Hi,
I recently bought this no-name 3W 365nm UV led from ebay. I connected it with a li-ion phone battery and it lit up fine. However I connected my multimeter to see how much current it was drawing and it showed 125mA. That's like only 0.5W :o
So is the led fake or am I doing something wrong here?
No information on LED, (you refer to "this", do you mean "a"), and no information on battery (voltage on load) provided. You say current draw was 125mA but if you don't specify the battery voltage and the value of any current limiting resistor fitted, also the resultant voltage across the LED, how are we meant to assist.
While it may be fake, you are doing something wrong. You absolutely cannot run such an LED just on a LiPo without current control.
Have you measured the LiPo voltage? I guess it wasn't fully charged and at around 3.6, and there are UV LEDs with forward voltages higher than that. If the LiPo would have been fully charged, you would almost certainly have fried your LED.
You cannot put a voltage on an LED and see which current comes out to get the rating. You have to know which current it can take, send it in with a constant current supply and then measure the voltage.
In this case, you can either use a constant current supply, or a higher voltage (I would suggest about 5V) with an appropriate (resistance and power rating(!), a normal little guy will not work) current limiting resistor.
I've bought alot of different LED's from eBay and I do not think any of them actually draw the current that it claimed it could. At the intensity of the light they put out, I was satisfied for what I barely paid to deliver that device 1/2 way across the world - since I could not USPS it to my neighbor down the street.
jackrae:
you refer to "this", do you mean "a"),
Sorry, it was supposed to be url but didn't show up. Here it is
http://www.ebay.in/itm/291979381993?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
and no information on battery (voltage on load) provided. You say current draw was 125mA but if you don't specify the battery voltage and the value of any current limiting resistor fitted, also the resultant voltage across the LED, how are we meant to assist.
Battery voltage was 3.7V. I didn't use any resistor because I thought it was a 3W led.
You absolutely cannot run such an LED just on a LiPo without current control.
My current control driver was rated at 700mA. I thought I'd check with a battery first.
Have you measured the LiPo voltage? I guess it wasn't fully charged and at around 3.6, and there are UV LEDs with forward voltages higher than that.
Li-Ion voltage was 3.7V. I also tried it with 3x fully charged AAA NiMH cells. Same result.
The LED forward voltage according to vendor was 3.4V - 3.8V.
I would suggest about 5V) with an appropriate (resistance and power rating(
Ok, I will check with 5V and post the result soon
I've bought alot of different LED's from eBay and I do not think any of them actually draw the current that it claimed it could.
Well my experience was the direct opposite. Except this one maybe:)
I have to correct my screen, it's late at night. Did you pay $280 for that? Is that 1 piece = a 5 or 10 pound box, stacked with sheets of them in there?
280 indian rupee for 2 of them including shipping. Approx 3.5 US Dollar.
I checked them at 5V with a 10k pot turned all the way down and current draw was at 0.5A.
Is that it's actual rated current draw or is it drawing too much and going to die?
Have you measured the resistance of your pot "all the way down"? Even 2 or 3 Ohm will limit the current. You cannot really tell anything from your tests.
Nobody can tell you how much your LEDs are able to take, but until they die, they will happily take what they get. Voltage-wise, they will do that VERY nonlinearly and also not in a stable way, which is why you have to control current, not voltage.
The LED physically looks the same as the 1w leds. You might be able to push 3w through it, how long it will last? Who knows.
I advise to push as little current as you need to be as bright as your happy with. When it gets too hot it will burn out.
If it hurts your finger, its way too hot.
If you can afford to buy 3 or 5 at a time, cost is usually lower per piece. This way if you burn one by accident, u have a spare, and know what its limit is. Its a generic led bought in bulk for low cost. Nobody tests them with precise tools, and the soldering sometimes is amature.
The resistance of the pot when it's all the way down is 1.8 ohms. - at 5V LED draws 500mA at this resistance.
LED draws 300mA when pot is at 6 or 7 ohms. Bith these current draw brightness looks almost the same but brightness drops considerably when at any higher resistance drawing 130mA.
Yepp, they look the same size as 1W ones. I'm actually happy with it's brightness at 0.5W, also I;m happy if it runs at 0.5W because I don't want to place it on a heatsink, however I just wanted to know whether it's a fake 3W LED.
I have run it only for a few seconds so I don't know how hot it gets. I didn't want to kill my LED by supplying too much power.
I can afford to buy more of them, but I don't want to buy fake ones. I just wanted to know if they're actually 3W LEDs and if I can run them at full power if need arises.
Battery voltage was 3.7V. I didn't use any resistor because I thought it was a 3W led.
Their is no way that comment makes any sense at all.
Do you think that a Li-ion battery is incapable of putting out more than 3W?
What saved the LED is that their are probably two or more in seriese in that thing you bought.
LEDs like this need driving with a constant current supply not a constant voltage.
The resistance of the pot when it's all the way down is 1.8 ohms. - at 5V LED draws 500mA at this resistance.
Is your pot rated for that? My guess is it is not and you are lucky you have not burned it out.
You seem to have little idea about what you are doing, I would suggest you do some reading about power LEDs.
Grumpy_Mike:
Do you think that a Li-ion battery is incapable of putting out more than 3W?What saved the LED is that their are probably two or more in seriese in that thing you bought.
No, I did just for 2 seconds to see how much it draws maximum. I was ready to sacrifice 1 led anyway.
Why didn't it draw more than 130mA if the battery was capable of high current output?
I did it just for 1 led, I didn't have enough voltage to power them in series.
Grumpy_Mike:
Is your pot rated for that? My guess is it is not and you are lucky you have not burned it out.You seem to have little idea about what you are doing, I would suggest you do some reading about power LEDs.
I don't know it's rating, it's a pretty big pot so I guessed it would do the job.
True, I read about power LED's and all of them suggest CC supply which I don't have.
No, I did just for 2 seconds to see how much it draws maximum. I was ready to sacrifice 1 led anyway.
And you seriously think that 2 seconds is too short a time to melt an LED? Or even damage it and reduce the output it will give.
I had done it with some cree LEDs and they had no problems.
Yea 2 whole seconds isn't really enough time. I practically have to take a nap while I wait for my arc welder to melt a tack between 2 pieces of 1/4" steel. Next time I'll hold a stop watch against how long it takes my eyes to blink before the house mains power cuts off when I short across the main circuit.
Can't you be bothered to accept the advice given in your previous posts re 2Watt resistor for driving your 3Watt LED. It was quite clearly stated that you cannot connect an LED directly to a battery without some form of current control.
But you obviously know better !
jackrae:
Can't you be bothered to accept the advice given in your previous posts re 2Watt resistor for driving your 3Watt LED.
Holy crap, it's the same guy. I even thought about linking that thread to explain it. It was clearly stated there ... This guy is really resistant to learning ...
jackrae:
Can't you be bothered to accept the advice given in your previous posts re 2Watt resistor for driving your 3Watt LED. It was quite clearly stated that you cannot connect an LED directly to a battery without some form of current control.But you obviously know better !
and some guy posted there that I don't need any.
I know that LEDs need current control. I was just experimenting with an extra unit to see what happens with the current draw.
DocStein99:
Yea 2 whole seconds isn't really enough time. I practically have to take a nap while I wait for my arc welder to melt a tack between 2 pieces of 1/4" steel. Next time I'll hold a stop watch against how long it takes my eyes to blink before the house mains power cuts off when I short across the main circuit.
Actually that depends on what kind of fuse you use. Once I had a friend replace a burnt mains fuse with a stay wire. After a few weeks I short ciruited something and the street electric pole fuse burned out instead of the one in my house. 
I had subjected some of my cree LEDs to torture tests and they all seemed to work fine even after I had powered them with a 18650 without any current control. So I thought I'd play with these and see the current draw.
This guy is really resistant to learning ...
I'd rather learn from experimentation. So far everything is working fine. Would you mind explaining that.
I'd rather learn from experimentation. So far everything is working fine. Would you mind explaining that.
In the other tread, people explained all this stuff. Why you need current limiting resistors, what the relationship between voltage and current is in LEDs and all that stuff. And now, you are still standing here supplying unlimited current to the LED, which won't tell you ANYTHING and claiming you "experiment".