LED color vs. dissipation

[DigiTan]

In an earlier post, I read that LEDs generally operate on the same current, but differ in their voltage drops depending on the wavelength. In terms of energy consumption, I had the impression that "low-energy" colors like red and orange need less power to provide the same brightness of, say...a blue LED.

Is this assumption correct? And how does each color rank in terms of energy consumption or transresistance? Also, what is the verdict on white LEDs?

Basically, I'm curious because I'm working on a circuit that uses a very small battery and I want to know if I should replace its white LED with some other type.

 

[audioguru]

Hi Digitan,
Your vision is the most sensitive to yellow-green. Red is distant from these most sensitive colours since orange is in between, therefore red doesn't appear as bright. Also, development has focused on ultra-bright LEDs using materials that make green, blue and white light. These ultra-bright LEDs require a supply of about 3.5V, which is more than two alkaline cells or a single lithium cell. They also need additional voltage for their current-limiting resistor. Therefore you need at least 4V for them when the battery is exhausted. Since most 6V batteries are regarded as exhausted when their voltage sags to 4V, a 6V battery is ideal for powering an ultra-bright LED through a current-limiting resistor.

Beware of ultra-bright LEDs with a very high mcd brightness number. They usually are focused in a very narrow angle. Good LEDs are 5000mcd and 30 degrees.

Is your very small battery capable of supplying the 20mA of current for an LED? How quickly will it be exhausted?

 

[zachtheterrible]

Beware of ultra-bright LEDs with a very high mcd brightness number. They usually are focused in a very narrow angle. Good LEDs are 5000mcd and 30 degrees.

What is wrong with that? In a flashlight, that would be perfect because you want a narrow beam. If it is too wide then the light will go all over the place and it won't work as well.

 

[audioguru]

Hi Zach,
If you hold a flashlight in your hand pointing to the ground directly in front of you when you are walking, a very narrow beam (6 degrees) would not show an area on the ground the size of your foot. A wider beam (30 degrees isn't very wide) would show 2 steps ahead.
For a long distance, a narrow beam is best but LEDs aren't bright enough to go far. I am not talking about high-tech LEDs like Luxion nor flashlights with many big battery cells and 24 LEDs. :lol:

 

[zachtheterrible]

oh :lol: I thought you were saying that the mcd rating has something to do w/ the viewing angle to . . . now im confused

 

[audioguru]

Hi Zach,
They make LEDs with a very high MCD brightness rating (20,000mcd) by simply focusing the beam into a narrow angle. That's cheating.

 

[williB]

Digitan ,is the LED just an indicator? or is there a reason to use white?

 

[Dr.EM]

On a similar note, can I just ask. If I was using an LED coupled directly to a CDS cell, a low viewing angle would be fine, wouldn't it? I plan on buying a 30 degree, 200mcd one for a simple optical compressor.

 

[Nigel Goodwin]

On a similar note, can I just ask. If I was using an LED coupled directly to a CDS cell, a low viewing angle would be fine, wouldn't it? I plan on buying a 30 degree, 200mcd one for a simple optical compressor.

I would suggest a low angle one would be BETTER, if you can get a flat faced one it will make life easier for you to mount then together as well.

 

[pike]

Also, what is the verdict on white LEDs?

From what I know these are blue LEDs with a photon filter layer applied to filter out the 'blue' and leave you with white. So theoretically they eat as much power as a blue LED (which is not a good thing!!)

Regarding that thing about decreasing viewing angle to increase mcd ratings, nothing beats a 5w Luxeon LED setup 8)

 

[audioguru]

Hi Dr. EM,
I think a fairly wide angle LED should be casting its beam on the entire active surface of the CDS cell for best efficiency. :lol:

Hi Pike,
An ultra-bright LED has a forward voltage drop of 3.5V and a max current of 30mA, therefore is 105mW. A 5W Luxion is 48 times more.
I think a Luxion would squash Digitan's little battery. :lol:

Hi Digitan,
I have read that a white LED is a blue one with a transparent yellowish phosphor on top. They are very bright-looking but since they are missing red, shining them on something that is red makes it look fairly dark. :lol:

 

[Dr.EM]

I tried the new LED, 30degree, green, super bright, and it does work significantly better. But, I want to make the compressor faster, by replacing the slow CDS with a phototransistor I bought.

 

[Nigel Goodwin]

I tried the new LED, 30degree, green, super bright, and it does work significantly better. But, I want to make the compressor faster, by replacing the slow CDS with a phototransistor I bought.

The CDS has the advantage that it's already a resistor - if you use a phototransistor you have the problem that you will have to redesign your circuit (perhaps quite a lot?).

How slow is your CDS?, I would expect it to be PLENTY fast enough for a compressor, which doesn't need to be that fast anyway? - obviously the attack needs to be faster than the decay, but CDS's are commonly used for the purpose.

 

[audioguru]

Hi Dr. EM,
How are you going to compress audio with a phototransistor?
Its output would be extremely distorted unless the level feeding it is very low. I had a cassette recorder that attenuated the signal then used a diode to ground as its compressor. With the diode conducting a DC current that was changed by the rectified average amplified output level, it compressed very well without noticeable distortion.

I tried an H11F1 optical-FET as a series, as a shunt then both as a compressor. It works very well when the input level is quite low. I couldn't use it in my Wien bridge oscillator because its distortion wasn't low enough but it might be fine for audio. :lol:

 

[Dr.EM]

Yeah, this is proving very difficult. The CDS speed is mabye a 10th of a second, but it needs to be more or less instant for use on drums etc. Its a nice sounding circuit even with the CDS, but this would be a nice improvement.

The circuit:

**broken link removed**

I have tried just sticking it in where the CDS is, don't ask why :lol:

Anyhow, I have "seriously" tried connecting it to an op-amp as an amplifier (I took this from a Mimms book where it is used as a lightwave reciever) then using the op-amps output on the gate of a FET, the Gate and sourse being used where the CDS is. Your going to tell me 1000 reasons why thats wrong
:lol: . Anyhow, it does'nt work, but it clicks and buzzes when I press a remote control near the sensor. Any much, much better ideas? :lol:[/url]

 

[audioguru]

Hi Dr. EM,
Here is a good, fast, FET audio limiter project:
**broken link removed** :lol:

 

[Nigel Goodwin]

:lol: . Anyhow, it does'nt work, but it clicks and buzzes when I press a remote control near the sensor. Any much, much better ideas? :lol:[/url]

Yes! - don't try and use a phototransistor for this purpose!.

Audioguru has posted a link to an FET design, FET's make quite nice variable resistors, and are often used in limiters and compressors.

 

[Dr.EM]

Very well then, cheers. I might try that limiter design out

 

[DigiTan]

Is your very small battery capable of supplying the 20mA of current for an LED? How quickly will it be exhausted?

So far, I've gotten 25mA from the battery and I'll expect a peak of about 40mA if it can manage. I'm using a 12V, 39mAh Vinnic alkaline, so I'm only planning to light the LED in quarter-second bursts every few seconds to save energy.

In response to williB's question, the LED is just an indicator, so I chose white just for high visibility. I might replace it with yellow or orange if white gives me a lot of battery life troubles. From the looks of the thread, I may switch to a bright yellow.


Here's another LED question: in the Digikey mail catelogs, I see they have "Red" LEDs and "High Eff Red" LEDs. Does "Eff" mean efficiency or there some other meaning to this?

 

[audioguru]

Hi GigiTan,
Your battery voltage is too high for powering a single LED efficiently because if the LED is a red 1.9V one then you are wasting 5.3 times the LED's power in the current-limiting resistor. If you use a 3.5V white LED, then the resistor wastes 2.4 times the LED's power.

You can get just about any peak current from a little battery if you put a capacitor across it. The battery charges the cap which can supply a huge current.

If you light the LED with a 1/4sec burst every 2 seconds, if the LED current is 30mA the average current is 3.75mA and your little battery will last only 10.4 hrs.
However if you light the LED with a 15ms burst like in my chasers every 2 seconds, if the LED current is 30mA the average current is 0.225mA and your little battery will last for 173 hrs. The very short burst of light will look just about as bright as a much longer burst and you can still save power if you flash it quicker.

A "high efficiency" LED is an old description for an LED that is a little brighter than an old dim one. A high efficiency LED might have a brightness rating of 2mcd while a modern ultra-bright one has a brightness rating of 500 or more mcd. When using an ultra-bright LED, you can use the enormous difference in brightness over an old LED by making a much brighter light, reduce its current so the battery lasts much longer or some of each.

Digikey stocks Agilent ultra-bright LEDs. :lol: