In this post we learn how to build a simple solar LED with battery charger circuit for illuminating high power LED (SMD) lights in the order of 10 watt to 50 watt. The SMD LEDs are fully safeguarded thermally and from over current using an inexpensive LM 338 current limiter stage.  The idea was requested by Mr. Sarfraz Ahmad.

Technical Specifications

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.
Sorry to bother you but I know about your capabilities and expertise in electronics and sincerity to help and guide the beginnings like me.I have seen this circuit some where for 12 vdc. 

I have attached to SMD ,12v 10 watt, cap 1000uf,16 volt and a bridge rectifier you can see the part number on that.When I turn the lights on the rectifier starts to heat up and the both SMDs as well. I am afraid if these lights are left on for a long time it may damage the SMDs and rectifier. I don not know where  the problem is. You may help me. 

I have a light in car porch which turns on at disk and off at dawn. Unfortunately due to load shedding when there is no electricity this light remains off till the electricity is back.

I want to install at least two SMD (12 volt) with LDR so as soon the light turns off the SMD lights will turn on. I want to additional two similar light elsewhere in the car porch to keep the entire are lighted.I think that if I connect all these four SMD lights with 12 volt power supply which will get the power from UPS circuit.

Of course it will put additional load on UPS battery which is hardly fully charged due to frequent load shedding.  The other best solution is to install 12 volt solar panel and attach all these four SMD lights with it. It will charge the battery and will turn the lights On/OFF. 

This solar panel should be capable to keeps these lights all the night and will turn OFF at dawn.Please also help me and give details about this circuit/project.

You may take your time to figure out how to do that.I am writing to you as unfortunately no electronics or solar product seller in our local market is willing to give me any help, None of them seems to be technical qualified and  they just want to sell their parts.

Sarfraz Ahmad

Rawalpindi, Pakistan


The Design

In the shown 10 watt to 50 watt 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.

Calculating the Resistor Values

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.

Need Help? Please send your queries through Comments for quick replies!