LED Turn On Voltage

C

chrisab508

New Member

• Dec 21, 2009

• #1

Hey Guys,
I have a bunch of green LEDs (not super bright), the plastic around the actual LED is physically green, but they did not come with a turn on voltage. 3V seems to work but they're not too bright, is there any way I can figure it out? What's the standard turn-on voltage for non super bright green LEDs?

Thanks,

- Chris

 

RadioRon

Well-Known Member

• Dec 21, 2009

• #2

I think that green is typically around 2.1 V. It is quite easy to measure it yourself. Connect the LED in series with a 150 ohm resistor and hook this circuit up to a 5V power supply. You should get about 20mA of current flow which is a common amount of current at which to specify the voltage drop of the diode. Just measure the voltage drop using a voltmeter.

 

MikeMl

Well-Known Member

Most Helpful Member

• Dec 21, 2009

• #3

Is it possible that they have a built-in resistor?

 

audioguru

Well-Known Member

Most Helpful Member

• Dec 22, 2009

• #4

Modern green LEDs are made the same as blue and white ones and have a forward voltage of about 3.2V to 3.5V. They are very bright.

 

C

Claude Abraham

Member

• Dec 22, 2009

• #5

LED lamps must be driven with current, not voltage. Allow for the 3.5V forward drop, and size the resistor accordingly. Most lamps are rated for 20 mA nominal. I always use 10 mA or less, for long reliable lifetimes.

 

Last edited: Dec 22, 2009

Mr RB

Well-Known Member

• Dec 22, 2009

• #6

If they have green plasitc they are most likely 20mA max. So connect an milli-ammeter and variable power supply (or just a 1k pot) and adjust it to 20mA then read the LED operating voltage.

I also agree with Claude you should only run it at about 1/2 its max current so 10mA is plenty.

 

Willbe

New Member

• Dec 22, 2009

• #7

The LED turn-on voltage is probably not controlled by the manuf. and may vary +/- 25%.