Flashing a high brightness LED

The effect you are describing happens when you send way to much current into an LED. These devices go from normal current to excessive current with only a few tenths of a volt change.

What is your voltage and what is the value of your series resistor?

Luhan

Is it this (depends on the 'circuit' but it's said to be constant current) or simply normal eye reaction to a too bright light?

Eyes are not suitable instruments to measure brightness. What you need is a flash-meter like:

You are correct (human vision quickly adapts to brightness). But, for hobby use, there are simple methods to measure brightness that might work here.

LEDs.

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Or maybe slow response from the supply providing the 'constant current' ?

Luhan

Are you sure the constant current source is not changing? You might want to put a 'scope across the LED and see if the voltage changes. Also, I don't understand about the constant current source being "ample". It's either right, or it's not.

Tam

Nico Coesel ha escrito:

I don`t see how a flash meter would help in the design, LEDs are meant to be seen by the eye, unless you are going for a home-made optical circuit. The person that sees the LED flash doesn`t give a damn what the flash meter says, if the naked eye detects that the light is inconsistent (or whatever you would call it) that's all that matters and that's what the design should revolve around.

I'm using an LM317 in constant current mode as per a data sheet....where output is drawn from the adjust pin of the regulator.

It calls for an input voltage that provides (3.45 x 2) the voltage drop of the LED's (2 in series) plus an additional 3 volts for the regulator.

I have 11 volts at 1 amp going into the LM317 input pin and show 300ma going through both the LED's.

Like one responder said, could it just be my eye sensitivity at such a brightness and color?

Or just the nature of the device?

It could also be my lens material which is acrylic (amplifying) the effect, I suppose.....it's does reduce as I move the lens farther away.

But considering the lens is a passive device?....it's just showing me what I normally wouldn't be able to see?

The eye has a response time. Be outside in bright light, and when you go in, you have to adjust to the lesser light. Be in a place with little light, and when you go out into the sun, you'll have to adjust.

The previous poster is suggesting that's what's going on. That it may not be a circuit issue, but that the original poster's eyes see a dimming because the eyes have adjusted to the light. The only way to rule that out is a light meter of some sort.

Mihcael

I have not "seen" this, but one can measure it, using the Luxeons. I think all LED's will dim as it heats up. The more heat, you get less light. You need to keep the sink below 25C to max out. The brightness shold remain fairly stable below ratings.

greg

You can use a photodiode and some LED's fed directly into a scope or meter.

greg

Here are some simple experiments you could try:

1. Bypass the 317 and connect a current limiting resistor directly to the

1. Turn the LED ON/OFF by switching the 11V instead of the load. This should also lend itself to some soft start scheme. If you are switching the load, I don't know what your current source is doing during the times it is running at no load.

Tam

All mentioned before...

1. The emitter could be dimming due to heating. When you apply current, this will heat the device causing the brightness to decrease. Are you properly heatsinking the device? Some high brightness emitters have a slug on the back for this purpose.

1. Your eye response. Use an instrument to monitor light output. Use a small emitter as a photodiode. Reverse bias it for wider dynamic range and faster response. Circuit is: series string of +power (5 to 10 volts), into reversed biased emitter, into resistor to ground. Resistor can be 50 Ohms to 1k Ohms depending on your response time needs. Connect oscilloscope across the resistor.

2. Your constant current supply isn't working as expected. Monitor the LED current with an oscilloscope.