What is Relay?
Relay is an electrical device which makes contact or disconnect between source and load circuit depending on magnetic energy of it. It has a magnetic coil. The coil is energized by a source externally then it makes short path or allows current flow. Removing source from relay coil it makes path open which disconnection two side circuit.
Figure: Relay not energized
Figure: Relay energized (Photo by jordi3736)
Above two pictures shows Relay operation. Relay contact is normally open without coil energized. Using external source coil energized and relay makes contact with secondary side circuit. In this way Relay operates.
Figure: PV Solar System
A circuit diagram is shown for PV solar system. A PV cell is connected with a battery. When sunlight excites PV the battery gets charge. The whole method is driving using some components. A controlling circuit or driving circuit is connected to a BJT having base 1KΩ resistor which protects the transistor from burning. BJT transistor collector point connects to a Relay. Two diodes are used one is connected with battery to stop reverse current flow from battery when the battery is fully charged. Another diode connected across the Relay coil which is used as freewheeling diode.
At the charging time
Figure: PV Charge controlling with the help of Relay
When controlling circuit sense battery charge low it sends 5V supply to the base of BJT then the emitter-collector path becomes short. BJT allows flow of current. The coil of Relay gets current from the control circuit which excites the coil and the contact of the Relay remains connected to the battery storage. Sunlight hits the solar panel and current flows via Relay to the battery until the battery fully charge. Driving circuit and battery common ground connection are sorted. In this way Relay operate to charge the battery with the help of control circuit and BJT. Driving circuit or controlling circuit is an intelligent circuit which maintains the whole system.
After the charge completion
Figure: Open condition after full charged battery
Control circuit senses that battery is fully charged. It sends 0 voltage to the base of BJT. 0V makes open path between emitter and collector path. No current can flow through the transistor. So the coil of Relay cannot remain energized, it closes the flow of current path between PV panel to battery. We know if we flow current through a coil it energized and when we stop current flow the coil does not loss energy easily, it releases energy to the connected circuit which can make difficulty for us. To solve the problem a diode is connected simply across the coil, the release of current of the induction coil flows within the diode which does not impact on the other circuit.
Common grounding problem Solution Using Opto Coupler
Figure: Common grounding problem solution Using Opto Coupler
What is Opto coupler?
Opto Coupler is a device with LED and transistor coupled. When LED gets signal from source it emits light which activates the transistor emitter-collector path as short circuit. It sends a small voltage to secondary side circuit. Opto coupler is also called photo transistor.
Figure: Battery charging using opto coupler
Total operation is executed by controlling circuit with help of some component such as Zener diode, opto coupler, MOSFET. When voltage level of the battery goes down driving circuit sense and send 5V signal to opto coupler LED. The LED emits light and the transistor activates which also supply voltage to the MOSFET gate. We used here N channel enhancement MOSFET. Getting positive voltage the electrons attracts to source terminal and creates an electron layers which makes a short path to flow current between source to drain. Thus current flows from PV solar cell to battery to MOSFET to PV. In this way battery gets charged. Battery, controlling circuit and opto coupler grounding point are shorted same. Zener diode, MOSFET, Opto coupler transistor, PV panel grounding remains same.
Figure: After charging completion
When the battery fully charged controlling circuit sense and sends 0V to the LED. The LED bulb goes off, transistor get no voltage. It cannot send any signal to MOSFET. There is no layer between two n channels; electron cannot pass through the MOSFET. It acts like open circuit. In this way battery saved from over load protection. Another problem when the battery gets fully charged it tries to discharge itselt to any path. If battery start discharging via same path as PV panel the system will break. To solve this problem a diode is connected in series to stop the reverse current flow from battery.