. A capacitor is used as an intermediate storage device. The voltage stored has to be conditioned into the required USB voltage and current voltage rating. A voltage buffer followed by an amplifying circuit serves that purpose. The voltage amplified will be regulated and is supplied to the target device battery through a switching mechanism. Each block is described in detail in the following sections.
2.1 PIEZOELECTRIC BUZZER
A piezoelectric buzzer is an electro-acoustic transducer which is capable of transforming ac voltages to pressure waves. It will in turn generate an ac voltage across its terminals when stimulated with pressure variations. It is made of piezoelectric crystals placed between two conductors. If the buzzer is subjected to vibrations we can obtain output voltage across its terminal in the range of few millivolts.
Figure 2: Piezoelectric and Converse Piezoelectric Effect
Figure 3: Piezoelectric Buzzer
2.2 RECTIFIER AND STORAGE CAPACITOR
The amount of energy scavenged using piezoelectric buzzer is alternating and is too low to directly power a load; hence an intermediate rectification and storage stage is required. We make use of a bridge rectifier to achieve this purpose. The four-diode bridge converts both polarities of the input waveform into positive voltage at the output. The full-wave bridge rectifier gives us a greater mean dc value with less superimposed ripple while the output waveform is twice that of the frequency of the input supply frequency. The average dc output level of rectifier can be made higher by connecting a suitable smoothing capacitor across the output of the bridge circuit. The capacitor connected to the output node acts as a charge reservoir, which smooth the output voltage and makes the circuit more like constant dc voltage source. There are certain conditions to be met for choosing a suitable smoothing capacitor. The working voltage of capacitor must be higher than the no-load output value of the rectifier and its capacitance value determines the amount of ripple that will appear superimposed on top of the dc voltage. If the capacitance value is too low then the
capacitor has little effect on the output waveform. If the connected smoothing capacitor value is sufficient and the load current is not too large, the output voltage will be almost as smooth as pure dc.
Figure 4: Bridge Rectifier with Storage Capacitor
2.3 VOLTAGE BUFFER AND AMPLIFIER
A voltage buffer is a unity gain amplifier which transforms electrical impedance from one circuit to another. The buffer stage is used to prevent the loading of a preceding piezoelectric buzzer by the succeeding amplifying stage. The piezoelectric buzzer output does not have capability to produce voltage or current corresponding to drive circuitry it is connected to. If we try to connect the capacitor directly to the amplifier stage loading might occur. The expected voltage may change and circuit might behave in undesired manner. The voltage follower provides a buffer, eliminating the loading effect. The intermediate buffer amplifier prevents the amplifier circuit from loading the first circuit unacceptably and interfering with its desired operation. There is no voltage gain during the buffer stage but there will be a sufficient amount of gain in current. We use op-amp configured in non-inverting mode to achieve the purpose.
Figure 5: Voltage Buffer
Gain Av= 1 + (Rf/R1). Since output and inverting input are short circuited, Rf=0
Since there is no R1 to ground, it can be considered as an open circuit and so R1 = ∞ Therefore (Rf/R1) = (0/∞) = 0. Therefore Av = 1 + (Rf/R1) = 1+0 =1. Since we need an output of 5 volts we need to amplify the stored voltage in the capacitor. For that we use an op-amp as a non-inverting amplifier. This configuration uses negative feedback to stabilize voltage gain.
Figure 6: Non Inverting Amplifier
Gain Av= 1 + (Rf/R1). We have Selected Rf=2.2KΩ and R1 = 1KΩ Therefore (Rf/R1) = (2.2/1) = 2.2 Therefore Av = 1 + (Rf/R1) = 1+2.2 =3.2 So now we
will have an output of more than 5 volts at the output.
2.4 SWITCHING CIRCUIT
For charging a mobile phone from USB port we require more drive current than what we can obtain from the previous sections. This problem can be solves by using the amplifier output to drive a transistor which in turn controls the load. Transistors can be used as current amplifiers in such a way that the collector current obtained as base current multiplied by transistor dc gain (hFE). This can be achieved using a PNP high side switch. But to isolate the previous stages from the higher voltages at base of PNP transistor we use a NPN transistor in front of it. That is a low side switching
before PNP stage. Whenever there is no current flowing into the base of the NPN transistor there would not be any current flowing into the base of the PNP transistor either.
Figure 7: Switching Mechanism
In our circuit we use the resistors at base of transistors to limit the current flowing into them. The resistor between emitter and base of PNP is used to pull the transistor to High (Off State) when NPN is off. NPN inverts the input to the PNP transistor. When the piezoelectric output is low the capacitor voltage will be low. So the base voltage of NPN will be low so it will be turned off. Whenever there is enough voltage at base of NPN it will start conducting driving PNP to Low (On State). PNP starts conducting and will source 9 volts to the regulator stage. The reason why we use high side switching is to be able to switch a high voltage using relatively low voltage.
2.5 VOLTAGE REGULATOR
The USB standard output voltage rating is 5 volts. In order to regulate the output from switching stage we use a 5 volt fixed positive voltage regulator.
Figure 8: Voltage Regulator
The 7805 positive linear voltage regulator is used in our circuit. A steady output voltage is maintained by varying the resistance in accordance to the load. The reason for choosing 7805 is because of its advantage of not requiring any additional components to perform the purpose. The capacitors are used to improve the stability and transient response of the output.
3 SCHEMATIC DIAGRAM
Figure 9: Schematic Diagram
4 PCB LAYOUT
Figure 10: PCB Layout
5 COMPONENTS USED
Figure 11: Components Used
The circuit was realized on both bread board and Printed Circuit Board (PCB) and was tested. The circuit was working and provided desired output as expected. If we can implant the mechanism on a shoe we can provide enough power to charge any mobile phone capable of charging using USB power on the go. It needs a person wearing the shoe only a few steps to start charging the device. The limitation of the circuit is that we can use it for