2.5V/5V white LED suggestions?

I’m looking to make a USB-powered mule light. In theory that’s a nicely regulated power source and a resistor is enough for roughly constant current. Though I’m not sure how close is the theory to reality. :wink:

Anyway, I hate how much power will be dissipated by resistors and I’m thinking about trying to direct drive 2 LEDs in series.
What would be my ideal LED?

  • 0.5+ lm at 2.5V
  • CRI90+
  • warm white
  • rosy
  • inexpensive

Such LED likely doesn’t exist….so what options did I find?
Pretty much 2:

  • XHP70.2 does 217 lm at 5.29V in koef3 test and 418 at 5.35V in Texas Ace test
    • even too powerful, a small resistor wouldn’t be a bad thing
    • very efficient
    • expensive
    • not rosy
    • it’s got tint shift and my mule won’t fix that
  • XHP50.2 does 206 lm at 5.38V in Texas Ace test
    • slightly smaller, cheaper, less efficient
    • overall not very different

That’s all that I have so far.
Any suggestions?

Try some high CRI XHP50A (or XHP70A) emitters, look for high CRI ones: https://www.arrow.com/en/products/search?q=XHP50A&filters=Manufacturer_name:Cree;In+Stock:Yes;

There is U-F2-27H, super warm, 2-step EasyWhite, and a lot of other high CRI emitters. While no rosiness warranty, it ensures super nice neutral tint.

Could also try mixing 2 different CCT 1S emitters, the resulting mixed output gets lower in the planckian locus.

Cheers ^:)

I didn’t think about XHP50. It doesn’t have a tint shift. Vf is higher, but probably still low enough? Are tints better?

5050 LED is quite large already, so tint mixing these just won’t work…

You want to minimize size but, can you afford to use a ∅20mm board? If so then you can tint mix, just grab some 4 chips 3535 6V 2S2P board(s): https://www.aliexpress.com/item/-/32727123202.html

Using a couple high CRI 3535 emitters close to the planckian locus should deliver a slightly rosy tint.

I once had the idea of using this configuration for an aspheric torch installing emitters in opposing corners. The result should be a nice combination of output, intensity, tint and CRI. ;-)

Cheers ^:)

P.S.: direct driving from a 5V source may mean relatively large brightness difference depending on source's output voltage.

Mr. Agro, how much brightness do you want for this LED? If it is not too much, just a resistor will work? Otherwise I think best to buy very simple tiny switching regulator and add 3V LED with resistor.

I want a light that plugs into microusb socket and is not much larger than the plug, a smaller variant of the following:

I don’t have strict size limits, but that’s ~5 mm wide and as short as I can make it. Which would be about 7.5 mm with XHP50.
I need a total of 1 lm, but 3 lm would be better.

Forum member Mr. Clemence has these nice high quality LED:

If you use one of this, from datasheet get 3 lumen with only 8.5mA of current. At 2.6V (from datasheet), the resistor only dissipate 17+mW, I think no problem at all.

TBH I’ve been thinking about Optisolis. And I think it’s a good way of doing such light. But I find 70 lm/W at that power levels to be very low, going 2s would nearly double efficacy.
Not that I’m so runtime-limited, it’s just the purist in me talking…

As to optisolis - shipping from Virence is very expensive which kills the deal. But I may abuse Arrow free shipping to get some cheap LEDs…
I could tint-mix using these or these

I hate the Arrow site. I found these LEDs on Mauser and checked if Arrow has them as well.

Remember that usb voltage can be any from 4.5 to 5.5V. Have you thought about one 7135 on the board?

You could always throw an LDO at it to knock down the voltage and keep it at a set level.

Another way of obtaining a certain mA level is a Current Regulating Diode. In this low of mA values, they’re not exactly cheap (like $2.50 at Mouser) but would be an easy way to get a set mA to the LED (s):

Here’s a 1mA

Agro, if you're after a very low level of output, why is efficiency that important? For a very low current the resistor in series thing will do, nothing will burn.

If efficiency is important then a tiny buck converter, period. No need for current regulation at low power levels, just tune its voltage output at the desired level. Since the led will barely heat up, there won't be any large difference in current flow between cold or hot status.

There's a large selection of high CRI emitters at Arrow. Check their range of XP-G3s, for example. The planckian locus tighter binned ones are those which number + letter suffix: E units are 5-step, G ones are 3-step. Remember, number + letter (not the other way around LoL).

Cheers ^:)

Thanks for the answers. I didn’t check up USB voltage accuracy before and wondered what would it be…
According to wikipedia that’s 4.45-5.25V for USB 3.0 and 4.40-5.25 for older standards. Quite bad. :confused:
No way 2s would work w/out some resistance added for stability.
Current limiting diodes indeed are expensive, but I found some 10 mA LDOs that should do the job:

Then I calculated how would a resistor work for Lumileds. I need 8 mA for the target brightness. 300 ohm gets me there with 5V.
At 4.4V I would be down to 6 mA. At 5.25 just over 9 mA. Not bad at all.

Efficiency is important because I’m a purist. Burning voltage down with resistance is not something I like, but burning about half the power is something I hate. Nevertheless…sometimes this is just the right thing to do considering other constraints. I don’t think that buck driver is worth the size and cost.

USB voltage right at the USB-A female connector will rarely be noticeably below 5V. Consider your voltage range to be more like 4.9 to 5.3V, Agro.


That’s not a problem….I intend to buy several values anyway and see what works best for me.
When I get 5-10 resistors for a single cent that feels like a giveway. Even if they stamp another cent of taxes on top of it.

I checked if one can have sense resistors at arrow this way, but found nothing cheap. Weirdly even 5% resistors with low resistance are usually at least around $.20 a piece, 100 times more than high resistance ones.

eBay is your friend Agro, some examples:


eBay is not my friend, they banned me and refused to tell the reason, instead saying “you know what you did wrong”. I don’t.
But aside from that…I have a few sense resistors from various sources, just thought that maybe I could stock up with some more for a giveaway price. It didn’t work up…:wink:

Re-register, they won't ban you unless you give them reason to. Mindset change and knowledge are your friends.


I tried. Got a ban within 1 hour. Was enough to make 1 order though.

Source: https://www.aliexpress.com/item/-/32615955837.html

Why are you getting banned?


P.S.: I got banned from CPF, but guess that is no problem. :-)