This section has multiple parts:
1) SAFE ELECTRICAL SPECIFICATIONS TO USE FOR ANY LED
2) LED DATASHEET SPECIFICATIONS
3) LED BRIGHTNESS VS CURRENT CURVES
SAFE ELECTRICAL SPECIFICATIONS TO USE FOR ANY LED:
Below are some simplified specifications for LEDs when you wish to do a resistor calculation for any size of LED.
The summary data below is based on reviewing the datasheets (below) and coming up with a common range of values. Hence these specification will cover almost all of the LED products you can buy. I say almost only because I cannot predict the future or there may be some less than well made LEDs out there.
SUMMARY OF LED SPECIFICATIONS FROM DATA SHEETS
Important Notes:
1) Minimum Power Supply Voltage: This means you must use a voltage supply that has voltage above this value to guarantee the LED will light up under any and all conditions. If you use the "Blue" wire from a decoder, your good to go. If you use the "voltage drop (Vf)" specification as the minimum power supply voltage, it may work in 99% of the cases but there is NO GUARANTEE it will work from part to part especially if different manufactures are involved. Murphy's law.....
2) Voltage Drop (Vf): If working with a supply voltage that is above this minimum, one uses the this specfiication to calculate the actual LED current.
3) Using the "bright" specification above may NOT result in the brightest possible LED for a specific LED. But it will be darn close. If you want the brightest possible light output, you will need to consult the datasheet for the specific LED your going to use.
4) Not all LED's are created equal. Different brands or models of what physically appears to be the same LED can have different brightness levels for a given current. Stated another way, some are more efficient than others.
5) The LED Voltage Drop does NOT mean you can hook up a power supply with the same voltage rating, drive the LED directly and have it work RELIABLY. Working and working reliably are TWO DIFFERENT THINGS. Why? Unlike a light bulb, LEDs are current driven devices and a SERIES RESISTOR IS REQUIRED to prevent burning up the LED due to excess current flow. For more information, go here to figure out the resistance value to use: Resistors for LEDs/Bulbs
LED DATASHEET SPECIFICATIONS.
Below are some LED specifications taken from various Manufacture's DataSheets. T1-3/4, T1, 0805, 0603 and 0402 sizes are represented.
Nominal Current: The nominal operating current the LED manufacture used to develop the LED specification. 20mA and 2mA are common industry standards currents to allow comparisons.
Max Current: The maximum operating current the LED can operate with without destruction. This is a DO NOT EXCEED specification.
Typical Voltage Drop: The typical voltage seen across a typical LED at the nominal current.
Max Voltage Drop: The worse case voltage drop of the LED you could theoretically get from the LED. You must have a power supply whose voltage is GREATER than this specification to guarantee the LED will light up.
Max Reverser Voltage: If you hook up the LED backwards with a voltage greater than this shown, you run the risk of destroying the LED. This is a DO NOT EXCEED specification.
Hi-efficiency: LEDs designed to put out the same amount of light as a "Standard" LED of the same size, color and MFG but do it at a lower operating current.
LED BRIGHTNESS VS CURRENT CURVES
LED brightness varies linearly with current.
References:
1) http://www.zap-tek.com/webpage/Elect/lsn_4/014_lamp_res.html