Is the current important for the led ?

Other Hardware General Electronics
1

Hi guys,

I want to connect the following led to arduino
http://www.aliexpress.com/item/wholesale-100-pcs-5mm-white-Round-Super-bright-emitting-diode-20000MCD-LED-Light-free-shipping/1947430546.html?spm=2114.13010208.99999999.261.ikM5M9

It takes 3.4V and needs 20ma (i think).
meaning i need a 80ohm resistant (because the voltage of arduino is 5v and (5-3.4)/20ma = 80.

What happens if i'll connect 100ohm resistor, the current will drop by 20% to 16ma, is it critical ? will it make the led less bright ?

2

It will be less bright, yes, but some leds are so bright anyway, I've found that even under 10mA they're still perfectly visible.

3

That LED is very, very bright (20,000MCD).
However, this comes at a price ... a narrow beam angle (20-25 deg).
So if you're viewing it at 60 deg, it will seem quite dim.
View it head on, and it will definitely be too bright to look at!

4

I think you're on the right track... but for the benefit of others reading this thread, in hopes of mastering the fundamentals involved...

a) Current is ALWAYS important... about on the level that "money" is "important" in business! (^_^)

b) Theory is all well and good... in theory. And a good starting point. But you often have to temper it with the results of tests. So, in your scenario, yes, I'd start with the 80s. But.

If you make the resistor too "big", the LED will be too dim, or even off, as you surmised. Make the resistor too small, and enough current will flow that you damage the LED or the Arduino, or both. If the specs spoke of the LED operating on 20mA, that probably means that 20mA is a sensible level to use. You could probably push it a bit for an even brighter light, at the cost of increasing the change of a premature (up to "instant"!) burnout.

It's not just the LEDs limits which have to be considered. Remember to pay attention to the limits on what current can be run through an individual Arduino pin, AND the limits on the safe total current, when the pins' currents are combined, too. You may need to drive transistors with the Arduino, and use THEM to drive the LEDs.

Onward!

Hook up your LED, through a decent ammeter. ("Decent" adequate. Doesn't have to be "five star")

Look and see what ACTUAL current you get with your resistor. As I said... theory is fine in theory.

Once you've made that start, you are in a position to "tweak" the resistor value you're using with that particular batch of LEDs, if necessary.

====
Further analysis....

In the original post, we had....

(5-3.4)/20ma = 80

Here's what TauTau was doing....

With LEDs, Ohm's Law doesn't "work" in a simple way.

The "5" was the voltage applied at the "top" of the circuit.
The 3.4 taken from it was the "voltage drop" assumed for the LED. That number is typical, but for some LEDs you need to use a slightly different number, to have things come out "right".

The 20 was the current he wanted to see flowing, based on what he'd been told about what would be "right" for that particular (model of) LED.

After doing the arithmetic, out came 80. That said "you need an 80 ohm resistor".

Within the limits of the currents which a given LED was made to operate on, the "voltage drop" number for a given LED (and the others with the same part number, in the case of LEDs from a sensible supplier) doesn't change. Not with different "overall" applied voltage. Not when the current flowing changes, which it will, if the "overall" voltage is changed.

I hope the above helps someone see what the early parts of this thread were saying.

5

It takes 3.4V and needs 20ma (i think).
meaning i need a 80ohm resistant (because the voltage of arduino is 5v and (5-3.4)/20ma = 80.

Something missing above...

From the atmega328p datasheet, the voltage at an output pin sourcing 20mA will be about 4.5V @ 25C. So now we have (4.5-3.4)/20 = 55Ω. Select the next highest standard resistor value which is 56Ω.

6

tautau123:
I want to connect the following LED to Arduino

It takes 3.4V and needs 20ma (i think).

We seem to have a language problem here. You use the word "needs".

If you read the description on that page, it says:

Max. Forward Voltage: 3.4V
Max. Forward Current: 20MA

Now it is granted that neither you nor the poor old Chinese fellow writing that description understands how to write measurements - "ma" is American for "mother" and "MA" is millions of Amps; the actual unit intended is "mA" meaning "milliamps" or thousandths of an Amp.

The more substantial problem is that you do not understand the abbreviation "Max.". It stands for "Maximum"; which means an amount that you must not exceed. In practice, this means that while you might choose to operate the device at this level of current, you generally should not and should deliberately choose a lower value.

And in fact, the response of the eye is such that a 20% drop in intensity is extremely difficult to distinguish. So 82 Ohm (which is an actually available resistor value) would be perfectly sensible if you require the very maximum light output, but in all likelihood, a 270 Ohm resistor limiting the current to less than a third of that value, will give you more than enough light and you may actually want to reduce it even further.

7

I'm lazy, so I just use a 330 (ish) ohm resistor for any LED when setting up a circuit. Then if I need extra brightness for illumination, I'll reduce the resistor to match ohm's law at the actual voltage, to get the max candellas out of the LED. However for an annunciator, I want it dim, enough to see, but not to distract, I'll use 1K - 10K ohms.

Pay attention to voltage drop when calculating minimum resistances! Red LEDs drop the least, while UV drops the most. It almost follows the spectrum (inversely proportional to wavelength.)

8

Thanks guys,

Is there a "visible current range" parameter for LEDs ?

something like :
the maximum is 20mA
but the minimum for the led to turn on and be visible is 5mA or 10mA ?

From your experience, if i'll give a LED 5mA will it still be visible ?

9

I had one recently down to 6mA iirc, still very bright. Depends what you mean by visible of course, since ambient plays such a role.

10

Is there a "visible current range" parameter for LEDs ?

This LED matches the details of the one in your link. Take a look at Fig 2. Note that forward current over 20mA is pulsed ... at 80mA pulsed, the intensity reaches about x2.2 = 44,000mcd

From your experience, if i'll give a LED 5mA will it still be visible ?

Looking at the same graph, it shows 0 intensity. Looks like you'll need 10mA or more.

If wanting to use this as an indicator LED at close range, I think you'll be disappointed. The viewing angle is very tight. The LED will appear turned OFF beyond 30 deg angle. Check the Relative Luminous Intensity vs. Radiation Angle plots. How bright will it be at only 35 degree angle? The picture from your link offers a clue, also shows some luminous reflection at lower angles ...

11

tautau123:
Thanks guys,

Is there a "visible current range" parameter for LEDs ?

something like :
the maximum is 20mA
but the minimum for the led to turn on and be visible is 5mA or 10mA ?

From your experience, if i'll give a LED 5mA will it still be visible ?

It will be visible at 10uA! LED Light output is usually accurately proportional to current,
our eyes are logarithmically sensitive to the brightness of light so even 1/1000th as bright (in
terms of photon flux) is still visible (indoors). A 10% brightness difference isn't noticable in
practice, 2:1 isn't very obvious, 10:1 is obvious.

There is one caveat, white LEDs colour response depends on the phosphor response,
which may depend on brightness depending on the phosphor used. White LEDs are
really a blue LED and a lump of yellow/orange phosphor to turn some of the blue light to
yellow/orange.

Semiconductor lasers are not linear in this way, as laser action requires a threshold
current density, unlike LED action.