LED Project Help

How they survived the spring top batts is the surprise. OK now. LEDs need a means to controll their current. You don't just connect them to batts. Easiest (but not so efficient) way to do that is by a suitable resistor connected in series. But to get the most out of it you must know more about the LED type specs like it's nominal voltage and accepted current. S.M.

Oh boy, you've done so many things wrong here that I'm tempted to tell you to put down the soldering iron and step away from the circuit.

I guess I should start by saying that it would help to know what the model of the LEDs are but since few LEDs have part numbers stamped on themselves you could at least tell us what color LED you once had. I ask this because an LED is a highly non-linear device that has different nominal forward voltage conditions depending on the color of the light. For an example a red LED has a 2V drop operating at 20mA. If you drop these numbers into the correct locations of the Schockley Equation you should notice that increasing the voltage across a diode more than the nominal operating voltage will cause a tremendous, damaging amount of current to briefly get drawn by a diode before the diode releases the inherent smoke hidden in all electronic devices.

But I can hear your wonder and confusion across the Internet. You already raised your supply to 12V with an alkaline battery pack, it was only when the 12V lead acid battery was applied that things died. Well you see I can guarantee that your alkaline battery pack was not producing 12V while connected to the LED array because of the limited power available in the alkaline battery pack. In contrast the total amount of power available from the lead acid battery toasted your LED array.

Now as far as what resistor you have in series with this array of LEDs I cannot propose. You see I do not know the organization of these LEDs or even their color to suggest an initial value.

More importantly your allegiance to Tim Taylor's quest for more power without a clear understanding of what you are doing makes me wonder if this will be just a waste of my time, many LEDs and your patience.

Hmm, google should be your friend..

http://led.linear1.org/1led.wiz

http://ledcalculator.net/

Rule of thumb when playing with common garden variety LED's is that for a 5Volt supply use a 470 ohm resistor, for a 12volt 560ohm resistor. Now you will need to "tune" the resistor value depending on the colour of the LED to obtain uniform brightness between LED's.

Best practice is to put an appropriate resistor on each LED, this is due to the fact that LED's will not play nice in parrallel together. There is the opportunity for one LED to draw more current than the other causing an imbalance in both power consumption and brightness.

All great answers. And just to round out all the info they were white LED's. lol... and patience is something I have plenty of... gotta have it when building this stuff!!

Welcome to CR4.

Your LEDs are toast! From what you say they were intended to run at around 4.5V (3 x AA batteries) and you've stepped them up to 12 V (or maybe even 13.8) and still expect them to run.

The simple solution (if you need to run 12V for everything else) is to set your LEDs in SERIES and that way, the 13.8V will be split (almost equally) across the LEDs and they will have 4.6V each and glow as they were intended to.

I suspect that even at 6V, your LEDs will have less than 100 hours total life.

The complete answer to your question would take "pages" to explain correclty, but as you can see from the comments prior to this, there is a level of complexity involved.

Heh Heh!

This is the best way in the world to learn. Many of us have done similar things! Its also a cheap way, as you blew maybe 10 cents of parts......

The advice you have already from other posters.....

Yeah you'll need some sort of current limiting resistor. Here is what I would do. Go get the highest rated LED's you want to spend your money on (the higher the rating the brighter they get). As for the resistor in series, I would use a potentiometer. Using a POT you can start with a high resistance (low brightness) and dial it down until you start getting closer to the maximum current rating for your LED's, and thus the brightest level that they can emit. Safest way to not exceed that current would be to hook up an ammeter in series to watch the current change as you adjust your POT until a suitable level is reached. Thanks for the extreme detail in your question, makes it easier to answer!

If he's going to pushing hi-brightness LED's to their limits, a pot may not hold up for very long. Most "garden" variety pots are somewhere in the 1/4-1/2 watt range. The power dissipation needed for a group of LED's in parallel could easily exceed that rating. The idea presented earlier about using a series/parallel setup on the LED's is the best way to go, you will not need to "waste" as much power in the current limiting resistor(s) that way. If you are interested in going that route, you will need to know the voltage drop and safe operating current for the LED's, to know what the series/parallel array should look like, as well as the proper series resistor to use.

Tom D.

Its actually best to know the exact type of LED and read in the data sheet just how much current is allowed and calculate the resistor value exactly for the voltage applied....

Look at the data sheet for pretty well any LED and you will see that the relationship between voltage and current is logarithmic:-

Although this goes straight towards the top that's because the dotted part of the line is outside the safe working spec for the device.

What this means in practice is that LEDs (diodes in general) have a forward voltage drop which is reasonably fixed for their normal operating region. In the case of the diode above the working forward voltage is 3½ Volts. This is how the data sheet specifies it:-

The relevant line is V_F : Forward Voltage : Green : 3.5 (Typical) 4.5V (Max) @ I_F=20mA

Most normal (i.e. not for lighting) LEDs are spec'd. to run at 20mA, and the amount of light they produce is linear relative to the current, so if you run them at 5 mA you get ¼ the nominal light output and if you run them at 30mA you get 1½ times the nominal light output. So if you know the voltage you're running them from and the forward voltage drop you can easily calculate the value of the series resistor.

If you don't have the data sheet you can get a pretty good idea of the forward voltage drop by making sure you limit the current to less than 20 mA, and measuring it (V_F)

In the above picture even if the LED was short circuit the resistor would only allow 20 mA through it, but, if you made up the circuit and measured the voltage across the LED you'd get about 3.35 Volts instead of 3.5 Volts. You could then re-calculate the resistor value to be

R= V/I = (12-3.35)/0.02 = 432.5 Ohms

If you then used a resistor that size you'd get pretty close to 19.6 mA through the LED, but, you might as well use the next common value i.e. 470 Ohms which will give you 18 mA.

12V 7.5Ah/20HR means 12V down to about 8V if you draw 0.375 Amps for 20 Hours.

If you try to draw 7.5 Amps from it it will last a lot less than 1 Hour.

Looking at your question again: I see you mention "yard" lights. Do you mean lights for a yard (outside area): hard to believe that three AAs would be man enough for that. (How big is this RC boat?)

But, if they're LEDs the same principles apply. The LEDs may already have small series resistors fitted to regulate the current for a 4.5V supply.

You and the guy with the links have helped a lot!! and thx to all of you for not being flamers that I'm accustomed to running into on other forums!!

To answer more-- "Yard lights" may have been a little misleading. Maybe "Landscaping" would be a better term. all in all it went like this. I have 5 "LED Arrays", 3 of them were LED lights powered by 3 AA batts and a solor cell, the other 2 were powered by 3 AA's and a cell, but they were independently powered. As I read this back to myself it sounds confusing.... hmm... ok... 3 lights were powered together, and 2 more were powered individually. there, that helps.

ok, so I opened up the landscape lights and inside, there are 3 LED's (in parallel) with a surface mount resistor (just 1) built into them. Either time, weather, or other means has made it so that I couldn't read the value on the surface mount resistor. Using all of you guys input, I'll remove the burnt LED's and most likely the resistor too, and will rebuild the entire circuit from scratch and just be using the original housing and lens diffuser.

Straying off a bit, the RC boat is appx 4ft long, and 3ft wide. Has a Catamaran style hull. Built from scratch out of 10in and 6in schedule-40 PVC Pipe and weighs in at appx 100lbs. I live on the gulf coast and like to shark fish. I built to drag my bait out for me after dark. (gets kinda freaky out in the gulf 600yrds off the bank in the middle of the night... lol) this is a pic of the earlier stages to give you an idea. my more recent pisc are on other puter, but I'll post it up if you want to see the LED arrays.

Cheap electronics, especially from the far east, use circuits like that, but they only work properly when the LEDs are exactly the same.....or they already have a small resistor already built into them, which is possible.

This small resistor effectively makes all the LEDs the same......

I can't 100% confirm that the LED's in each landscape light are exactly the same make/model, but I feel 98% sure that they are. All the light fixtures are the same brand, size, everything; just 2 slightly different styles of mounting. But that's not to say that they switched horses midstream on the LED Batch if ya know what I mean. Visual inspection isn't the best source, but is all I have and since LED's really don't have ID #'s stamped on them I can't confirm 100% likeness, but I feel that they are most likely the same LED values across the board. Bad part is I have no datasheet for the LED specifics. At least with the new ones I'll buy, I'll know the Data and will be able to make a better informed decision/circuit build.

By the way, I've included another pic but with the landscape lights mounted since that's what we're talkin about. They came from Wal-Mart, and I'm sure that some of you have seen them before. The Solar Cell mounted on top was the one that came with the 3-pack of lights. It no longer holds batteries of it's own and instead now will trickle charge my Servo battery pack (which is a seperate 5v circuit (no probs on that side yay!)).