I am replacing a few simple control panels.

I had to do some reverse engineering since the person who designed and built these has retired and we believe passed away.

The panels are 30 plus years old.

They use just standard 5mm T 1 3/4 LED's as indicators.

The LED's are still working fine.

Say I use a standard LED (just looked one up at DigiKey) ... 2 V forward and 20 mA.

The circuit is 12 volts.

So, doing the math I get 500 ohms ( R=(12-2)/.02 )

I would pick a standard 510 ohm resistor.

My question ... for the sake of longevity ... do you run these at the full 20 mA?

Or do you run them at say half that ... or ?????

I see the previous designer used 1K ... so he is only supplying them say 10 mA (assuming the Vf is about 2 volts).

I want these to last a long long time ....

Is it normal to supply 1/2 the current?

Is there some kind of lower limit you should not go below?

Thanks!

 

Lower limit, not generally. If the LEDs seem bright enough at 10ma then stay with a 1K resistor or higher.
LEDs are normally operated at 20ma to achieve the rated amount of illumination in their specifications

 

My question ... for the sake of longevity ... do you run these at the full 20 mA?

Not for indicators. I'll often operate standard brightness at a few mA.

If you have issues with ambient light, you can use ultrabrights at low current.

 

Generally speaking, the closer you get to the max rated current, the shorter the service life. If they're bright enough to see and lasted a very long time with a 1k (lower current), then keep running a 1k.

 

Awesome guys! Since all my "hobby" projects don't have to run 24/7 for years on end, I just normally run LED's at 100%

When I started thinking about it, maybe this was a bad idea.

i will keep the 1K

Thanks!

 

Do you run your car at red-line all the time?

 

An Avago 3mm HLMP 2V 2mA put out 2.1micro-candellas of light which is faint by today's standards.
Talking about today standard leds such as a 5mm superbright drawing 20mA,
A small voltage regulator board with the reguator fixed at the ( Vf ) voltage forward and resistors to adjust led current.

It is the light emitting diode's higher junction temperature that degrades the junction and sometimes the light output.
The manufacturer might recommend an aluminum heat sink. Depending on the process and materials one can estimate
by temperature which is approximately 15% less current. We can say that leds like to run cool to last long time.

The datasheet operating conditions usually show the minimum. typical and maximum given in volts and milliamps.
The typical voltage and typical current is a close approximation to aim for. The voltage regulator may or may not need a heat sink.
A temperature measurement is always a good idea all over the whole circuit

 

Since all my "hobby" projects don't have to run 24/7 for years on end, I just normally run LED's at 100%

Hobby or not, we still don't tend to operate any components at 100% of their ratings.

 

The 20 mA current was the rated current for the rated life of the LED providing the rated light output. Operation at a lower current will extend the lifetime and reduce the light output.
So it is always a trade-off between brightness and lifetime when choosing the current. For a stable system with a reasonably constant voltage supply, a simple resistor to limit the current is totally adequate.

 

I set the LED operating current at a level that gives adequate brightness for the given situation. Depending on the efficiency of the LED, 2-5mA is usually sufficient in most cases.

 

I am replacing a few simple control panels.

I had to do some reverse engineering since the person who designed and built these has retired and we believe passed away.

The panels are 30 plus years old.

They use just standard 5mm T 1 3/4 LED's as indicators.

The LED's are still working fine.

Say I use a standard LED (just looked one up at DigiKey) ... 2 V forward and 20 mA.

The circuit is 12 volts.

So, doing the math I get 500 ohms ( R=(12-2)/.02 )

I would pick a standard 510 ohm resistor.

My question ... for the sake of longevity ... do you run these at the full 20 mA?

Or do you run them at say half that ... or ?????

I see the previous designer used 1K ... so he is only supplying them say 10 mA (assuming the Vf is about 2 volts).

I want these to last a long long time ....

Is it normal to supply 1/2 the current?

Is there some kind of lower limit you should not go below?

Thanks!

No accurate way to answer your questions without an LED datasheet spec that shows the various curves for the LED you intend to use. If you really want to get the max lifetime out of new LED's, choose LED's that have good datasheets so you can review the specs before making a choice.

 

...do you run these at the full...

Never. Always at ~50%.

 

I set the LED operating current at a level that gives adequate brightness for the given situation. Depending on the efficiency of the LED, 2-5mA is usually sufficient in most cases.

This is my practice as well.

Don't know if anyone else has ever seen this but I somehow think I've seen some LED's with data sheets stating the MAX current at 30mA. More recently I've been using super-bright (SB) LED's. Running them at 20mA makes them awfully bright and annoying. I've seen SBLED's run as low as 5mA. MrChips states he's run them as low as 2mA, so there you go. And as others have said, use the brightness suitable to the conditions where they will be observed.

My truck dashboard is LED illuminated. I tend to run it on it's nearest dimmest setting since I don't like a lot of glare when driving at night. I just wish I could dim the screen on my radio/navigation screen.

 

My truck dashboard is LED illuminated. I tend to run it on it's nearest dimmest setting since I don't like a lot of glare when driving at night. I just wish I could dim the screen on my radio/navigation screen.

Have you tried placing an LDR in series with the LED and series resistor? You would need a low resistance LDR. This should make the LED dim at night.

 

Toyota factory entertainment system. Not a job I want to pull out the head unit, open it up and then modify the system. I'll live with it. When it gets too bright and distracting I just close my eyes.

 

A dimmed indicator light (in this thread) is looked at. A bright flood light is used to light up something.
You do not want to look at a bright floodlight.

 

20nyears ago I designed the "high brightness" LEDs into a product because I could drive them directly from a CMOS gate thru a resistor, and the gate output was still high enough to drive the following circuit elements. Those LEDs were not getting much current at all.
So certainly the best choice for drive current depends on the application, that MAX value is the "redline"value, not the target value. An amazing number of folks do not seem to understand that.