How can I limit current for a led?

Question

How can I limit the current when the source voltage is the same as that which the led recquires? That is, same voltage but different current.

Answer 1

You have a sticky situation there.

If the voltage is EXACTLY what you need, then you have no need for a current limiting resistor.  But...

If your voltage source is not-so-well regulated, you will have problems.  Here is the deal:

Take a garden-variety red LED.  It is supposed to have a 2V drop, and will usually pass about 20mA at 2V.  But if the source voltage went up only 0.3V more, the current would shoot up to more than three times, and blow the LED right up.

As an engineer, I would have to suggest that you use an incandescent bulb in that application.  Or, if you have to use a LED, you could rig up an elaborate scheme (or a small DC-DC converter chip) to boost the voltage higher, so you could use a fixed resistor as a current limiter in series with the LED.

You did not say anything about the stiffness of your source.  If your voltage source is rather "spongy", you will get some natural current regulation due to the ESR of the supply.  But if it is a rigid voltage, expect to have problems powering your LED.

Answer 2

Craig is correct, you cannot operate LEDs in that mode, they need a current limiting resistor and a higher voltage supply.

The other answers are wrong.

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Answer 3

The LED will only draw the current it needs from the supply if the source voltage is the same as that of the LED. There is no need for a series resistor in this case.

If you limit the current by adding a series resistor, then part of the source voltage will be dropped across that resistor, taking the remaining voltage below that at which the LED functions, ie the LED will not light.

Refer to ohm's law calculation of  I = V / R for series resistors. Express the LED in terms of  V and I for it to function and treat it as though it were a resistor in the circuit. Then calculate the effect of adding a series resistor.

Answer 4

Current limiting resistors do so indirectly. They actually cause a voltage drop from the supply so the voltage across the LED is within the allowable range. Thus, perfectly OK to operate the LED with the design voltage. By Ohms's law, the current will automatically be appropriate.

But, note my warning about excess voltage in the other posting!

Also, you can put a dropping resistor if you wish, but brightness of LED will be much reduced.