Forward voltage of blue 3 mm led

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fixit7

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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?

 

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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    fixit7

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Hi,

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

Klaus
 

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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    fixit7

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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.
 

Hi,

I have no data sheet.
Click to expand...
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
 

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.
 


Thanks for all the help.

My voltage source is 3 volts. Will I need a resistor in the circuit?

Thanks,

Andy
 

If you directly apply a voltage below Vf, the LED will not light up.
If you directly apply a voltage above Vf, the LED will burn out.
You MUST use a resistor or current limiting circuit to prevent damage.

If you do not know Vf, use a voltage a little higher than it is likely to be, say 5V and feed it through a resistor big enough that you can be sure the current will not damage the LED. Lets assume with 5V that you use 1K, that means the current cannot possible exceed 5mA and will probably be far less. Wire it up, the LED should light, measure the voltage across the LED, this will give you a reasonably good idea of what Vf is. From there, use the formula in the earlier posts to find the best resistor value to use for you application. Note that Vf does increase slightly with forward current and for 3mm LEDs the maximum rated current is usually no more than about 20mA.

Brian.
 
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    fixit7

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betwixt said:
You MUST use a resistor or current limiting circuit to prevent damage.
Click to expand...

Can't really stress how important this is. Once I saw a red LED slowly burning out from a direct connection to the power source without any current limiting resistor. Almost tripped myself from just rushing over to disconnect the power source. :bang:

Another way of knowing the specific forward voltage value for your LED is to use an adjustable voltage divider in your circuit and tinker with the potentiometer until you can see the LED turning on. You can measure the voltage flowing through the output of the voltage divider from there.

Just to add on with what the others have mentioned, if you are planning to keep the LEDs on for a very long time then do be mindful of the power dissipation levels.
 
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