I am grateful for your help.
Please keep in your mind that I am in 64 and just started taking interest in electronics just to have good time in this old age. Basically I am certified mechanical engineer from Germany 35 years ago and worked overseas for many years and left many years ago due to personal problems back home.
In the shown SMD solar LED light circuit with automatic charger above, we see the following stages:
A solar panel
A couple of current controlled LM338 regulator circuits
A changeover relay
A rechargeable battery
and a 40 watt LED SMD module
The above stages are integrated in the following explained manner:
The two LM 338 stages are configured in standard current regulator modes with using the respective current sensing resistances for ensuring a current controlled output for the relevant connected load.
The load for the left LM338 is the battery which is charged from this LM338 stage and a solar panel input source. The resistor Rx is calculated such that the battery receives the stipulated amount of current and is not over driven or over charged.
The right side LM 338 is loaded with the LED module and here too the Ry makes sure that module is supplied with the correct specified amount of current in order to safeguard the devices from a thermal runaway situation.
The solar panel voltage specs may be anywhere between 18V and 24V.
A relay is introduced in the circuit and is wired with the LED module such that it's switched ON only during the night or when it's dark below threshold for the solar panel to generate the required any power.
As long as the solar voltage is available, the relay stays energized isolating the LED module from the battery and ensuring that the 40 watt LED module remains shut off during day time and while the battery is being charged.
After dusk, when the solar voltage becomes sufficiently low, the relay is no longer able to hold its N/O position and flips to the N/C changeover, connecting the battery with the LED module, and illuminating the array through the available fully charged battery power.
The LED module can be seen attached with a heatsink which must be sufficiently large in order to achieve an optimal outcome from the module and for ensuring longer life and brightness from the device.
The indicated limiting resistors may be calculated from the given formulas:
Rx = 1.25/battery charging current
Ry = 1.25/LED current rating.
Assuming the battery to be a 40 AH lead acid battery, the preferred charging current should be 4 amps.
therefore Rx = 1.25/4 = 0.31 ohms
wattage = 1.25 x 4 = 5 watts
The LED current can be found by dividing its total wattage by the voltage rating, that is 40/12 = 3.3amps
therefore Ry = 1.25/3 = 0.4 ohms
wattage = 1.25 x 3 = 3.75 watts or 4 watts.
Limiting resistors are not employed for the 10 watt LEDs since the input voltage from the battery is on par with the specified 12V limit of the LED module and therefore cannot exceed the safe limits.
The above explanation reveals how the IC LM338 can be simply used for making an useful solar LED light circuit with an automatic charger.