# Forward voltage of blue 3 mm led

1. ## Forward voltage of blue 3 mm led

I have this package of 30 3 mm blue leds.

I do not recall where I bought them.

The pic shows one of them powered by a 18650 battery.

I was wondering what the forward voltage is?

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2. ## Re: Forward voltage of blue 3 mm led

The forward voltage of a little blue LED is about from 2.6V to 3.6V at 20mA. Each one will have a different forward voltage, but most will "typically" be 2.8V.
An 18650 Lithium Ion cell is 4.2V when fully charged which will quickly burn out any LED. A current-limiting resistor should be used to limit the current to 20mA:
1) (4.2V - 2.6V)/20mA= 80 ohms, use 82 ohms. Measure the voltage across the LED.
2) If the voltage across the LED was 3.6V then the current-limiting resistor value can be (4.2V - 3.6V)/20mA= 30 ohms.

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3. ## Re: Forward voltage of blue 3 mm led

Hi,

Why not
* look into a datasheet
* measure it
by yourself?

Klaus

4. ## Re: Forward voltage of blue 3 mm led

Originally Posted by KlausST
Hi,

Why not
* look into a datasheet
* measure it
by yourself?

Klaus
I have no data sheet.

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5. ## Re: Forward voltage of blue 3 mm led

Like most diodes, LEDs are constant voltage devices. That means the voltage across them remains fairly steady as the current through them changes. Their brightness is related to the current, not the voltage and it will increase with the current.

In your photograph, I presume (blinded by the light!) there is already some series resistor in the connection or the LED would most likely burn out when connected across the battery. Ideally, you need a variable power supply with a current limit circuit. You set the current limit to say 10mA for a small LED like yours up say 500mA for a high power LED and slowly increase the voltage. When the current reaches 10mA, you read the voltage and that is the Vf you are looking for.

If you do not have an adjustable power supply, make an initial guess at the voltage, it would be somewhere around 3V and find a power source a few volts higher, maybe a 6V or 12V battery. Work out the series resistor to limit the current using this formula:
(source voltage - 3)/desired current in Amps
So if you used 12V and 10mA the resistor would be 9/0.01 = 900 Ohms. Connect the LED and resistor across the 12V so it lights up. Then using a testmeter, measure the voltage across the LED itself, that is the Vf.

Note that LEDs like the ones you show are rarely rated for more than about 20mA.
Brian.

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6. ## Re: Forward voltage of blue 3 mm led

What is the voltage of the 18650 battery cell while it is lighting the LED?

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7. ## Re: Forward voltage of blue 3 mm led

The voltage is 3.7 volts for the 18650.

I have a wall power source that outputs 3.6 volts that I will probably use.

8. ## Re: Forward voltage of blue 3 mm led

Hi,

I have no data sheet.
Yes, I know, that's the problem.

But there are many blue LEDs, and every manufacturer provides datasheets.
Just go through a couple of them to see how much the values differ, or how good they match.
Don't you think it's worth a try?

Klaus

9. ## Re: Forward voltage of blue 3 mm led

3.7V is the "storage voltage" when the Lithium Ion battery cell is half discharged or half-charged. You are lucky that the blue LED did not instantly burn out. A wall power supply that is only 3.6V is too low for enough voltage to be across the current-limiting resistor.

10. ## Re: Forward voltage of blue 3 mm led

Thanks, I will do that.

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