How to find out LED characteristics?

[jorrit]

I recently got a few blue leds from a friend. When I connect them to 5V with a 600 Ohm resistor they shine brightly (i.e. they even blind when looking straight at them). However, normally when the led would support 20mA I should be able to use even lower resistors. However I don't want to damage them. So is there a way that I can find out the actual type of the led so that I can use the optimal resistor for the best brightness?

Greetings and thanks in advance,

 

[audioguru]

A blue LED is about 3.5v. From a 5V supply, a 600 ohm resistor would supply it with only 2.5mA.

Most 3mm and 5mm LEDs have an absolute maximum continuous current rating of 30mA. The more is the current, then the brighter they are. Try a 68 ohm resistor with the LED and the 5V supply and measure the voltage across it and use Ohm's Law to calculate the current.

 

[jorrit]

A blue LED is about 3.5v. From a 5V supply, a 600 ohm resistor would supply it with only 2.5mA.

Yes, that's what I thought. So I was surprised to find out that it burns pretty bright already with that. But I'll experiment and measure the voltage. Thanks for the answer.

Greetings,

 

[Oznog]

Sadly, no. LEDs are not marked with part numbers in any way and there is no way to test their characteristics such as the max sustained current rating. And the current rating does vary somewhat. The light output and focusing varies a LOT.

It's a problem. I have a handful of LEDs scattered around my workbench that were taken out of their numbered packages and the only thing I can tell you about them is the color and a rough estimate of beam width when I give them some juice.

All you can do is err on the side of caution. 20mA for a blue in a 3mm or 5mm pkg is probably the most you want to run it continuously. 30mA is ok for momentary applications. If many are packed close together, it will increase the ambient temp so you may need to decrease the current to prevent heat degradation of the emitters.

 

[Hero999]

Often the maximum current doesn't give you peak efficiency, it's often better to use a lower current, use less battery power and make the LEDs last longer than aiming for maximum brightness.

Some white LEDs even glow dimly when I hold on leg and touch the other on an earthed object like a radiator.

 

[Andy1845c]

Some white LEDs even glow dimly when I hold on leg and touch the other on an earthed object like a radiator.

Really? What causes it?

 

[audioguru]

White and blue LEDs can be destroyed with static electricity.

It must be your body picking up strong mains, radio or TV signals and the LED rectifying them and causing it to light dimly.

 

[Andy1845c]

It must be your body picking up strong mains, radio or TV signals and the LED rectifying them and causing it to light dimly.

Wow. Never heard of that.....

If thats the reason, I would guess it only works if you live under high voltage lines or near a powerful radio or TV station

It would be pretty cool to see. I'll have to try it.

 

[Hero999]

Luckily ESD damage is quite rare in our humid British climate.

audioguru is correct, it's electrmagnetic radiation causing it to glow which is interesting because I'm not aware of any powerful transmitter in the area, there again there a probably loads of mobile phone masts nobody knows about.

I don't know what power is bieng supplied to the LED, (a few :mu:W maybe?) but it still works.

 

[Oznog]

I've seen LEDs glow when I touched them and the room is darkened. Actually IIRC I touched one lead to a DC power supply and was still holding the other LED lead (but never touched the power supply's other lead) it would light itself up.

 

[audioguru]

Like you hearing, your vision's response to brightness is logarithmic so you can see in sunlight and in moonlight. The iris has some automatic gain control for very bright light.
So you can see an LED with an extremely small current.

 

[Hero999]

I've seen LEDs glow when I touched them and the room is darkened. Actually IIRC I touched one lead to a DC power supply and was still holding the other LED lead (but never touched the power supply's other lead) it would light itself up.

That's probably capacitive coupling between you and the primary of the mains transformer.