What do we know about the DMM back-light LED life cycle?

[Kiriakos-GR]

I need some modern opinions about the latest DMM back-light solutions.
Like the ones found on Fluke - Agilent - or other instruments. 
Dave does lots of reviews, but is there any way to evaluate the quality of its back-light design,
so to be baptized as successful or not ?


What truly bothers me are those questions :  ( By excluding the factor of battery consumption)
1) Can I use the back-light as much I like, even 10 hours constantly ?
2) Does those SMT LED's have issues with their operating temperature ? 
3) Is it possible to loose their brightness in time ? 
4) How many hours of operation is their limit ?
5) Is their build quality detectable by any way... ( So to be able to tell if they are cheap toys or an trusted part ?)

[Fox]

Hi Kiriakos,

the answer to your questions depend on the specs the designer wanted to achieve,
like do they asume a short time usage e.g max. 2min light duration with the maximum brightness or have they dimensioned the LED for constant usage.
 
to 1. If the LED  is driven within the specs of the LED that would be no Problem ( standard LED about  max. 20mA)

to 2. SMD LED have the same issues as THT LEDs, they have a max. operating temperature. If you refer to the heat the LED produces themselves, so it depends on the power of the LED. Normal LEDs driven within their specs don`t produce noticeable heat (at lest when you don`t pack them thight in big numbers)

to 3. Yes every LED looses brightness over time, especally white ones

to.4. For that one you have to look in the datasheet of the LED.

to 5. not that i now of, they loo all pretty much the same, maybe if you look at them with a magnifying glass so you can see the chip and the bondwires.

[mikeselectricstuff]

LED lifetime really isn't an issue for the sort of timescales DMMs have them on, and even if it were most would be easily replaceable.

 

[Bloch]

Hi Kiriakos,

Are you sure the DMM are using led ?

In my work we use some 3inch and 6 inch displays. They use a kind of fluorescent light (150V). And they should last 10000 hours.  But we found out that is not the case if used in below 0 degrees celsius.

[tekfan]

Everything you ever need to know about LEDs:
http://www.national.com/en/videos/abd_driving_high_power_leds_pt1.html

Long but worth watching.
RIP Bob Pease

[Kiriakos-GR]

Hi Kiriakos,

Are you sure the DMM are using led ?

In my work we use some 3inch and 6 inch displays. They use a kind of fluorescent light (150V). And they should last 10000 hours.  But we found out that is not the case if used in below 0 degrees Celsius.

My DMM it uses LED,  I get 40mA consumption above the standard when its activated.
About the ( kind of fluorescent light ) I think that called as cold cathode type, and yes all my Olympus digital cameras,
haves inside an special high voltage circuitry for it. 
And Its true that this technology haves issues with very low temps (flickering) .
And so yes, your observations are correct. 

About my case, I run tests right now, I have all ready active the back light for 10 hours,
and monitor the battery consumption.   
No flames yet 

[westfw]

cheap white LEDs can have pretty horrible lifetimes, especially if overdriven.  Something about phosphor poisoning.  I had some nightlights that died pretty much completely in less than a year.   Brand-name white LEDs are better.  There's no way of telling how good the LEDs inside your meter are :-(  The lifetime of more conventional LEDs is MUCH higher.  I actually replaced the broken white LEDs in those nightlights with other colors (blue, orange, true-green) that I had lying around, and they've been running with no noticeable degradation ever since (about 7 years?)

[Kiriakos-GR]

Well the LED on the back light display, it is a different animal.

The quality of the circuitry and the LED it is very important.
My test ended successfully, 14 hours operation with out issues.

And I had one good reason to worry about the back light on the specific DMM.
You can totally disable any power saving modes on it, main and back light.
And so there is the possibility to forget it on for hours.
By proving that the back light it is well made, I will just kill the batteries if I forget it on,
and this is an acceptable price to pay, if you are careless.