More efficient to use 2 series LEDs or a resistor?

[kalel]

In case of working on some kind of 5V USB powered lights (nothing specific in mind), would it be better to use a single LED and a resistor, or 2 LEDs in series?

Condition is that the LED used has a forward voltage of ~2.5-3.3v or so, and I mostly mean the 20-30 mA standard 3-5-8-10 mm LEDs. It's likely that the 2 LEDs won't glow at full brightness, but the question here is if that would make them any more efficient (more light per power used). If so, that might be interesting when considering battery powered (perhaps by power bank) sources.

There is one serious problem however, I understand that having only 2 LEDs in series could cause a short circuit should either one of the LEDs break. I'm not sure how that could be addressed without a resistor.

[Corporate666]

If you put two LED's in series, that doesn't do anything to limit the current.  The voltage/current graph of an LED isn't linear - it's anything but.  So you will essentially see little to no brightness with linearly increasing voltage... and then hit a tiny tiny narrow voltage range where you have close to a linear voltage/current relationship, then any extra voltage will cause the current to skyrocket.

So you still need a resistor.

If you are interested in most light for the power consumed, then two LED's in series are more efficient, but if you only need a certain amount of light (for an indicator in a USB device, for example), then you are much better off driving either with PWM or with a switching supply.  There are easy-to-use chips out there that do this for a wide range of current needs.  Diodes Inc, Allego and many others have such devices - there's literally thousands of others on Digikey. 

Bottom line - any power you burn up as heat isn't producing light and that's inefficient.  There's also the fact that many LED's are most efficient at very low drive levels on a "how easy to see the LED with your eyes vs. power consumed" basis.

[thm_w]

USB voltage can go down to 4.4V, so two 2.5V LEDs in series won't work reliably.
Unless you are going for the lowest cost or simplest design, use a constant current LED driver as Corporate has said.

You can get a constant current buck module from ebay for <$2, that would work well with a single LED. I'm sure some boost drivers exist as well.

[kalel]

USB voltage can go down to 4.4V, so two 2.5V LEDs in series won't work reliably.
Unless you are going for the lowest cost or simplest design, use a constant current LED driver as Corporate has said.

You can get a constant current buck module from ebay for <$2, that would work well with a single LED. I'm sure some boost drivers exist as well.

It's true that even relatively small voltage changes can affect the light levels significantly. Also, the buck module sounds like a perfect high quality solution for the purpose.

However, at low currents of some 20-30 mAh LEDs (unless there are a ton) I assume the voltage shouldn't drop much below 5.0v on a standard USB power supply. I did not verify this and could be wrong of course, but I would expect a normal USB supply (or power bank) not to go as low as 4.4V without a higher current load.

Although, the simplicity of having 2 LEDs and no additional components also sounds a bit appealing when/if possible to use, with some LEDs that match the relatively narrow requirements.

[mariush]

Some cheap usb phone chargers could output up to 5.6..6v at very low currents (for example they use 5.6v zener diodes to limit the maximum voltage)
Some very cheap usb cables would cause a lot of voltage drop at high currents so the power supply may be tweaked for more voltage ex 5.2v .. 5.5v knowing that at 300-500mA the cable the power supply comes with will drop some voltage on it .. but at 20-30mA you won't have much voltage drop.
Some usb outputs can be as low as 4.5v ..

Basically 5v from usb is not reliable.
You can put 2 red leds in series (~1.7v forward voltage) or green/amber leds , basically any led that has up to around 2.2v forward voltage in series with just one resistor to limit the current and you'll probably be fine. Won't be that much efficient , a buck regulator or led driver won't be super efficient, it would still have some losses, so it won't be much better than a plain resistor if your led current is low (let's say less than 5-10mA)
Blue or white leds are trickier as those require around 3v forward voltage... can't really put them in series as the 6v would not work with 5v.

Use led driver ICs that can do boost mode (raise whatever input voltage you give up to 6-7v required to exceed the forward voltage and get the amount of current you want through the leds connected in series). You typically set the peak current using a single resistor and the led driver boosts as much as needed.