Gadgets like phones, iPods, smartwatches, etc. have become an important part of our life. They all face one problem, and that is the need to charge after regular usage. It becomes a major concern when you are in a place where electricity is not available. One of the solutions to these kinds of problems is to depend on the renewal energy sources. There are different types of renewable energy sources like wind, tidal, solar, etc. In today's project, we are going to use solar energy to charge our mobiles. To convert solar energy into electricity, we will need solar panels. We will see how a solar panel works and design a solar mobile phone charger circuit to charge our mobile phone as well as to protect the battery from overcharging.
- Solar panel (6V, 80mA) – 2
- Micro USB cable -1
- LM317 Voltage Regulator - 1
- BC547 NPN Transistor -1
- Small Breadboard
- Potentiometer (10K)
- 1N5819 Diodes - 2
- Resistors 100 Ohms and 150 Ohms - 2
- 5.6V 1N4734A Zener Diode - 1
Working of the Solar Panel
Solar cells are usually made out of silicon wafers. The silicon atoms in the solar cells form 4 strong bonds with its neighboring silicon atoms. By having these strong bonds, the electrons will stay in one place, and no current flow is seen. These solar cells usually have two layers of semiconductors. The top layer of the solar cell is doped with phosphorous to convert it into an n-type semiconductor, and the lower layer is doped with boron to convert it into a p-type semiconductor. The N-type layer has excess electrons, and the p-type layer has extra holes. When light particles strike the solar cell, the photons present in the light will have enough energy to knock the electrons from their bond, leading it to move towards the N-side, and the hole (formed by the absence of an electron) will move towards the P-side. The movable electrons are then collected at the thin metal material present at the top of the solar cell. If an external circuit is connected to these metal materials, the electrons will flow into the external circuit and then reach the conductive aluminum sheet present at the back of the solar cell. The electron then settles in the hole which is present in the P-type layer of the solar cell. Each solar cell has a voltage of 0.5V to 0.6V. The solar cells are connected in series to get the required voltage. Usually, 12 solar cells connected in series are sufficient to charge a mobile phone. There are three types of solar panels. They are Monocrystalline, polycrystalline, and thin-film. In our project, we are going to use two 6V 80mA solar panels. We are connecting the two solar panels in series to get a voltage of 12V and 80mA. The below pic shows the single mini solar panel which can generate an output voltage of 6V with a max current of 80mA.
The below pic shows the series connection of two mini solar panels, which can generate an output of 12V with a max current of 80mA. You can increase the output current by connecting extra solar panels in parallel and each parallel connection must be having two solar panels connected in series to supply 12V. So to get an 800mA output current, you will be needing 20 solar panels.
LM317 Voltage regulator
LM317 is a variable voltage regulator. By using LM317, we can vary the voltage up to 37V with a max current of 1.5A. To get the variable output voltage, the below circuit is used.
The output voltage can be calculated by using the below formula:
Vout = 1.25(1 + (R2/R1))
Now, by varying the value of the resistor R2, you can vary the output voltage.
Note: Even though the output voltage is dependent on the external resistors connected to the LM317, the input voltage should be greater (minimum of 3V) than the desired output voltage.
I have used an old USB to micro USB cable to charge the mobile phone with our solar mobile phone charger circuit. I have removed the USB, and now the cable contains a micro USB connector, which is used to connect to the mobile phone and 4 wires on the other end of the cable. The micro USB cable consists of 4 pins. Two for transferring power and another two for transferring data. The pinout of the micro USB cable needed for transferring power is shown below.
After knowing the pinout, it's time to know the wires connected to these pins on the other end of the cable. To determine which wire is connected to which pin, I have used a multimeter in continuity mode. In this way, I found the wires needed to connect to the output of our circuit.
Solar Mobile Phone Charger Circuit Diagram
The circuit diagram shown below consists of voltage and current regulation along with the over-voltage protection circuit. The connections are as follows: the anode terminal of the diode (D1) is connected to the positive terminal of the solar panel, and the cathode terminal of the diode (D2) is connected to the input pin of the LM317 voltage regulator. The output terminal of the LM317 is connected to the anode terminal of the diode (D2), and the cathode terminal of the diode (D2) is connected to the cathode terminal of the Zener diode. The anode terminal of the Zener diode is connected to the base of the BC547 transistor through a 100 Ohm resistor. The collector terminal of the BC547 transistor is connected to the output pin of the LM317 voltage regulator through a 150 Ohm resistor. The emitter terminal of the BC547 transistor is connected to the GND. The adjust pin of the LM317 is connected to the potentiometer’s variable end and the collector terminal of the BC547 transistor. One of the potentiometer’s fixed end is connected to the GND. The wire connected to the VCC pin of the micro USB cable is connected to the cathode terminal of the Zener diode and the wire connected to the GND pin of the USB cable is connected to the GND.
Working of this solar powered cell phone charger circuit
The working of the solar mobile charger circuit is simple to understand. At first, place the whole setup in a place where you can get the maximum solar rays. To get the desired output voltage from the circuit, adjust the potentiometer (Use a multimeter to measure the output voltage of the circuit). Once we got the desired voltage (5V will be sufficient to charge a cell phone), connect the micro USB to the mobile phone. If there is proper solar radiation available for the solar panel, the phone will get charged.
Let us see how our circuit protects the battery from overcharging. Before understanding the protection from overcharging, let us understand a little bit about Zener diode. Zener diode is similar to that of a normal diode, but the only difference is that when connected in reverse bias, at certain input voltage, the Zener diode will start conduction. The voltage at which the Zener diode conducts in reverse bias is called reverse voltage or Zener voltage (Vz). If a Zener diode of Vz 5V is connected in the reverse bias and applied an input voltage higher than that of Vz, the Zener diode will start to conduct even in reverse bias mode, but the voltage parallel to Zener diode will always be 5V. Now, coming to the overcharge protection, If the user sets the desired output voltage (by varying the potentiometer) to 5V and choose a Zener diode of Vz = 5V, the circuit works fine until the battery at the charging end is below or equal to 5V. Once the voltage of the battery at the charging end is more than the 5V, the Zener diode will start to conduct in the reverse bias (as the Zener voltage is 5V). This makes the transistor BC547 to operate in forward bias mode, which cuts off the R2 resistance from the circuit and the output voltage from our circuit will be 1.25 volts (from the formula of LM317, keep R2 =0). This voltage is not sufficient to charge our battery. In this way, our circuit will not charge our battery once it reaches the required voltage, and our battery is protected from overcharging.