LM317 and LED

[audioguru]

If you add a resistor in series with each LED then you must increase the supply voltage.
The max allowed current in most ordinary LEDs is 30mA. I have some Luxeon SuperFlux LEDs with 4 pins that are rated at 70mA but at 50mA or more they get very hot.

If you have two 3.4V LEDs in series they need 6.8V and the LM317 current regulator needs an additional 3.25V so the minimum supply voltage will be 10.05v and will not work with only 6V.

The circuit shown can be wired in many ways since everything is in series.

 

[Hero999]

If it's a high powered LED, I would recommend, making the supply voltage as near to the LED voltage as possible if you can, but not too near obviously as a minute change in voltage will lead to a huge change in LED current. What I mean is if it's possible use a 5V supply to power a single 3.5V LED instead of a 12V PSU. This wastes less energy in heat disipated by the resistor or regulator. Don't forget that the LM317 requires 3.25V to 4.25V of headroom (depending on the current) when used as a current regulator so it isn't always a good solution.

 

[audioguru]

You can power an LED or a few LEDs in series from 1000V if you limit the current properly because the LEDs set their own voltage.

Your calculation for the resistor in an LM317 current regulator earlier in this thread was wrong. You used the calculation of a 330 ohm resistor in series with a 3.75V LED powered from a 12V supply producing 25mA without an LM317.
Some online calculators are wrong. Do the simple calculation yourself.
An LM317 current regulator needs a voltage of at least 3.25V for itself in addition to the voltage needed by the LEDs.

 

[MikeMl]

...
The circuit shown can be wired in many ways since everything is in series.

How do you get equal brightness out of two LEDs wired in parallel?

 

[Hero999]

You don't, you should wire them in series, or connect each LED with its own series resistor.

To be picky no two LEDs are the same so the brightness will vary subtly even if they're connected in series.

 

[audioguru]

How do you get equal brightness out of two LEDs wired in parallel?

Look at the schematic. You match their forward voltages by testing and documenting the forward voltages of hundreds of LEDs and matching them.

Cheap Chinese flashlights (torches for limeys) have many matched LEDs in parallel. You cannot see a difference in brightness. But it will be difficult to replace one.