Driving a lot of white LEDs

Hi, I am fairly new to the LED lightning concepts. Although I do understand the basic constraints and how to drive LEDs. I am trying to build a LED based flood light. I am using around 120 white 5mm good quality LEDs. I have used series and parallal combination to design my cluster and to operate it at 12V.
However I am facing problems during testing. According to the LED manufacturer the LEDs should last for 50,000 hours but in my case, the LEDs start flickering and a few have also burnt out after only a few days operation. The light intensity also decreased considerably after only 48 hours of operation.
I am using current limiting resistors and am driving the LEDs at 20 mA as mentioned by the manufacturer as optimum current. I am using a 12V 2A DC Voltage supply. I have got some information that I have to drive LEDs with a current controlled driver. These drivers are fairly expansive and there are ICs that can drive around 4LEDs and that solution is also very expansive.

Can anyone give me any suggestions or solutions to help me with this design? I' ll be grateful.

Welcome to the CPF!

If you are putting 20mA to the LEDs, it should not die or flicker. You said they are good LEDs. What brand are they?

hotbeam said:

Welcome to the CPF!

If you are putting 20mA to the LEDs, it should not die or flicker. You said they are good LEDs. What brand are they?

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Have you measured the current? Measure the voltage across the resistor and use ohm's law to figure out how much current is actually going to the LEDs

are they really of good quality or it is the blabberings coming from the manufacturers?

also, are the leds running cool enough? they do create heat you know...

Remember that I=V/R, so if your DC supply (V) is stable, then using just the right resistor (R) will create a stable, and steady current source (I).

If, for example you are putting 3 LEDs in series, they will have a combined Vdrop of about 10.5v, so you would need to "use up" the excessive 1.5v across the resistor. Since you need the current to be 20mA TOTAL, the proper value for R would be 1.5/.020, or 75ohms; you can use 2 x 150ohms in parallel to get the same value.

If you are using 75ohms in series with 3 x 20mA LEDs on a 12V source, you will be running without risk of wholesale failures.

Remember too, that a lot of wall warts are unregulated, and will vary output from 14v down to 12 depending on load. To account for a 14v source, you would need R to be ((14-10.5)/.02)=175ohms!

Also, any LEDs vary. If (individually or in strings) connected in parallel, some LEDs will have lower Vf than others, which will cause them to suck up more current than others and therefore burn out faster. Some will also burn out faster even at the same current. To truly limit current, you'd probably need to have some resistance in series with each of your in-series strings of LEDs.

There is a great article that tells you exactly how to do what you are trying to do:

http://www.iinet.com/~solarwashington/newsletters/0503/LEDdesign.A03.rtf

I have created an Excel spreadsheet that calculates all the values so people can check the math. Please report any discrepencies.

There are two approaches.

1. Limiting Resistor

For a 12V power supply you need to drive the LEDs in series/parallel. You will only be able to drive two LEDs in series with such a low voltage, if you don't want the LEDs to shut off when the power supply voltage goes below 11 volts, so you will have 60 parallel branches of two LEDs in series in each branch. That's a lot. So you will need 60 300 ohm 5% 1/4 watt resistors. Your power supply will have to be able to supply up to around 1.6 amps allowing for a maximum possible voltage of 15 volts. A car battery would have no problem supplying this but a typical wall-wart would, so be sure to check the current rating if you are using a wall transformer.

2. Current Limiter

The same restrictions apply here as well but now you will need 60 LM317 voltage regulators and 60 resistors so it would be prohibitively expensive. The main differences would be a resistor value of 62 ohms and a power supply that supplies 1.2 amps.

It looks like your only real option is the simpler limiting resistor circuit.

In either case, heat build-up should not be much of a problem as long as you don't confine everything to a sealed container. Be sure to allow reasonable air-circulation around components so heat does not become a problem. You can test this by letting everything run for several minutes and feeling the LEDs and resistors to see if they get hot to the touch. If they do, you need more air circulation.

WARNING: Disconnect the power first, especially if it is from an AC power supply.

The variances between LEDs of the same batch isn't that bad. I've build about a dozen of these larger LED arrays using over 500 LEDs and I've only had 4 individual LED failures. These lights are used 24/7 and their output is still over 80% after 14 months.

The circuit uses current sensing on ONE string of LEDs and uses that voltage in a buck regulator to power the remainder. The largest I have constructed is 144 LEDs using this method.

Amer said:

I am using current limiting resistors and am driving the LEDs at 20 mA as mentioned by the manufacturer as optimum current. I am using a 12V 2A DC Voltage supply.

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Is the DC voltage supply regulated? Many of them (like some wall-warts) are not regulated, and the voltage will be higher if there is not enough load...



I am quite interested in what you're doing, as I am investigating into making some white LED lights, by swapping red LEDs on discarded bus brake lights like this:


Each one has a circuit board with 50-60 LEDs, plus a seperate regulator circuit (which I need HELP to figure out what it does).

The amber (turn-signal) ones are quite bright too.