Hi all,

For a LED having Vf=2V, If=20mA,

>Whats the ON current for this LED??
>Is that necessary to limit current to 20mA?? so much current !!!!

Thanks in advance.

 

The MAXIMUM led current is "If", amps forward,20 ma.
I run them at 10 ma because that's plenty to see them as an indicator light and probably increases their life span. They get pretty weak at 5 ma, but now you know how to treat them.

 

Some have 10mA, others are 20mA, and still others are 25 or 30mA.
High powered ones can be 100mA or 350mA and up to 1000mA.
Plus lots of ones in between.

The DATASHEET for the specific LED will give you the forward current (If)

It is often recommended to aim slightly lower in designing the power supply circuit.

Say 18mA, for LED's that have a 20mA forward current rating. This can increase lifespan of the device quite substantially.

You MUST provide some means for limiting current. Series resistors are the usual method. Constant current configuration of voltage regulators is another.

 

Thanks for reply,
Now I am final stage....

@ #12 - according to you 10mA is good for 20mA LED
@ Kermit2 - according to you 18mA is good for 20mA LED

How to decide that??

 

Because 18 is LESS than 20. It really is that simple.

Someone else may decide 15mA is what they want, and I choose 18mA. All part of the wonderful world of circuit design.

YOU decide what to do.

 

Fire it up and look at it. If it's bright enough at 10 ma, use that. If it's bright enough at 18 ma, use that. Just don't go over 20ma.

 

There are many times I don't worry about at all, I just throw a 1KΩ resistor because I have a ton of them, and know it will be under the rated current. LEDs are pretty rugged devices (other than connecting them backwards), it is easy to overthink them.

Many cases, where you are using them for illumination, you want the max, Given they have 10's to 100's of thousands of hours of life if you cut their lifespan by a few thousand hours by overdriving them slightly you would never notice.

They tend to announce clearly when you are abusing them. Changes is color is a clue.

 

An LED that is rated at 20mA has a maximum allowed current of 30mA or 40mA. If you operate it at "only" 20mA then it will last for hundreds of thousands of hours if is is name-brand.

If you want the LED to be at its rated brightness then use 20mA. If you want it to be dimmed a little then use 10mA.

 

so much current !!!!

That's why I use "low current" 2mA LEDs.

creakndale

 

Of course a current-limiting resistor will be needed.

Subtract the LED operating voltage from your working voltage to get the amount of voltage the resistor has to drop. Then use Ohm's Law calculate the value of the resistor. After that, use Joule's Law _P_ to calculate the wattage of the resistor.
I E

 

How much you choose to limit the current depends on the use for the LED. If it's for a simple indicator, then a lower current may suffice (10mA - 15mA), but if it main function is
to provide light for seeing things, then 18mA - 20mA might be better. For longevity, do not go over 20mA(If). Bottom line, I am agreeing with AudioGuru's assessment of (If).

 

They tend to announce clearly when you are abusing them. Changes is color is a clue.

This is true! If your red LED suddenly goes a bit orangey... then yellow... look out!