Voltage on a white LED.

[Herschel Peeler]

Just because the data sheet says 3.5 V at 20 mA it does not mean it must be used that way. Here is an exercise riving a white LED at different currents. Above is the schematic used. I started at the low end and increased current until the LED lit up. To my surprise the LED was almost always lit, even as low as 0.05 mA, yep 50 uA and it still lit a bit. Attached is a spreadsheet, an Excel file converted to pdf, that lists current in the first column, the voltage across the LED at that step and the wattage of the LED at that step. I had no way of measuring Lumen. At the end of the excel file is a graph of the results.
The LED was listed at 18,000 mcd at 20 mA. I went all the way up to 50 mA, which is about what I suspect my LED flashlight runs it at. My flashlight is just three AA batteries and four such LEDs. No resistors or current limiting.

 

[dorke]

You shouldn't be surprised at all.
The game when driving LEDs is the relations you get between current-Lumen-efficiency .

Look at the graph(a high current LED):
You get more Lumens as the current is increased(Red),but the efficiency drops(Blue).
A designer needs to find the "sweet-spot" for his application/needs.

Care must be taken for 2 things:
1. Don't exceed the Max allowed current.
2. Prevent sufficient cooling for the LED to prevent thermal runaway.

 

[Herschel Peeler]

You shouldn't be surprised at all.
The game when driving LEDs is the relations you get between current-Lumen-efficiency .

Look at the graph(a high current LED):
You get more Lumens as the current is increased(Red),but the efficiency drops(Blue).
A designer needs to find the "sweet-spot" for his application/needs.

Care must be taken for 2 things:
1. Don't exceed the Max allowed current.
2. Prevent sufficient cooling for the LED to prevent thermal runaway.

View attachment 29818

Re: heat sinks

On T5 size LEDs for instance you can solder a bit (0.5 sq in) of copper foil to the cathode lead.

 

[BobK]

10000 lumens at 20mA. That is some efficient LED! Maybe you meant 10000 mcd?

Bob

 

[Herschel Peeler]

10000 lumens at 20mA. That is some efficient LED! Maybe you meant 10000 mcd?

Bob

Thank you, yes, 10,000 mcd.

 

[Audioguru]

Please post the manufacturer's name and part number of the very bright LED or its datasheet. I suspect it is an ordinary fairly dim LED in a case that focusses it to make a very narrow beam.

I have a cheap Chinese LED flashlight (a torch but without a flame) that can adjust the focus of its beam to be dim and wide or bright and narrow.

 

[Herschel Peeler]

Please post the manufacturer's name and part number of the very bright LED or its datasheet. I suspect it is an ordinary fairly dim LED in a case that focusses it to make a very narrow beam.

I have a cheap Chinese LED flashlight (a torch but without a flame) that can adjust the focus of its beam to be dim and wide or bright and narrow.

The bag said RL5-W18030. I buy them by the 100. I didn't note the manufacturer. "Superbrightleds.com" it says
Sorry for the error in the original post. "10,000 Lm" has been corrected to 18,000 mcd".

 

[(*steve*)]

Sorry for the error in the original post. "10,000 Lm" has been corrected to 18,000 mcd".

For those who don't know the difference, lumens are a measure of total light output, candella are a measure of brightness.

LEDs are generally made brighter by focusing them (you'll note they have a smaller half-angle).

LED efficiency (measured as lumens per watt) has been increasing since LEDs were invented. This also increases the brightness for a given half-angle or allows for the same brightness and a wider half-angle.

 

[Herschel Peeler]

For those who don't know the difference, lumens are a measure of total light output, candella are a measure of brightness.

LEDs are generally made brighter by focusing them (you'll note they have a smaller half-angle).

LED efficiency (measured as lumens per watt) has been increasing since LEDs were invented. This also increases the brightness for a given half-angle or allows for the same brightness and a wider half-angle.

I acknowledge the error, know the difference and deserve the comment.

 

[Audioguru]

The white LEDs are 18,000mcd with a 30 degrees viewing angle and their "white" is very blue.

 

[Colin Mitchell]

LEDs like yours are rated at 17mA and should not be driven at more than 25mA.
You will get no life out of it at 50mA.

 

[Herschel Peeler]

The white LEDs are 18,000mcd with a 30 degrees viewing angle and their "white" is very blue.

Yes, 5,000K is "blue"

LEDs like yours are rated at 17mA and should not be driven at more than 25mA.
You will get no life out of it at 50mA.

If the objective is longest life you are correct.

 

[(*steve*)]

Yes, 5,000K is "blue"

5000K is daylight

Anything less than that is warm (3200K is a typical warm white), but practically something less than 5000K is often referred to as "cool white" even though 5000K is actually pretty neutral. Monitors are often set up for about 9000K. A clear blue sky starts about 10,000K and can be as high as 16,000K.

Again, practically, "cool white" in lighting terms is a white that is warmer than daylight, but which "feels" cool (bluish) in a household setting.

 

[Herschel Peeler]

5000K is daylight

Anything less than that is warm (3200K is a typical warm white), but practically something less than 5000K is often referred to as "cool white" even though 5000K is actually pretty neutral. Monitors are often set up for about 9000K. A clear blue sky starts about 10,000K and can be as high as 16,000K.

Again, practically, "cool white" in lighting terms is a white that is warmer than daylight, but which "feels" cool (bluish) in a household setting.

Good. Our living room and bedroom lighting are warm white. Kitchen and working (sewing and electronics) areas are cool white, mostly LED. Color representation is needed in the working areas.

 

[Audioguru]

Here are parts of the datasheet for the white LED: