Do L.E.D.s dim with age?

[Frosty]

I posted this elsewhere, so please forgive if you read it twice.

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I couldn't find my headlamp while packing for a weekend trip so I went to Wal-Mart and bought another. Same kind as my old one (the $12.77 kind).

It came with batteries and the light seemed very bright.

Before I could pack my new light, I found my old one. It wasn't nearly as bright as my new one. One might assume that the old one needed batteries, but I'm an engineer and I just can't help myself sometimes.

I put the new batteries in the old light, and the old batteries in the new light. SURPRISE! Even with the old batteries, the new light was still brighter, though not by quite as much.

Both headlamps look identical, and I doubt much re-engineering goes into a twelve dollar item.

Now I am faced with four possibilities:

1. L.E.D.s grow dim with age
2. L.E.D.s grow dim with use
3. My old light was a production anomaly, and is not as bright as other units
4. My new light is a production anomaly, and is brighter than other units

The last two I tend to discard because of the great difference in brightness. Headlamps are merely LEDs connected to batteries with a switch. The only real variable are the LEDs themselves and I suspect they do not vary that much in production.

Intuitively, I lean toward No. 1, but have no data to back this up.

Anybody know?

 

[ken]

Headlamps are merely LEDs connected to batteries with a switch.

many l.e.d. lights are "modulated" this helps the batteries last even longer - if you want to see this just put them near a running fan and you will see the "strobe effect" - the diffefent brightness ranges in the headlamps aren't from the voltage being different for each one, but from a change in the modulation frequency (or on off cycle) your eyes dont see this (just like you dont see the 60 frames per second of a tv picture) - the only way to see this is by illuminating a moving object (such as a spinning fan) - as far as i know l.e.d.'s are not effected by age (but they aren't supposed to burn out either and i have chenged a lot of them).

 

[DougPaul]

Frosty:

> 1. L.E.D.s grow dim with age

No--their shelf life is not an issue.

> 2. L.E.D.s grow dim with use

Yes, but _very_ slowly (~10000+hrs) unless you overload or overheat them.

> 3. My old light was a production anomaly, and is not as bright as other units
> 4. My new light is a production anomaly, and is brighter than other units

There is production variation and defective units. If you look at multiple LED headlamps, you can see that LEDs vary by a small amount.

But more likely is that the 2 headlamps had different LEDs in them. They look the same from the front, but might be different models.

LEDs can also also have different optics. The clear epoxy acts as a lens--some are designed to have a broad beam and some are designed to have a narrower beam. The LED itself is a tiny bit of silicon down inside.

I saw this happen with the Petzl Tikka. An old version was dim and drew 60 ma (1 milliamp = 1/1000 ampere), a newer version was much brighter and drew 250 ma. (Yes, I measure the current drawn by my headlamps--it gives me an objective measure of the battery life. An AA cell has about 2400 mah (ma-hrs) and an AAA cell has about 800 mah of capacity.)


Ken:
The choppers (circuits that turn on and off rapidly) are used to reduce the intensity of an LED light by reducing the average current. (A chopper is more efficient than reducing the current by putting a resistance in series with the LED.) Reducing the average current also increases the battery life.

Doug
Electrical Engineer

 

[Pete_Hickey]

Batteries more important with LEDS

Since LEDS require very little power, they can run with almost dead batteries. Now, this may not be the the case in the OP (although Frosty did notice switching batteries made some difference)

With incandescent bulbs, it is obvious when the batteries are fading. There is a smoewhat quick drop in the amount of light produced. With LEDS, there is a very slow gradual loss of light.

I use LED based lights every day in the winter for my bicycle commute, and I've learned to periodically swap batteries to see if it becomes noticibly brighter. If so, it's time to switch the batteries.

 

[MichaelJ]

Some LED lamps have a small circuit inside (don't ask me how it works, I barely passed my EE courses in college; a friend explained all this to me) which is some type of simple voltage regulator that at the cost of overall battery life keeps the LED as bright as it can until the last gasp of the battery. And some just dim as the batteries age.

But what you're seeing sounds like simply improved production.

 

[SAR-EMT40]

The choppers (circuits that turn on and off rapidly) are used to reduce the intensity of an LED light by reducing the average current. (A chopper is more efficient than reducing the current by putting a resistance in series with the LED.) Reducing the average current also increases the battery life.

I know that I don't have to tell you this Doug. The choppers, or modulators do actually reduce the average current, as stated, but the primary reason they are used is so that the LED can be pushed much harder and increase the intensity of the LED without overheating because the duty cycle is reduced, and therefore the power that has to be dissipated in the form of heat. That means that you can make the LED much, much brighter without burning it out.

Keith

 

[jbrown]

It could very well be the type of LED, i.e. the actual specs of the LEDs in question. If they were different intensity, one at 8000 mcd and one at 10,000 mcd, you'd probably notice a difference. It just matters which LED the manufacturer is using. (mcd=millicandella=measure of brightness.)

 

[DougPaul]

With incandescent bulbs, it is obvious when the batteries are fading. There is a smoewhat quick drop in the amount of light produced. With LEDS, there is a very slow gradual loss of light.

There is another, and possibly more important cue with incandescent bulbs: the color of the light changes as the batteries weaken. In contrast, the color of the LED light is unaffected by the condition of the batteries.

What you observe is also a function of the battery type: Lithium, NiCad, and NiMH cells have a relatively rectangular discharge curve--that is the voltage stays relatively constant until the battery is exhausted. Alkaline cells have a tapering discharge curve--that is the voltage drops slowly as the cell is used up. So, depending on the battery type, the light will stay constant and then die relatively quickly or taper slowly.

Some LED lights have regulator circuits that will compensate for voltage variations. These lights will tend to have a constant output and then die quickly even with alkaline cells.

Doug

 

[arghman]

Some LED lamps have a small circuit inside (don't ask me how it works, I barely passed my EE courses in college; a friend explained all this to me) which is some type of simple voltage regulator that at the cost of overall battery life keeps the LED as bright as it can until the last gasp of the battery. And some just dim as the batteries age.

Switching regulators store energy when electromagnetic coils are temporarily connected across a battery, then release that energy into the LED; this process is usually done at a high frequency (tens or hundreds of thousands of times per second) and controlled to maintain constant voltage or current. They do waste some energy, but can be made very efficient.

I'd pick a switcher over a lamp with a "dumb" circuit (LEDs connected directly to battery, or connected to battery via a resistor which wastes power). They may cost more but will probably in the end give you a longer usable run-time; if the LED dims, I'm going to have to replace the battery even though there may still be some useful energy left in it.

 

[DougPaul]

Switching regulators store energy when electromagnetic coils are temporarily connected across a battery, then release that energy into the LED; this process is usually done at a high frequency (tens or hundreds of thousands of times per second) and controlled to maintain constant voltage or current. They do waste some energy, but can be made very efficient.

Switching regulators need not use an electormagnetic coil (inductor)--they can use capacitors instead.

A headlamp with a regulator also has the advantage that one can get full intensity from NiMH batteries (1.25 volts vs 1.5 from alkaline).

Actually there are 3 possible functions going on (possibly in the same circuit, possibly separate circuits): voltage step-up, regulation, and a chopper-based dimmer.

Take, for example, the BD Zenix IQ: it uses 2 AA cells giving 3 volts. White LEDs require 4 or more volts--so it must have a voltage step-up. I get the same light intensity with NiMh and alkaline cells--it has a regulator. It has intensity settings: high with no chopping, and 2 lower settings with a chopper. (You can see a chopper by waving the headlamp around quickly. A steady line of light indicates always on, a dotted line of light shows the chopper.)

LED lights with 3 or more cells don't require a step-up and could, but don't usually have a regulator, and may have a chopper dimmer to get multiple intensities. (Example: Princeton Tech Aurora: 3 cells + a chopper.)

A chopper can also be used as regulator (by varying the fraction of the on time), but only for the dimmed modes. (Since a chopper cannot be on more than 100% of the time it cannot brighten the light to compensate for a weak battery.)

Doug

 

[paul ron]

Comparative shopping...

LEDs don't dim by age and very slightly by use if overvoltaged to make them seem brighter. They do dim as the batteries get old.

They come in many different brightnesses and voltages from 1.5VDC to 12VDC. What you should be concerned with when buying a headlamp is how many batteries, how long will they last and which lamp will give you the brightest LEDs in a comfortable white light. Voltage regulators and choppers are all nice circuits the manufacturers are going to use in marketing competition.

I buy plain white LEDs from electronics outlets and have run them on 1.5V and 3V button cells for months with no regulators or choppers n pulsers. Yes you can make LEDs with these devices but they aren't needed and don't improve the light or longevity for our purpose. These gimicks will only add cost and weight to your light.

When I was looking for a camping headlamp, I tried em all at the store. I took all of their LED headlamps into their store room and lit em up. The newer Tika plus wasn't as bright as the older Tika 3 LED light, the newer one also had a weird blue tint to the LEDs light and do I really need a flasher option? My choice was the simple 3 LED Tika using 3 AAAs, the batteries last almost a year with the heavy use I give it. I use it around the house, as an area light in the lean-to or tent, Scout camp, cooking around the campfire, working on my car, looking through my closets... etc.