# Comparing LED efficiency.



## Factus (Apr 13, 2011)

Hi all. first post here.

I am making an LED suit for South Africa's version of 'Burning Man' and need some help on comparing efficiency.
There seem to be very complicated ways to work out lumens/W with spectrometry etc but I cant find anything on keeping supply constant and comparing brightness..

But for a newbie like me, is there any way to limit current and separately pop two different LED's on the same current source and see which is brighter for the current it is recieving? If one makes more light on less current or the same current... you follow me..

These are just simple 5mm LED's, nothing special but the manufacturer assures me the one is more than douoble the effeciency which i should clearly be able to see. BTW I have experience with test loads from building my lasers.

I may not be on the right track because I know current draw relies on voltage but it was the first thing that came to mind to keep constant. Or maybe some kind of pot, turn it, get the same brightness as the other and then compare the value of the pot somehow. 
I have tried connecting lots to a teeny button cell (which i'm sure has a low max discharge) and then seeing which were brighter but in the process forgot which set of LED's are which - clearly doesnt help!!! besides, all of them seemed the same brightness but then again maybe i did not reach max current discharge. Very lost!

Both LED's are rated at standard 3.7v.

open to any ideas!
many thanks


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## AlexLED (Apr 13, 2011)

Hi Factus,
the simple answer should be: yes, use a constant-current power source (adjustable power supply, LED driver, ...) set at desired current (more precise, the current you plan to use in your application !) and measure the brightness, for example with a lux-meter. This should give you a good comparison.

Of course, there is a more complicated answer as well: differet LED (and even different batch/bins) differ in spectrometric distribution, voltage at a given current and the efficiency (lm per watts) even depends on the current ! So, even if the current is the same, different forward-voltages will result in different watts and thus different efficiencies ....

Hope that helps !


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## Factus (Apr 16, 2011)

Thanks very much al! I deduced that they are pretty much the same.

One more question: 
I am building a ne555 timer circuit to try to achieve a strobe effect for these guys. As I will be pushing over 250ma (which is max current rating) I obviously need to look at my options.
1) Saw something about using a transistor as the 'switch' and then when a small current from the timer goes through it it closes another circuit which makes the leds light up? Maybe I am way off.
2) Putting the chips in parallel.
3) Heatsink the chip?? or maybe 250ma is max WHEN heatsinked, I dont know.


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