LED running at 100mA?

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realflow100

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Joined 2012
#1
I've bought a cheap 1$ LED flashlight from walmart (which is IMPRESSIVELY bright by the way.)

I wanted to make a kind of LED lamp light for my desk to run off USB with it

I used two 10 ohm resistors in series (i think they are carbon film or some type of metallic film of sorts. not wirewound. theres no inductance and they are silvery colored coating.)

going straight to the LED
run from 5v USB

its at 3v with just around 100mA running to it.. about 0.3W
It's originally run from 3 AAA's with two 8 ohm resistors in parallel on the circuit board.

I took the lensing and stuff off of it and all the housing and stuff

Soldered some leads to the back of the circuit board (the LEd is a surface mount type. no way of getting the actual LED itself off the board unless you got a heat gun.)

with the resistors going directly to the positive pad. plenty of solder to act as heatsinking just in case.

Putting my finger on the LED while its on gets pretty hot. almost uncomfortable but not quite. it seems to be stable voltage wise and current wise.

i guess its a 2.9 - 3v LED
Says about 45 lumens.
Eveready EVML33A-S Compact Metal 1 LED Flashlight
is one place to get it from.

With the reflector its got a crazy powerful distance beam.

Easily lights up an entire bathroom enough to read a book at almost any angle just by itself with or without the reflector.

Is 0.3W too much for it? it's still working after a good period of time testing.

Also found another place to get one. its on amazon. I cleaned up the link a bit.
https://www.amazon.com/Flashlight-Aaa-Led-Mtl-Compact/dp/B00EFKV1C2
 
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#3
No device is tripping.

The two 10 ohm resistors in series are reducing the voltage from 5v USB to 3v dropping 2v delivering 100mA to the LED at 3v
It runs the same from a 5v 400mA portable drill charger. as well as from computer.
And a usb power bank. runs about the same from each of these.

I wanted to know if 100mA is too much for the 45 lumen LED.
 
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#5
realflow100 said:
I wanted to know if 100mA is too much for the 45 lumen LED.
Click to expand...
White LEDs intended for general light sources are broadly classed as 1W or 3W devices. They are actually blue LEDs with a yellow phosphor and are designed to attain the most lumen output at 3.3V; 1W devices at 100mA, and 3W devices at 300mA. Flashlights like yours use 1W LEDs, so 100mA is expected. As a side note, older dollar LED flashlights used ordinary 20mA LEDs which were trickier or less efficient to power with stock batteries.
 
#7
Are you sure because it gets really nearly uncomfortably warm touching the bare LED.
It normally didn't even get slightly warm running off the batteries when it was run before.
and running at 100mA at 3v max its only at 0.3W and still heating quite a bit.
I tried running it at 400mA for a few seconds and it got SEARING UNBELIEVABLY HOT almost instantly.
and the voltage was limited to below just 3.1v by the LED itself dropping the voltage down of the power supply to 3.1v started at 3.2v then dropped to 3.1v seconds after.

its just a single surface mount LED square flat chip soldered to a small round PCB no significant heat sinking at all. unless you count the leads of the LED soldered to the PCB and the PCB traces as heat sinks.

so I used resistors and it's going okay.
I tried 10 ohms but it was way too hot running with zero change in brightness output to be honest.

so 20 ohms it is and what i use for it now.

I'm not sure what kind of LED it is. its a white-ish slightly warm light with an ever-so-faint greenish tint but you can only see it if you put a diffuser in front of it and cover the LED's face up.
if I put a white thin piece of plastic over the LED smack against the chip. it suddenly becomes VERY WARM colored.

I think it works best as a flashlight rather than a floodlight. it just doesnt quite have enough lumens as a floodlight without the reflector.
but with the reflector its a pretty good flashlight.. lights up just as good as daylight in a small area pretty much. no problem.
 
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#8
realflow100 said:
Are you sure because it gets really nearly uncomfortably warm touching the bare LED.
Click to expand...
One of the biggest problems that had to be overcome before LEDs became popular (and now cost-effective) for general lighting was heat dissipation. Yes, running a 3W LED at close to 1A without heatsinking kills it in seconds. Philips original Luxeon LEDs were bonded on a "star" aluminum plate, intended to in turn be placed on a big heatsink. Without the star, the bare LED itself is usually bonded with a thermally conductive adhesive to an insulated heatsink, where the insulation is a very thin fiberglass PCB where the connections lie, which is thermally one with the heatsink. That said, the 1W LEDs used in flashlights nowadays are run much below the 300mA maximum they are capable of to reduce the need for heatsinking or even eliminate it. They will still produce heat galore where one can't touch them comfortably for more than 10secs if that. One of my hobbies is creating custom LED/heatsink assemblies to replace existing fluorescent lamp fixtures for domestic aquariums. I never run 1A on the 3W LEDs I string up, limiting it to 600mA at most, on top of generous heat sinking (the decrease in brightness had it been 1A is barely noticeable). Another note: battery-based LED flashlights not only have minimal to non-existent heatsinking, the only thing they can do to still drive down costs is to use mere dropping resistors to limit current. Freed from the constraints of batteries, I would use a true constant current source and/or efficient DC-DC inverter.
Since LED bulbs are now cheap and commonplace, I suggest you get a 60W equivalent (actually consumes 9 to 12W) at walmart, screw it in & turn it on, and after a few minutes touch it all around. While not as hot as the incandescent it's meant to replace, it's still hot enough at the base where you can't touch it for more than a few seconds. That should give you an idea of how "hot" they are realistically permitted to be.
 
#9
it stays around 100mA of current even if the voltage I use is 5.3v or 4.8v
so I just stuck with two 10 ohm resistors in series.
The resistors only get mildly slightly warm.
but the LED is pretty hot. not totally searing hot but pretty hot.

at 100mA I dont feel comfortable giving it more. its already at pretty much maximum brightness. i doubt its a 1W LED because its super ridiculously small like a millimeter to a few millimeters. Tiny thing. less wide than a regular old style indicator LED
and its square.
I'm only giving it 0.3W and its really pretty toasty.
at 0.5W its SEARING HOT

I actually have a 5-6W LED bulb in my room right now and it lights up pretty good.
I removed the plastic diffuser from the front because it was reducing the light output too much.
the base of it gets pretty darn hot. And my room is chilly breezy cold!
 
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