# Circuit to drive 12V 14.4W LED strip light from 20W 18V Solar panel

#### grah

Joined Sep 14, 2023
6
I’m new to solar panels and LED strip lighting. I am hoping to use a 20W 18V max solar panel to run a one metre 12V 14.4W LED light strip without a battery. I do not care about variable light output. I want the system to use sunlight from one side of my house to provide light to a hydroponic garden on the other side which is in shadow for the first half of each day. Suggestions for a DIY circuit to control current and voltage to suit the LED strip or directions on where I can find a circuit will be greatly appreciated.

#### BobTPH

Joined Jun 5, 2013
8,120
14.4W would not be very effective for growing plants. Ask anyone who grows pot inside how many watts of LED lighting they use.

Sunlight has 1360W of power per square meter. And LEDs convert less than 1/3 of the power into light, so your 14.4W LEDs produce maybe 4W of actual light.

This is enough to light up about 29 square cm at the level of sunlight. So if you want to grow plants in a 5.4 cm (2.1inch) square, it would be equivalent to sunlight.

#### Jerry-Hat-Trick

Joined Aug 31, 2022
451
How about abandoning the solar panel and, if line-of-sight will allow, using a mirror or mirrors to reflect sunlight onto the other side of the house? One piece of marine ply covered in aluminium foil should work, several pieces to form a rough parabolic reflector would be better. You then need to be able to rotate the "mirror(s)" in the vertical and horizontal axes to track the sun and make the reflected light illuminate where you want it. Mechanically not trivial, but electronically quite easy - first step, would this work for you?

#### grah

Joined Sep 14, 2023
6
14.4W would not be very effective for growing plants. Ask anyone who grows pot inside how many watts of LED lighting they use.

Sunlight has 1360W of power per square meter. And LEDs convert less than 1/3 of the power into light, so your 14.4W LEDs produce maybe 4W of actual light.

This is enough to light up about 29 square cm at the level of sunlight. So if you want to grow plants in a 5.4 cm (2.1inch) square, it would be equivalent to sunlight.
Thank you. I am interested in finding a circuit to employ the panel and strip light I have on hand. Scaling up the circuit to handle a larger panel and more than one metre of LED light strip will be my next step.

#### grah

Joined Sep 14, 2023
6
How about abandoning the solar panel and, if line-of-sight will allow, using a mirror or mirrors to reflect sunlight onto the other side of the house? One piece of marine ply covered in aluminium foil should work, several pieces to form a rough parabolic reflector would be better. You then need to be able to rotate the "mirror(s)" in the vertical and horizontal axes to track the sun and make the reflected light illuminate where you want it. Mechanically not trivial, but electronically quite easy - first step, would this work for you?
Thank you but my interest is in circuitry to match solar panels with led light strips, starting with the components I have on hand.

#### BobTPH

Joined Jun 5, 2013
8,120
There are tons of buck converter boards available cheap that will do that.

#### Ian0

Joined Aug 7, 2020
8,949
Just connect enough LEDs in series so that their total forward voltage drop corresponds to the peak power voltage of the panel, and enough in parallel to withstand the peak output current. If you have the right number of LEDs you don't need any circuitry.

#### BobTPH

Joined Jun 5, 2013
8,120
Just connect enough LEDs in series so that their total forward voltage drop corresponds to the peak power voltage of the panel, and enough in parallel to withstand the peak output current. If you have the right number of LEDs you don't need any circuitry.
He is using strips with resistance installed for 12V operation.

#### Ian0

Joined Aug 7, 2020
8,949
He is using strips with resistance installed for 12V operation.
It would be interesting to plot the load-line on the solar panel curve and see if it would actually be overloaded.

#### BobTPH

Joined Jun 5, 2013
8,120
It is a totally impractical project. To get significant extra lighting, it’s going to take 5-10 times the area of solar panels as the area he wants to illuminate.

#### Ian0

Joined Aug 7, 2020
8,949
It is a totally impractical project. To get significant extra lighting, it’s going to take 5-10 times the area of solar panels as the area he wants to illuminate.
Of course it is: LED efficiency is 40-50% panel efficiency 21%, so it will generate the same amount of light on a 12th of the area of the panels, or a 12th of the amount of light on the same area as the panels.
It's a fair bet that the area that is in the shade will be no more brightly illuminated after the LEDs have been added. After all, there is a significant amount of light that comes from the sky that is not in the direction of the sun (otherwise the sky would be black)

#### BobTPH

Joined Jun 5, 2013
8,120
Of course it is: LED efficiency is 40-50% panel efficiency 21%, so it will generate the same amount of light on a 12th of the area of the panels, or a 12th of the amount of light on the same area as the panels.

If the panel coverts 21% of the light to electricity and the LED converts 50% of that electricity to light, you get 10.5% if the light hitting the panel out the other side, so it would take nearly 10 square meters of panel to light 1 square meter at full sun.

Also, the figure I found for efficiency of LEDs in turning electrical Watts into light Watts was 30%. And the DC to DC conversion will reduce it by another 10%.

So my calculation was:

.21 x .30 x .90 = 5.7% of light transferred.

Perhaps I exaggerated with the word significant. With 5x the area you would get 28.5% if full sun, which would definitely be significant.

#### Ian0

Joined Aug 7, 2020
8,949

If the panel coverts 21% of the light to electricity and the LED converts 50% of that electricity to light, you get 10.5% if the light hitting the panel out the other side, so it would take nearly 10 square meters of panel to light 1 square meter at full sun.

Also, the figure I found for efficiency of LEDs in turning electrical Watts into light Watts was 30%. And the DC to DC conversion will reduce it by another 10%.

So my calculation was:

.21 x .30 x .90 = 5.7% of light transferred.

Perhaps I exaggerated with the word significant. With 5x the area you would get 28.5% if full sun, which would definitely be significant.
I was using 40% - then it's right. I have seen some claims from LED manufacturers lately that they have hit 50% efficiency, but I doubt those LEDs would find their way into LED lighting strip!
If we are talking about 12V LED strip, then there is a resistor in series with 3 LEDs making another 75% yield.
Whatever the figure really is, it's poor!

#### BobTPH

Joined Jun 5, 2013
8,120
Supposedly, grow lights can tailor the spectrum so that a higher percentage of the light can be utilized by the plant than it would get from sunlight. This might make it more feasible. interestingly, since green light is reflected by the plants it is not utilized, and the grow lights leave it out, producing a magenta tinted light. And the plants look black if that is the only light.

#### BobTPH

Joined Jun 5, 2013
8,120
I was using 40% - then it's right
Oh. I read your post backwards, that you only need a panel 1/12 the area of the garden. Now I see that we were basically in agreement except over the amount of light that would make it useful.

#### DickCappels

Joined Aug 21, 2008
10,104
The thread starter has already said that they just want to get a solar panel to light his LED strip and then take it from there. They have notmentioned wartts once

#### AnalogKid

Joined Aug 1, 2013
10,792
At the risk of addressing the original question . . .

Do you want to build a circuit from scratch, or wire up something pre-made, such as a voltage or current regulator module from ebay?

A 20 W / 18 V panel delivers 1.1 A, while a 14.4 W / 12 V LED strip required 1.2 A. So with your starting point numbers, the panel cannot (or is not rated to) deliver the required current. One solution is a simple linear regulator that drops the panel voltage to 12 V and lets the LED strip draw whatever it wants. My guess is that an overcurrent of less than 10% will not damage the panel, but I'm sure there are others around here with more panel experience. In this scenario, the peak power dissipated by the regulator circuit is 7.2 W, which a medium-sized heatsink can handle in free air with no fan.

Next up - a buck regulator. This is a switching power converter that utilizes energy storage in a magnetic field to increase the output current while decreasing the output voltage. With this type of circuit (or ebay module), you easily can get 1.2 A at 12 V without over-currenting the panel. Also, it dissipates much less energy that a linear regulator, often not requiring any heatsink.

Please post a link to the LED strip product page, its datasheet, etc.

ak

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#### Ian0

Joined Aug 7, 2020
8,949
It it's an Ebay 18Volt panel it is probably 18V open circuit for the purposes of specmanship, and its peak-power voltage is around 13V. Need to check the datasheet!

#### BobTPH

Joined Jun 5, 2013
8,120
A 20 W / 18 V panel delivers 1.1 A, while a 14.4 W / 12 V LED strip required 1.2 A. So with yur starting point numbers, the panel cannot (or is not rated to) deliver the required current.
The panel alone cannot, but the panel plus a buck converter, as I recommended in post #6 can.

#### grah

Joined Sep 14, 2023
6
At the risk of addressing the original question . . .

Do you want to build a circuit from scratch, or wire up something pre-made, such as a voltage or current regulator module from ebay?

A 20 W / 18 V panel delivers 1.1 A, while a 14.4 W / 12 V LED strip required 1.2 A. So with your starting point numbers, the panel cannot (or is not rated to) deliver the required current. One solution is a simple linear regulator that drops the panel voltage to 12 V and lets the LED strip draw whatever it wants. My guess is that an overcurrent of less than 10% will not damage the panel, but I'm sure there are others around here with more panel experience. In this scenario, the peak power dissipated by the regulator circuit is 7.2 W, which a medium-sized heatsink can handle in free air with no fan.

Next up - a buck regulator. This is a switching power converter that utilizes energy storage in a magnetic field to increase the output current while decreasing the output voltage. With this type of circuit (or ebay module), you easily can get 1.2 A at 12 V without over-currenting the panel. Also, it dissipates much less energy that a linear regulator, often not requiring any heatsink.

Please post a link to the LED strip product page, its datasheet, etc.

ak
Thank you ak.
Yes, I would like to build a circuit from scratch.
I had considered what I believe is your first suggestion - with a voltage divider circuit between two resistors? - but was concerned about unregulated current.
A buck regulator sounds safer and would provide good preparation should I later move on to a larger panel.
Specs for my panel and LED strip follow:

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