The discussed automatic change over relay circuit was requested by Mr.Karimulla Baig. The circuit normally charges the connected battery at constant current through the power received from the solar panel, and reverts to DC power from an AC/DC adapter in the absence of solar energy (during night time). Let's the read the request in more details:

Technical Specifications

"Dear Sir,I tried reaching you for a long time but unfortunately i was not able to reach you please please kindly help me in designing the below circuit.

Please help me in designing the change over circuit for my battery charger. where i want to charge my 6V 4.5Ah battery from solar and AC mains when ever there is no power from solar i need to charge my battery from AC mains.

i have made the both chargers of both AC mains charger and solar Charger and i need a change over for this kindly help me in designing the change over circuit.

The problem what I'm facing is there will be always voltage across the panel even though no current, I'm facing the problem to change it to mains.

Regards, Karimulla Baig"


How the Circuit is Designed to Work

Looking at the proposed circuit diagram, we see three basic stages, on the left an IC 741 circuit, at the center a voltage regulator stage using IC LM317, while on the top an AC/DC adapter circuit.

The AC/DC adapter circuit is a simple rectified transformer power supply, designed for providing 7V DC as long as there's mains power available.

The IC317 circuit is a regulator circuit, configured for generating a constant current, 7 volts output to the 6V battery which is connected at the given points.

The pot with the LM317 IC may be adjusted to produce the required charging output for the particular battery.

The most important part of the circuit is the IC 741 stage, which is set up as a high voltage trigger circuit.

The associated preset is adjusted such that the relay activates when the solar panel voltage is above 7 volts.

The activation of the relay means the regulator circuit and the battery receive the voltage from the solar panel via the N/O contacts of the relay.

However, the moment the panel voltage drops below 7 volts, the relay switches OFF, connecting the DC adapter power with the regulator circuit, and now the battery starts getting charged through the AC/DC adapter voltage source.

The above results confirm perfect functioning of the entire circuit just as required by Mr. Baig.

R1 = Reference voltage/charging current = 1.25/Chg.Current

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