Solar Inverter/Charger Circuit for Science Project

The following article explains a simple solar inverter circuit consisting of it’s own battery charger and an automatic changeover relay system for switching the battery to the inverter in the absence of solar energy. The circuit was requested by Ms. Swati Ojha.

The design:
The circuit mainly consists of two stages viz: the inverter, and the automatic relay changeover.
During day time for so long the sun light remains reasonably strong, the panel voltage is used for charging the battery and also for powering the inverter via the relay changeover contacts.
The automatic changeover circuit preset is set such that the associated relay trips OFF when the panel voltage falls below 13 volts.
The above action disconnects the solar panel from the inverter and connects the charged battery with the inverter so that the output loads continue to run using the battery power.
Circuit Description:
Resistors R1, R2, R3, R4 along with T1, T2 and the transformer forms the inverter section. 12 volts applied across the center tap and the ground starts the inverter immediately, however here we do not connect the battery directly at these points, rather through a relay changeover stage.
The transistor T3 with the associated components and the relay forms the relay change over stage  The LDR is  kept outside the house or at a position where it can sense the day light.
 The P1 preset is adjusted such that T3 just stops conducting and cuts off the relay in case the ambient light falls below a certain level, or simply when the voltage goes below 13 volts. This obviously happens when the sun light becomes too weak and is no longer able to sustain the specified voltage levels.
However as long as sun light remains bright, the relay stays triggered, connecting the solar panel voltage directly to the inverter (transformer center tap) via the N/O contacts. Thus the inverter becomes usable through the solar panel during day time.
The solar panel is also simultaneously used for charging the battery via D2 during day time so that it charges up fully by the time it gets dusk.
The solar panel is selected such that it never generates more than 15 volts even at peak sun light levels.
The maximum power from this inverter will not be more than 60 watts.A MOSFET  based solar inverter can be witnessed HERE

Parts List for the proposed solar inverter with charger circuit intended for science projects.
R1,R2 = 100 OHMS, 5 WATTS
R3, R4 = 15 OHMS, 5 WATTS
T1, T2 = 2N3055, MOUNTED ON SUITABLE HEATSINK
TRANSFORMER = 9-0-9V, 3 TO 10 AMPS
R5 = 10K
R6 = 0.1 OHMS 1 WATT
P1 = 100K PRESET LINEAR
D1, D2 = 6A4
D3 = 1N4148
T3 = BC547
C1 = 100uF/25V
RELAY = 9V, SPDT
LDR = ANY STANDARD TYPE
SOLAR PANEL = 17 VOLTS OPEN CIRCUIT, 5 AMPS SHORT CIRCUIT CURRENT.

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