Hybrid Solar Charger

D. Mohankumar is an associate professor at Government College for Women, Thiruvanthapuram, Kerala.


LED2 indicates charging of the battery. Capacitor C3 is provided for clean switching of transistor T1. Diode D2 protects T1 from back EMF and diode D3 prevents the discharge of battery current into the circuit.

When output from the solar panel drops below 12 volts, output of the comparator turns low and the relay de-energizes. Now the battery gets charging current from the transformer-based power supply through the normally-closed (N/C) and common contacts of the relay. This power supply comprises step-down transformer X1, rectifying diodes D4 and D5, and smoothing capacitor C4.

Fig. 3: An actual-size, single-side PCB for the hybrid solar charger
Fig. 3: An actual-size, single-side PCB for the hybrid solar charger
Fig. 4: Component layout for the PCB
Fig. 4: Component layout for the PCB

Download PCB and component layout PDFs: click here

Construction and testing

An actual-size, single-side PCB for the hybrid solar charger is shown in Fig. 3 and its component layout in Fig. 4. After assembling the circuit on PCB, enclose it in a suitable box. Use high-gauge (thick) wires to connect the solar panel and the battery to the circuit.

To test the circuit for proper functioning, remove the solar panel from connector SP1 and connect a DC variable voltage source. Set some voltage below 12V and slowly increase it. As the voltage reaches 12V and goes beyond, the logic at test point TP2 changes from low to high. The transformer-based power supply voltage can be checked at test point TP3.



  1. What is the advantage if we are charging the solar cells with the A.C current? It’s just storing the energy and simply acting as common inverters we use in our everyday life.

  2. transformer secondary connected across two diodes that direction of diode is correct? if will change any one diode direction what it will happen?

  3. Hi, i have two queries,
    1) why the transformer is full time on in all time? Can we turn off transformer supply when it transformer is not in use.
    2) This circuit automatically turns ON charging of battery, but can it automatically turn OFF battery charging when battery is full??

  4. @efylab.Can I use my car alternator in place of the solar cell and then connect the hybrid solar charger to it? Will it charge my 100amps battery?

  5. Actually i made this circuit on GPB (general purpose board) , the o/p is approx 17v and also battery is charging ; however,realy is not operate and charging of battery is not automatically stop

  6. This circuit is really working fine but in cloudy condition sometimes relay is chattering rapidly from solar to mains. When light intesity changes from low to high or vice-versa problem disappear and yhe circuit again works accordingly on the intensity of the sunlight. Could you solve me out please.


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