# Is voltage across important?

## Main Question or Discussion Point

I suspect this is a very stupid question but:

Why is voltage across a component (say, an LED) important? Isn't current the only important factor?

If it says on the package 2.1V, 20mA. What does this mean?

Does it not matter if I connect it to a voltage of 5V over 250Ω or 3V over 150Ω?

Thanks guys!

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I believe the 2.1v would be the forward voltage, the voltage the LED will run at when it's limited at 20mA, like if you used a constant current power circuit. Check this site http://led.linear1.org/1led.wiz for a nice LED calculator.

Integral
Staff Emeritus
Gold Member
20ma is the max current rating of the diode. If you do not limit diode current to less then 20ma the life of the diode will be seriously compromised.

vk6kro
The voltage across a LED when it is operating is very important.

It tells you how many LEDs you can put in series across a 12 volt supply to make Christmas lights, for example.

It tells you if you could power the LED from a single 1.2 volt NiCd battery.

You have to put limiting resistors in to limit the current, of course, and if you have plenty of voltage to spare, you only have to worry about the current. But it is those times when you are trying to run something off a low voltage when it really matters how much voltage is across each LED.

Also, different coloured LEDs have different voltages across them, so a colour that will not work could be replaced by one that will.

First the voltage definition is the potential difference between two ends. the current is the flow of the electrons. let me tell you some simple example think about the over head tank - the voltage is the pressure difference between tank and the tap but current can be compared with flow of the water. with out potential difference (voltage) the flow of electron (current) can not exist.