LED Connection

I want to connect more than 25 LEDs to 9 V battery with low power consumption.

Dear muwa,

First thing to do is to measure the voltage needed to light up one of the leds, using a dc power source.

Normally the leds light up between 0,7 and 2,8 Volts.

Measure the intensity consumed when the led light is OK for your application.

Normally I use 10 mA.

Once you have all this data, just use a resistor for each led to limit the current to 10 mA and for the voltage drop required.

Connect them in parallel.

Another thing you must use is a voltage stabilizer between the 9 Volts battery and the current limiting resistors, because the battery will be loosing voltage.

Use a 7805, it will give you 5V stabilized. In the datasheet you will find info on how to connect the 7805, and how to include filtering caps. Calculate the total intensity 25·10 mA=250 mA. Ensure that the stabilizer can handle this, normally a 7805 can handle 1000 mA, so you will be at 25% usage, it is OK.

Use a diode for inverse polarity protection between the battery and the stabilizer.

Remember that the leds have polarity.

Perform this calc, R=(5 Volts-Voltage drop required)/ 10 mA; where voltage drop required is the voltage at which
the led lights up and is consuming 10 mA for example.

Best regards,
Jorge
www.solartroning.org

Then use large series limiting resistors. In other words, run the LEDs at a low current. Or, for the most efficient solution, use a current regulated boost converter to step up the voltage so you can run them in series. How high a voltage you will need, will depend on the type of LED (different colored LEDs have different forward voltage drops)
If you decide to run them on 9V, then use a series/parallel arrangement. The number of LEDs you can run in series across 9V will, once again, be dependant on the forward drop of the LEDs. Use a resistor on each series leg to limit the current down that leg.

BUT, you can't defy physics. For a certain brightness, you need a certain amount of power. The best you can do is use the most efficient means of delivering that power to the LEDs, possible -- and that would be some kind of current regulated switch mode system. Also, you can fool the eye into thinking the LEDs are brighter than they really are, by pulsing them at a high current so they flash brightly. Not sure what the optimal duty cycle/frequency is, and that might vary with color.

Running them in parallel with one resister per LED is about the least efficient way of doing this, and not recommended if power conservation is an issue. Also, the 7805 has two strikes against it for this application:

1. Huge power loss do to the drop from 9V to 5V
2. The 7805 has a worse case quiescent current of 8ma!

If linear is the only way to go for you, then consider some kind of current regulator, not a voltage regulator.

Hi Steve,

Thanks for your point.

You are right, it will not be efficient.

Best regards,
Jorge.

Because this question "How to power an LED" comes up so often I decided to do a blog on the subject. Here's the URL:

* www.sinistercircuits.com/Blog/LEDs_LightemWithoutSmokinem