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 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 o 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.
To harvest the energy thus produced, the piezoelectric element has to be attached to a vibrating environment and use an interface circuit to transfer the produced energy into the load. The piezoelectric buzzer is an example of piezoelectric transducer. The ease of implanting the device makes it the suitable choice for harvesting the energy produced during human physical activity. Therefore “A Portable Mobile Charger using Piezoelectric Energy” is designed to charge the mobile phone battery whenever it is needed, especially in an emergency situation.
The circuit operates by using available vibration source surrounding our life to generate its own electricity and thus can be used to charge mobile phone batteries and is be handy when the need arises. This allows people to move around with their daily activities while keeping their mobile phone fully charged. In the recent years, standardized chargers and connectors for mobile phones and other portable gadgets have been introduced. USB as been the standard for charging pretty much any small electronic gadget nowadays.
The standard USB power rating will be V mamma which means that a device connected will receive V and be able to draw a maximum of mamma. The bottom line is that if a circuit capable of converting the piezoelectric energy generated into the above mentioned ratings, then it is suitable to act as a USB charger regardless of different makes of mobile phones. With the exceptions of certain companies blocking USB charging, we can be sure that any phone supporting the feature can be charged using this. This Portable Mobile Charger using Piezoelectric Energy is a cheap and convenient alternative to conventional charging methods. 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. Piezoelectric Buzzer Rectifier & Storage Capacitor Switching Circuit Voltage Regulator Target Device 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 millions. A capacitor is used as an intermediate storage device.
The voltage stored has to be conditioned into the squired 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 millions. 5 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 working voltage of capacitor must be higher than the no-load output value of the citified 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 6 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. 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 colorimetric 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 s no voltage gain during the buffer stage but there will be a sufficient amount of gain in current. We use pop-amp configured in non-inverting mode to achieve the purpose. Figure 5: Voltage Buffer Gain OAF 1 + (RFC/RE). Since output and inverting input are short circuited, 7 Since there is no RI to ground, it can be considered as an open circuit and so RI -? Therefore (RFC/RE) = (mm) = O.
Therefore 1 + (RFC/RE)= 1+0 . Since we need an output of 5 volts we need to amplify the stored voltage in the capacitor. For that we use an pop-amp as a non-inverting amplifier. This configuration uses negative feedback to stabilize voltage gain. Figure 6: Non Inverting Amplifier Game 1 + (RFC/RE). We have selected RFC=2. SKY and RI = Therefore (RFC/RE) = (2. 2/1) = 2. 2 Therefore 1 + (RFC/RE)= 1+2. 2=3. 2 So now we will have an output of more than 5 volts at the output. 2. 4 SWITCHING CIRCUIT 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 (wife). This can be achieved using a PEN high side switch. But to isolate the previous stages from the higher voltages at base of PEN transistor we use a NP transistor in front of it. That is a low side switching 8 before PEN stage.
Whenever there is no current flowing into the base of the NP transistor there would not be any current flowing into the base of the PEN 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 PEN is used to pull the transistor to High (Off State) when NP is off. NP inverts the input to the PEN transistor. When he piezoelectric output is low the capacitor voltage will be low. So the base voltage of NP will be low so it will be turned off.
Whenever there is enough voltage at base of NP it will start conducting driving PEN to Low (On State). PEN 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.