next topic »

[on:]

Hello i was wonder if i know different leds take different voltage and current but what ones are the same?
is red the same as in green or blue how about what and orange and purple leds?


Joseph

[Reply #1 on:]

In most cases they will all be slightly different, even within the same brand, color, and batch.

The LEDs most alike will be the "white" ones based on Blue or UV LEDs and one or more phosphors.

[Reply #2 on:]

I believe they call it band gap, each color (except white and pink and colors that are produced from phosphorous) / wavelength has a unique voltage, generally shorter the wavelength lower the voltage.

White is simply blue and doped phosphorus, so blue and white have a similar forward voltage drop.

[Reply #3 on:]

I think it is the other way, shorter wavelength, higher energy on the photons and higher voltage. If the OP wishes to control the different color LEDs to emit equal brightness, then use some measurement to determine brightness and adjust what resistor you use with a pot instead of fixed resistor. Was that why you asked about voltage?

[Reply #4 on:]

thank you all that answer a lot of my thoughts on this subject.

[Reply #5 on:]

To some extent "brightness" is also down to the perception of the viewer. 

[Reply #6 on:]

The bandgap is a function of the semiconductor used, so the forward voltage is really
just a function of material and current-density.

The material chosen, though, should have a bandgap close to the photon energy
for efficient operation, so in general the forward voltage in volts is close
to the photon energy in electron-volts.  The precise colour depends on the
precise doping scheme used, often quantum wells I think.

Most white LEDs are blue or violet pumped phosphors, so are generally like blue
LEDs.

Very roughly:

1.4V - infrared
1.7V - red
2.2V - orange/yellow
3.0V - green/blue
3.5V+ - violet/ultravioet

In general LEDs wired as photo diodes are most sensitive to the emitted colour
but also respond to shorter wavelengths.

As with all diodes the forward voltage depends on current density and temperature too.

[Reply #7 on:]

Have a look at MarkT's posting above.
Now read the datasheets of some LED's to look for a chart of Voltage vs Current, etc.

The physics meaning of "red" is about 1.7 eV per photon, and surprise surprice a red LED starts to work when supplied with a bit over 1.7V.  Likewise green is about 2.4 V and blue a little more. 


Constant Current Driver
==================
One way to even up the outputs is to pick an "ultrabright" of each colour, and set up on pinboard to run each with "constant current" by placing a "bog standard npn bipolar transistor" such as a BC635 below it.  10kOhm from a d_out pin of the arduino at 5V supplies about 0.4mA to the gate of the npn.  That defines the current I_ce through the npn to be about x40 that, which should be about right for the LED's; 16mA each irrespective of colour.  You'd probably find that humans find the green most noticeable, so expect some tweaking from that by changing the 10k values a little.  Lastly, to be kind to your npn, measure with a multimeter the actual voltage drop on each LED, which should be about 1/2 a volt more than its tun-on minimum.  Choose resistors to go above each LED to 5V so that there is about 1V left to make sure that your npn is on. 

For example 5V - (1V transistor  +2.6V blue_minimum +0.5V blue_more ) = 0.9V surplus
16mA at 0.9V is about 55 Ohms above the blue.  Inserting that should not change the behavior of the test circuit, but avoids much of the internal heating in the npn.

Lastly, and only do this with spare parts which you can afford to blow and well away from your arduino, try cooking an LED by supplying 10mA to the npn gate implying up to 400mA LED drive current and direct 5V with no extra resistor.  You might get the LED so hot that its colour redshifts enough to notice.  Look up 610nm orange and 614nm orange-red for an example of the small change to expect near to catastrophic overloading.  It is worth doing that once, so that if you ever see it again you know the warning sign of "this LED is cooking and is about to blow".  There is a certain small ammount of cooking which an LED will tolerate and go back to normal if left to cool down.

[Reply #8 on:]

Hello i was wonder if i know different leds take different voltage and current but what ones are the same?

They're all different. That's why lighting LEDs isn't as simple as connecting a voltage, you need to fix the amps, not the volts.

is red the same as in green or blue

No.

how about what and orange and purple leds?

No.

Here's some typical values: http://www.ebay.com/itm/201088894985

Note that there can be a huge voltage variation even in LEDs of the same color, eg. red can be from 1.8 to 2.5V.

This why thinking in volts is wrong for LEDs.

LEDs work in amps, not volts.

[Reply #9 on:]

No such thing as amps without voltage.

(Think a 1000volts in static next to no amps involved) the resistor drops the rest of the volts.

[Reply #10 on:]

No such thing as amps without voltage.

Of course not, but that's not the point, is it?

[Reply #11 on:]

Of course not, but that's not the point, is it?

Well kinda....

I have a 1.5vdc 100amp power supply off of 240ac....

100 amps through an LED = kaboooom   (but it won't will it?.... no)

[Reply #12 on:]

thank you MarkT  and everyone else what about white led what voltage does that take?

[Reply #13 on:]

As I said most white are blue LEDs pumping a phosphor.  Some are violet pumping
a phosphor.  A few have two colour LEDs pumping a phosphor.  The datasheet for
the LED will show the spectrum which gives it all away...

[Reply #14 on:]

thank you help me a lot.