LED POwer Consumption

[freakytiki4u]

I am trying to find out how long an LED will last on a 9 volt battery if anyone knows how to figure this out. (I do not) The Information on the LED is as follows:

* 9 volt 3mm Red / Blue Alternating
* 24 gauge red and black wires, 8 inches long
* Input voltage: 5~7 Vdc.
* Low power consumption (<20mA)
* Very bright! 5000 Mcd

Any help would be greatly appreciated. The LED will be wired directly to a 9 volt battery. I am also interested in wiring in a photo voltaic device to make it only come on at dark if that can be done.

[selldoor]

Re: LED POwer Consumption

How come its 9 volt when it says input voltage is 5~7 volts.
How long is piece of string??
Are we talking standard PP3 battery. ? Alkaline /Zinc Chloride
10 for £1 / £1 each . Duracell would have you believe theirs will last much longer.
You would need to know the capacity of the battery mah and presu,ably divide that by
<20ma. May also be affected by temperature. - connect it up and see?

[mariushm]

Re: LED POwer Consumption

That's not a LED. That's a circuit with at least two leds, a red one and a blue one.

Get back to us with more information, or some links to where you bought that thing from, or at least some pictures of the thing.

Note that it says input voltage 5-7 volts. Depending on how that circuit is made, the leds may burn if you use a 9v battery so we have to know how the leds are powered.

In THEORY ... it says there low power consumption (20 mA) ...
Here's a datasheet for an alkaline 9v battery :

The 20 mA power consumption fits into the TOY category, so the battery will discharge from 9v to about 6v in about 18 hours. In real world, I think your leds will last about 12 hours.

an alkaline battery will have a capacity of about

[Longbow]

Re: LED POwer Consumption

Good beginning to an LED thread...but no followup. 20 mA is usually maximum current for a standard 5 mm led. You won't get much AA battery life at that current level. Fortunately these diodes emit plenty of light for front panel use at 1 mA, sometimes less. Flashlights would be a different story.

What is missing here is the basic understanding that an LED is a current device and some form of current limiting must be used (i.e. a resistor). So instead of battery and led, think battery, resistor, led.

[redwire]

Re: LED POwer Consumption

It sounds like a dual LED red/blue with a built-in flasher IC? Like those cheap Chinese blinky things.
Assuming 20mA drain down to 4.8V, that's 30 hours. But they drive the LED's hard so I think it will draw more current at 9V, because I've never seen dropping resistors, just a chip-on-board. A pic would help

[Longbow]

Re: LED POwer Consumption

There is no way to predict this 30 hour figure without a manufacturer's plot for a specific battery output over time - and at a specific current draw. Maybe someone has this information. I certainly don't. We don't know if the led has a limiting resistor. If so, the brightness (and current draw) will decrease as the battery voltage decreases. Some circuits rely on the internal resistance of the battery to limit current - risky.

If this is a more sophisticated circuit it may have a PWM led driver IC. Driver IC's keep the current constant, resulting in shorter battery life, with the led operating at the same brightness level. Maybe you can use the mA-H rating on the battery in this case.

Flashlights that can supply big currents (4 - 6 D cells) do use limiting resistors of 1 ohm or less along with the large high power led-on-heat sink. Even then, the current is limited by the internal resistance of the batteries. Not an impressive design direction, but everyone is going for more and more brightness at any expense.

[Snav]

Re: LED POwer Consumption

Sounds like your driving a flasher chip which might run at much less power. Possible to get 30hr if you current limit it.

You can use a 100ma lm317 with a 62ohm resistor and feed constant current of 20ma for max battery life. Using as adjustable voltage you can measure the actual current and calculate a better resistor.

Resistor goes between Output and Adjust pins. Input from battery with output to LED from adjust pin. 0v connects directly to battery.

Fig 21 on Page 8 of the attached datasheet shows the wiring. The pot is for fine tuning but not needed.