How to Define value of Resistor used for LEDs

[DigitalWork]

Hi Mentor,

I am a beginner in electronics. I want to know that what value of resistor to be used for LEDs. Also when LEDs are in parallel and when they are connected in series.
What all happen internally means how much current flows . How much voltage gets drops. and does the current flowing in source changes if it enters LEDs.
Means like if LED has forward voltage as say 2.7V and 20mA current,then if source can provide more than this current then does the LED can take more than 20mA current. And if source can't provide 20mA then will LED not glow.How the resistor value calculated.

Thanks in Advance
DigitalWork

 

[betwixt]

A LED is a constant voltage device, that means it takes almost no current until you apply "Vf", it's forward conduction voltage but from there on it tries to hold the voltage constant by drawing more and more current. So to drive them properly you need to satisfy two conditions, 1. you have at least Vf as your supply voltage and 2. you limit the current to a safe level.

Putting it a different way, if Vf of your LED is 2.7V and you connect it directly to a 2.6V supply it may not light up at all but if you connect it to a 2.8V supply it might burn out.

What you have to do is ensure the supply is greater than Vf and use a resistor in series with the LED to prevent it drawing too much current. The resistor is calculated using Ohms Law, R = V/I. The 'V' is the voltage difference between Vf and the supply voltage and I is the current you want the LED to draw. For example if you have a 12V supply and you want the LED to draw 20mA you would need to drop (12 - 2.7) = 9.3V at 0.02A (20mA) so the value would have to be 9.3/0.2 = 465 Ohms.

Brian.

 

[kam1787]

http://ledcalc.com/

BTW it is usually a bad idea to put LEDs in parallel unless each LED has it own resistor - otherwise the slight differences will result in one LED grabbing more of the current and burning out, and then the next LED and the next

- - - Updated - - -

Hi Mentor,
And if source can't provide 20mA then will LED not glow.How the resistor value calculated.

the atypical LED will emit some light even at 1 or 2mA. See http://ledcalc.com/ for how to calculate the resistor values.

 

[Audioguru]

An LED has a range of forward voltages because they cannot make them exactly the same like light bulbs. Your "2.7V" LED might be 2.5V, 2.7V or 2.9V.

The LED with the lowest forward voltage will use the most current so to avoid burning it out, use a resistor value that will provide 20mA.

Use a fairly high supply voltage so that your LEDs have almost the same brightness.
If you use a 6V battery to light a few LEDs then for 20mA in the 2.5V LED the resistor value is (6V - 2.5V)/20mA= 175 ohms which is not a standard value./ If you use 180 ohms then the current in the 2.5V LED is (6V - 2.5V)/180 ohms= 19.4mA. The 2.9V LED will have a current that is (6V - 2.9V)/180= 17.2mA which is almost the same brightness as the 19.4mA LED.

But if you use a 3V battery and calculate (3V - 2.5V)/20mA= 25 ohms but use 27 ohms then its current is (3V - 2.5V)/27= 18.5mA. the 2.9V LED will have a current that is (3V - 2.9V)/27= 3.7mA which will appear very dim.

Another problem with using a supply voltage that is too close to the LED voltage is if the supply is a battery. A 6V alkaline battery (fore AAA or AA cells in series) is 6V when new but drops to 4V as it is used. Your 2.9V LED with the 180 ohm resistor will still light up. But if you used only two cells then their 3V will drop to 2V and the LEDs will not light.

 

[DigitalWork]

Hi Mentor,

Thank you for clearing my doubts and giving basic knowledge about choosing resistors and LED function.Now I can imagine a clear picture.Thanks
As you have mentioned that LEDs cannot be made same like bulbs where in bulbs we don't require constant forward voltage why LEDs can't be made like bulbs?

And if small voltage difference is taken say as Source voltage to be 4V and Vf for LED be 3.2V then also high current will flow and LED will burst?
Upto how much current (minimum ) LED will glow bright?

And how does voltage remain constant by taking more and more current.what does that mean?
If the Forward Current mention to be 20mA and if more than 20mA current provided will it burn the LED?

 

[Audioguru]

A light bulb is a certain length and thickness of tungsten wire. It is easy to make them all exactly the same.
An LED is a diode that has a certain forward voltage and is difficult to make them all the same.

Some light bulb are huge and can take a huge current. But some are small and take a small current.
Some LEDs are ...... (as above).

Electrical and electronic parts have datasheets printed by their manufacturers so that engineers can see their spec's and use them properly.
The datasheet for an LED has its part number, its absolute maximum allowed current, its recommended current, its forward voltage range at a few currents and many more spec's.

Most ordinary LEDs are 5mm in diameter and have a recommended operating current of 20mA. 30mA or more continuously will burn them out. They can be pulsed for short durations at higher currents.
The datasheet graph of the typical forward voltage for a red LED (the forward voltage is determined by the color of an LED, look in Google) that I use is attached. It shows that at 1mA its forward voltage is 1.6V, at 10mA it is 1.7V, at 20mA it is 1.8V, at 40mA it is 1.9V and at 100mA it is 2.3V. If the current is doubled then its forward voltage is not doubled because it is a diode.

Our vision is slow so that a pulse 30ms or less in duration looks dimmed. So the width of pulses (PWM) is used to dim LEDs as seen on cars and buses. The pulses occur at a frequency high enough that the LED does not appear to blink on and off (but it is blinking on and off).

 

[DigitalWork]

Thank you Mentor it helped a lot.

One line I did not understand is :-A LED is a constant voltage device, that means it takes almost no current until you apply "Vf", it's forward conduction voltage but from there on it tries to hold the voltage constant by drawing more and more current.

Does that mean current will increase till it reaches Vf of LED . So as mentioned in datasheet Vf to be let 2.7V and current to be 20mA. Then if we limit currernt to be 20mA then it will not reach 2.7 forward voltage as we earlier only limited current to 20mA.
Thanks in advance

 

[Audioguru]

One line I did not understand is :-A LED is a constant voltage device, that means it takes almost no current until you apply "Vf", it's forward conduction voltage but from there on it tries to hold the voltage constant by drawing more and more current.

Does that mean current will increase till it reaches Vf of LED?

No.
The LED current is almost nothing (and its brightness is almost nothing) when the voltage applied to it is less than Vf. When the applied voltage is higher than Vf then the current increases a lot when the applied voltage increases only a little.
See the graph I posted. When 1.6V is applied then the current is only 1mA and the LED is dim. With 1.8V applied the current is 20mA and the LED is bright. With 2.3V applied the current is 100mA.

So as mentioned in datasheet Vf to be let 2.7V and current to be 20mA. Then if we limit current to be 20mA then it will not reach 2.7 forward voltage as we earlier only limited current to 20mA?

If the LED has a forward voltage of 2.7V at 20mA and you limit the current to 20mA then the LED will limit the voltage across it to 2.7V.
You can use 5V and limit the current to 20mA with a series resistor then the LED will have 2.7V across it and the resistor will have 2.3V across it.
You can use 100V and limit the current to 20mA with a series resistor then the LED will have 2.7V across it and the resistor will have 97.3V across it.