View Full Version : Need Help With LED Project

10-28-2001, 11:06 AM
(Standard apology if I've posted in the wrong forum)

Probblem-On string 2, only the large, red ones light.

Details-As part of my Halloween costume, I'm building long strings of light-emitting diodes. The Led's are hooked up in paralel along 2 long leads, ladders style. The first string works fine. On the second one, only the red LED's would light. I checked and all LED's were connected properly. All were still functional. I couldn't find any shorts. I disassembled that string. The string I finished last night has the same problem. The greens don't light. The yellows don't light. Even the small reds don't light. I humbly beseech the Dopers to aid me.

10-28-2001, 12:39 PM
Long strings of LEDs, eh? Might I ask how many, what their forward voltage drop is, and what exactly you are using to light them?

LEDs, like all diodes, need a specific voltage across them to operate, and for LEDs—as opposed to normal diodes—the value is usually much higher (up to 2 volts in some bastards!).

Too much current passing through a diode won't "pop" it necessarily, but may short it, and anything in parallel with it will then not operate.

Also, you might simply be drawing the maximum curent allowable from the (presumed) battery and it isn't enough to power all those diodes. Current is additive in parallel, so too many LEDs = bigger battery.

If you could, attempt it by adding one LED at a time to the mock-circuit and see what happens. What style battery are you using?

So many questions!

10-28-2001, 01:02 PM
One important question is the type/size/voltage of the battery you're using, as LED's are very voltage dependent (they have a set voltage drop across the device).

10-28-2001, 01:13 PM
This has probably already occured to you, but you might also check to be sure that you have them all attached in the right direction. Maybe all of the green ones from the second batch got their leads put on backwards, or something.

10-28-2001, 03:48 PM
First, thanks for your help

Second- I'm using Rayovac 3 volt, lithium, disc batteries.
Roughly 20 led's are on the circuit. Some are from Radioshack, but the package gives no specs regarding voltage or anything else. The rest are salvaged from all manner of junked electronics.

I tried removing the red Led's. The string then operated normally. I replaced them and the large reds were the only ones to light up. They're not from the same batch. They are spaced along the string (not all at the end or beggining, and they are not adjacent to eachother). I'm using regular, rosin core, lead-free solder. The wires stranded copper-the cord from some junked appliance.

I can complete the project using green and yellow. But, I'd still like to know what caused this.

10-28-2001, 05:09 PM
The problem is, you're connecting them in parallel - and red LEDs start to conduct at a lower voltage than green ones do. It's actually because the photons they emit are less energetic. Infrared LEDs use lower voltage still, and blue ones take much more.
I think you're lucky you didn't burn them out. LEDs draw almost no current as you gradually turn the voltage up from zero, and then suddenly when they hit a threshold they conduct much more with only a tiny increase in voltage. The general scheme people use is to drive the LED in series through a resistor, and supply a voltage to the combination such that the desired current will pass through the resistor when the LED drops its operating voltage and the resistor drops the rest. You should check the LEDs you are using, but as a general rule many LEDs are good for about 20 mA (in other words 0.02 A). If you don't have the information, and don't want to burn any out experimenting, step back to perhaps 5 mA and you will almost certainly be safe.
You can run all your LEDs in series and put 5 or 20 mA through the chain, or if you don't have enough voltage you can group the LEDs into subsets that total somewhat less voltage than you have available. Just pick your resistor to conduct 5 or 20 mA when you put the rest of the voltage across it. If you don't have another source, you can use these guesses: red LEDs will drop 2 or 2.5 V and green will drop 3.5 or 4 V.

10-28-2001, 06:08 PM
Napier has it right… LEDs are basically current devices. Your typical garden variety LED, regardless of color output, can be run anywhere from about 5 mA to around 20 mA, depending on the brightness level you want. Throughout that range, however, the voltage is fairly "stiff", i.e. the voltage doesn't change much as the current varies. For red LEDs the voltage drop is around 1.7 V. For shorter wavelengths the voltage drop increases. This is why the green and yellow LEDs in your costume didn't light.

When ganging LEDs, it is common practice to hook them in series, not parallel. This way, the current through each is identical, regardless of the color. However, if you're using a voltage source this topology will usually require a series resistor.

Circuit example:

Let's say I have two red LEDs and one green LED. I want to gang them together, I want them to have high brightness, and I want to use a 9V battery. The "high brightness" requirement means the current will be 20 mA. The voltage drop across each of the red LEDs is approx. 1.7 V, and the voltage drop across the green LED is approx. 2.1 V. If we put them in series, the voltage drop across all three LEDs will be 5.5 V. Since the battery is (nominally) 9V, we'll need a "dropping resistor" in series with the rest of the LEDs. The resistor will have a voltage of 9 - 5.5 = 3.5 V across it. Since the current is 20 mA, the resistance should be R = V/I = 3.5/0.02 = 175 ohms. The next higher standard value is 180 ohms, so we'll use it. The resistor will be dissipating a power of P = V^2/R = 3.5*3.5/180 = 68 mW, so a ¼ watt resistor will work fine.

One obvious disadvantage to this technique is that the voltage drop can get quite large as the number of LEDs increase. The solution to this problem is to use multiple "series networks." Each series network connects to the one (common) voltage source, and each will have it's own dropping resistor.

If you want to get even fancier, I would suggest using linear current sources instead of dropping resistors. An LM317 can be easily configured as one.