## Determining LED Characteristics

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Guys, i'm fairly new to this, but have been lurking for a while :) I have looked all over the place, but don't seem to be able to find a suitable answer!

I have two bags of two-pin 5mm LEDs - no datasheets, no markings. Several are diffused and coloured, so should be easy enough to guess Vf (i.e. Normal Red ~1.8V, Green ~2.2V etc.) But they are mostly clear :?. For the coloured ones, i'll start at the guess voltage, and increment the current with limiting resistor(s) until I get a brightness that i'm happy with (or burn them out!).

Apart from buying a bench supply with precise control over voltage and current, and tweaking the current until I get a sharp difference in brightness, is there a better way to figure out Vf and VMax without testing to destruction?

The only other answer I can think of, is to just chuck them away and buy a load more where I know the properties (and make sure in future I keep them separate and marked).

Any tips or help would be very much appreciated.

Last Edited: Fri. Oct 31, 2008 - 05:55 PM

Use a 10 mA current source (some circuits in Wikipedia) and check voltage in the led.

Felipe Maimon

Thanks Felipe! I hadn't thought about constant current sources and measuring the voltage drop across the LED. If there are any high-brightness LEDs in there, as a general rule, will 10mA be enough current to light most LEDs?

Usually high brightness LEDs can be used with 20 mA. 1 mA is enough to lit a led, even old ones, so 10 mA will give you a good output to check on the Vf.

Felipe Maimon

Testing with a current source will just tell you Vf at that current. It will change some depending on the forward current. If you really want to test them, and you have a large enough quanity, I would suggest sacrificing one. Just use a voltage source of your liking, a minimum current resistor if protection of the voltage source is needed, a POT and the LED in series. Measure the current using a DMM and you should be able to find an acceptable brightness. If you aren't too aggressive with your test you probably won't kill it either. There is no way to tell the rated current, what you likely need to know instead of Vmax, so that you can get the maximum brightness.

anyway how can we measure brightness.
i mean with what current can we get 100% brightness and is there any easy way to impeliment current source arrays so we can run it with an avr and controll a 16x16 matrix of leds!

I love Digital
and you who involved in it!

Quote:
an avr and controll a 16x16 matrix of leds!

Two words:

Holtek HT1632

Hi Firestorm,

Take your +5V supply and test them with a 470 or 330 ohm resistor. Those should be reasonable values for starters.

Toss the bad ones. Categorize the others by color, and the size of series resistor you used to get a reasonable brightness.

JC

Ali_dehbidi wrote:

i mean with what current can we get 100% brightness

Definition required ... or perhaps the word "rated" needs to be in there somewhere. Most LEDs will be rated as so many mcd at such and such forward current. But it is usually within a statistical range. That is why some critical applications insist upon the items being supplied from the same manufacturing batch.

Any LED can be forced to emit more light than specified, but it will only do so until it fails ... usually in a very short time due to excessive heat.

Cheers,

Ross McKenzie ValuSoft Melbourne Australia

Thanks for all your replies and help.

I ended up filling my breadboard with the LEDs several times, and used a 5V supply and a 1K Pot with a 180R series resistor on a flying lead to quickly check the colour and brightness from ~5mA to ~28mA. I marked 5mA increments on the Potentiometer base, and measured Vf for each LED with my DMM when I had a nice output. Took me about an hour...

They all lit, were all kinds of colours, and are now in "labelled" bags :roll: