So basically is there a minimum current for leds to be lit? I have a blue led that is very well lit with less than 0.4mA and it seemed strange to me.
LED currents are often quoted at, or near, maximum. This means they can put the maximum brightness in the data sheet, which makes the figures look more impressive.
Light brightness, like sound intensity, is not linear. That is why when you use analogWrite on an LED the dim values give noticeable brightness differences, but once you get about half way it seems to be already on full brightness and there's little difference between around 50% and 100%.
For the same reason, if you halve the current you don't halve the brightness. A small current and a large current can have a very similar visual intensity. Only once you start getting the current low enough do you start to see much change in the brightness. Once the current drops far enough the LED will fade out to nothing.
If you take your 0.4mA blue LED and sit it next to another one the same running at 20mA you will see a difference. The difference may not be as much as you expect given the difference in the currents.
I am asking because I want to find out what resistor I should pick so the led won't light. It is required so the led won't be affected by a pulldown resistor in my circut.
Vinter:
So basically is there a minimum current for leds to be lit? I have a blue led that is very well lit with less than 0.4mA and it seemed strange to me.
No there is no minimum current. Any current flowing will produce photons. If there are enough for the human eye to detect is another matter.
Vinter:
I am asking because I want to find out what resistor I should pick so the led won't light. It is required so the led won't be affected by a pulldown resistor in my circut.
You better explain what you want to do. It sounds like you are going about things wrong if this is your question.
Vinter:
I am asking because I want to find out what resistor I should pick so the led won't light. It is required so the led won't be affected by a pulldown resistor in my circut.
No resistor. It would need to be open circuit.
Grumpy_Mike:
Vinter:
So basically is there a minimum current for leds to be lit? I have a blue led that is very well lit with less than 0.4mA and it seemed strange to me.No there is no minimum current. Any current flowing will produce photons. If there are enough for the human eye to detect is another matter.
Vinter:
I am asking because I want to find out what resistor I should pick so the led won't light. It is required so the led won't be affected by a pulldown resistor in my circut.You better explain what you want to do. It sounds like you are going about things wrong if this is your question.
I tried to take your pulldown resistor advice in my charlieplexed cube circuit but it affects the way it's working. Check my possible wiring problem thread if you want for details.
Vinter:
I tried to take your pulldown resistor advice in my charlieplexed cube circuit but it affects the way it's working. Check my possible wiring problem thread if you want for details.
I'd suspect a mistake in the wiring, the pull-down resistors don't go from LED to GND, IMO, they shouldn't light any LED.
OK, I've just read DC42 answer in the other thread.... forget my post ;
majenko:
Light brightness, like sound intensity, is not linear.
No, that's strictly speaking wrong, Perception of brightness and intensity is non-linear, but
brightness and intensity themselves are ultimately measured in W/m^2 which is linear!
(Pedantry is its own reward I find 
Both eye and ear are roughly logarithmic in response - so a factor of 1000 is seen as three
times as strong as a factor of 10. With your eyes night-adjusted you might be able to see
an LED glow with less than a microamp!
LED light output is pretty close to linear with forward current except in white LEDs (where
the non-linear response of the yellow fluorescent compound affects brightness and colour
temperature).
Compare with tungsten filament bulbs where as the current falls the filament gets cooler and
eventually produces no visible light at all (but still infrared).
MarkT:
majenko:
Light brightness, like sound intensity, is not linear.No, that's strictly speaking wrong, Perception of brightness and intensity is non-linear, but
brightness and intensity themselves are ultimately measured in W/m^2 which is linear!
(Pedantry is its own reward I find
That's what I meant 
[quote author=Nick Gammon link=topic=183526.msg1359430#msg1359430 date=1376992043]
No resistor. It would need to be open circuit.
[/quote]At some high resistance value, wouldn't the voltage dip below the Vf of the diode and thus not lit?
tylernt:
[quote author=Nick Gammon link=topic=183526.msg1359430#msg1359430 date=1376992043]
No resistor. It would need to be open circuit.
At some high resistance value, wouldn't the voltage dip below the Vf of the diode and thus not lit?
[/quote]
No, because you weren't talking about lowering the voltage, you were talking about raising the resistance. 5V supplied to a 2V red LED, for instance, as you raise the resistance, the current will drop and the voltage will go down a bit on the LED, but not very far.
For the voltage on the LED to drop to zero, it would have to be either connected to an infinite resistance, or be shorted out.
polymorph:
tylernt:
[quote author=Nick Gammon link=topic=183526.msg1359430#msg1359430 date=1376992043]
No resistor. It would need to be open circuit.At some high resistance value, wouldn't the voltage dip below the Vf of the diode and thus not lit?
No, because you weren't talking about lowering the voltage, you were talking about raising the resistance. 5V supplied to a 2V red LED, for instance, as you raise the resistance, the current will drop and the voltage will go down a bit on the LED, but not very far.[/quote]So 5V -> 100 mega-ohm resistor -> red 1.8Vf LED -> ground will result in over 1.8V being present at the LED?
Electricity is weird.
It should result at 3,2 volts being present. Resistance only affects current. Volts and resistance act like constants in a circuit at least this is how I understand it. I think the drop changes a bit because of heat and other factors but not much at all.
Those straight lines represent the current vs voltage line of the resistor. Now picture that starting point as 5V and the line getting lower and lower as the resistor increases in value. As you can see, the voltage across the LED drops only very slowly as the current drops.
As illustrated on this page, the voltage curve of an LED varies. It isn't always a nearly vertical line.
Hello in my breadboard notice that using a BC574, the voltage in the off leds is form 0.1 to 0.2, i think that this voltage is passing trhought the transistor that is connecte in the negative side, any thoughts?
Measured in what way?
tylernt:
So 5V -> 100 mega-ohm resistor -> red 1.8Vf LED -> ground will result inover1.8V being present at the LED?
Yes
Electricity is weird.
No. It would be if it worked the way you think.
It is a common beginner mistake to think that a resistor reduces voltage per say. A resistor limits current. Using two resistors can appear to reduce voltage but only due to the current flowing through them.
A 100 Meg resistor connected to 5V at one end still has 5V at the other end if no current is flowing through it. Just like a 1 Ohm resistor connected to 5V at one end still has 5V at the other end if no current is flowing through it. No current is flowing if it is not connected to anything else.
Grumpy_Mike:
tylernt:
Electricity is weird.
No. It would be if it worked the way you think.
It is a common beginner mistake to think that a resistor reduces voltage per say. A resistor limits current. Using two resistors can appear to reduce voltage but only due to the current flowing through them.
A 100 Meg resistor connected to 5V at one end still has 5V at the other end if no current is flowing through it. Just like a 1 Ohm resistor connected to 5V at one end still has 5V at the other end if no current is flowing through it. No current is flowing if it is not connected to anything else.Basic Ohms law. "The voltage across a resistor is proportional to the current flowing through it." No current = no voltage. Or, to put it another way, V=IR. If I = zero then, whatever R is, V will always be zero.