Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Basic LED / Solar Panel Question

Status
Not open for further replies.

katrah

Newbie level 4
Joined
Oct 21, 2016
Messages
6
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Location
Midwest
Activity points
67
I'm very new to electronics. I would like to build a simple solar panel rated at 12 volts 3 watts to light 4 LEDs. Each individual LED is rated at 3.4-3.6 volts 350 mA. My intent is to wire the LED's in series making them a total of about 3.5 volts 1.4A. My reason for this is so that the LEDs begin to dim on/off under even very low lighting conditions.

So if I'm doing my math right this setup should look like this:

R = [12v -3.5v] / 1.4A = 6.07 ohms

At 6.07 ohms resistance under max lighting conditions from the solar panels, I shouldn't need a resistor, am I correct? At what point would one want to use a resistor with LEDs? And is there anything else I'm not considering with this design that my newbie self is unaware of?
 

Your LEDs are in parallel not in series. Parallel is not recommended because their forward voltages are not exactly the same like incandescent light bulbs.
LEDs have a range of voltage. Yours might be from 3.3V to 3.7V. In parallel, a 3.3V one will hog all the current then the 3.7V one will not light until the 3.3V one burns out.
Also, your 6 ohms resistor will be huge and hot since it dissipates almost 12W. Of course you need the resistor because it limits the current.
The little LEDs also get hot. How will you cool them?

Why do you need the LEDs to light when it is sunny outside? Are they in a dark room that has no windows?
 
  • Like
Reactions: katrah

    katrah

    Points: 2
    Helpful Answer Positive Rating
Yes, I apologize, I said series when I meant parallel. So no one puts LEDs in parallel? Any recommendation on where I can learn more about this because I've spent many hours trying to Google for answers and I never seem to find them. It's all written for people who already know how this stuff works.

From what I've been able to tell from what little I can find online about LEDs and resistors, I thought 6 ohms was very, very little. It sounds like I'm very incorrect?

I'm not sure why it matters that I want LED's to light up with the sun. I find it curious when people question why someone wants something that they themselves cannot think of a reason for. My reasons have nothing to do with the question. But if you must know I'm trying to make supplemental lighting for food plants in a northern facing window. I also want to learn more about electronics and I thought this was a simple enough project. But as I said, finding answers for someone very new like me has been a hair pulling experience.
 

LEDs are often parallel connected in low voltage LED "bulbs", but they are probably forward voltage binned. So it's basically feasible, intensity uniformity depends.

What I really don't understand is how you want to get 1.4 A out of a 12V 3W solar panel. According to my math, maximum rated current is 3W / 12V = 0.25A. Means you don't even get the intended current per LED in series connection. Connecting 3 or four LEDs in series circuit to the panel without any resistor seems to be the best you can do.
 
  • Like
Reactions: katrah

    katrah

    Points: 2
    Helpful Answer Positive Rating
I'm aware that the amperage of the panels are well below the rating of the LEDs. I decided on these panels due to size/price. I can only make this "so big" before it's just... well, silly. ;) I did make a prototype of this using 2 of the same LEDs in parallel with panels rated at 12v, 1watt, .083A. I felt like the light this omitted was surprisingly bright (I've also been using reflective mylar to help increase the lighting a bit). I also intend to experiment with different voltage vs. wattage and note how many volts the solar panels should be for my low lighting situation as well measuring the LUX output. Hands on learning I suppose.

My BIG question was about resistance and what type of range is typically okay for an LED. I'll explain maybe we I feel confused. The best thing I could find about choosing a resistor for LEDs was this video: choosing a resistor to use with an LED. From watching this it sounds like 6 ohms is absolutely nothing to worry about. But... it's one thing for me to prototype something that's mathematically incorrect and only use it for a few minutes, it's another for me to make a permanent item for me to experiment with in terms of how well this may (or may not) suit the purpose I've created it for for several months.
 

All "choosing a resistor to use with an LED" math is presuming a low impedance voltage source which you don't have. Instead you have a current limited source, as long as the maximum solar panel current is lower than the acceptable current of the connected LED circuit, no current limiting resistor is required.

The optimal LED circuit can be graphically designed by drawing load lines into the solar panel I-V diagram.
 
  • Like
Reactions: katrah

    katrah

    Points: 2
    Helpful Answer Positive Rating
All "choosing a resistor to use with an LED" math is presuming a low impedance voltage source which you don't have. Instead you have a current limited source, as long as the maximum solar panel current is lower than the acceptable current of the connected LED circuit, no current limiting resistor is required.

The optimal LED circuit can be graphically designed by drawing load lines into the solar panel I-V diagram.

Thank you so much! That answers my question!

(I'm still up for any recommendation on websites or eBooks on LEDs for beginners if anyone has something to suggest!)
 

IF the power source is stable (the PV output isn't!) the resistor is calculated by the formula:

R = (PV voltage - LED voltage)/LED current. The result being in Ohms, you got the calculation correct.
W = (PV voltage - LED voltage)*LED Current. The result is in Watts.

If you wire LEDs in series, add their voltages together, the current flowing through them will be the same, it passes through one to reach the next. If you wire them in parallel, unless you use specially matched ones, the current will take the easiest path which is through the LED with the lowest forward voltage. Even with matched LEDs there will be some that 'hog' more current than others.

The problem in your case is the PV output is limited, although it may be rated at 12V and 3W that is under test conditions, in practice, a lower current will result in a slightly higher voltage and beyond a certain current load, the voltage will start to drop. That makes the calculation a lot more complicated, especially if you factor the varying daylight levels as well. However, to produce 3W from the PV at 12V, the current from it can not be higher than I=(W/V) = 0.25A which is lower than the LED limit so you can leave the resistor out altogether and rely on the PV not being able to produce enough output to damage them.

Another problem is the voltage you need. Assuming the worst case of 3.6V per LED, you can't use all four in series, that would need at least 4*3.6 = 14.4V which is more than the PV can produce. That leaves these options:
One LED with it's own series resistor and three in series with their own series resistor,
Two parallel circuits of 2 LEDs, each with their own series resistor.
Four LEDS, each with their own series resistor.

Given the limited power you have available, there will be little difference between the options.

Brian.
 
  • Like
Reactions: katrah

    katrah

    Points: 2
    Helpful Answer Positive Rating
You did not say how you will cool the LEDs. Usually their small metal stars are bolted to a finned metal heatsink.
 

Okay, thanks guys. I understand now that if my panels are producing less current than the forward current of the LEDs that I do not need a resistor.

My only other question is then if I were to say use different LEDs or different solar panels (as I said I'd like to experiment with different setups and measure the LUX ratings) and the potential current from the panels exceeded that of the LEDs at what point is a resistor needed? From watching that video it seemed as though something as little as 6 ohms is nothing to worry about, but I don't want to assume that's the case.
 

The resistor is necessary even if it's value seems very low. In high current circuits it is quite common to use resistors of 1/100 Ohm or even smaller.

The same formula as I posted in message #8 applies. In a simple circuit just calculate and use the appropriate resistor. If you use a much bigger PV panel the situation changes and there are ways to electronically limit the current so the LEDs are never run beyond maximum current even when the voltage may vary and be much higher. In essence the circuits work like automatic resistors that adjust their value to keep the current flowing through them constant, regardless of the voltage. Before thinking about using such a circuit, consider it will cost significantly more than a resistor and you need a few volts 'overhead', in other words if your LEDs need 12V you have to start with say 15V or more.

Brian.
 
  • Like
Reactions: katrah

    katrah

    Points: 2
    Helpful Answer Positive Rating
Wonderful! Thanks so much for the help Brian. I just purchased a "beginners" electronics book since finding some of these finer details just from Google searches seems difficult and often misleading. Hopefully this book will serve me better than Google searches and YouTube! lol
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top