5.5v DC supply on a 12v LED ?

Hi thanks for reading.

I'm building (from the ground up) a set of 3x6" > Amplified 3 watt stereo Planar full-range speakers to be powered from a Cell Phone. I have most of the required parts which are many.

One of the parts of the system I'm installing is a Blue LED (Amp On) Indicator. I'm running the system from a USB Charger or a Cell Phone USB Port at 5.5 volts. I went forward and ordered the Blue LED's and they haven't arrived yet so I can't test.

These LED's are from E bay with little info of course. They are said to be standard, Blue 2mm 12 volt LED's. (Not Ultrabright) 

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My question to you who may know:

Are (Rated) 12 volt LED's likely to light from a 5.5 volt power supply?


I guess my confusion is between the 12v claim and the LED Color Standards for a Blue LED..
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Without knowing the complete situation, here is the resistor value I've been able to come up with based on *LED Color Standards (Blue) and Supplied Power (5.5v):

(*Blue) LED calculator: Current limiting resistor value:
Source voltage = 5.5v
LED voltage drop = *3.5
LED forward current = *20mA
(=)
Current-limiting resistor needs to drop 2 Volts. The resistor value is 100 Ohms, the next higher standard value resistor is 120 Ohm. The resistor will have to dissipate 0.04 Watts, the resistor should be rated 0.067 Watts, a 1/8 Watt resistor should do it.

Final: 120 Ohm, 1/8 Watt. (color code: brown red brown, Silver).
(100 Ohm: Brown, Black, Brown, Gold).
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Thanks for the help! I hope I made some sense here ;)


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caitlinsdad2 years ago

12v LEDs are usually found in big instrument panels or for automobile use. Electronic project parts LEDs are usually made to be lit up around 3 volts. The resistor besides regulating the optimal voltage to light up the LED, prevents burnout from the power surge when power is applied. The 12v LEDs might light up with usb 5.5 v but should be really dim if they light at all.

Rexrich (author)  caitlinsdad2 years ago

Ok Thank you caitlinsdad!

Well bad news cause now I have to find 6 volt Blue LED's and they need be very small for this app. 2 mm is as big as they should get. Flat top is also preferable.

Any idea where these can be bought?

Digikey, Jameco, Mouser, Adafruit and ebay but you get the import stuff that may be unreliable. It's generally the 3v LEDs that you are looking for and the resistor drops the power supply voltage. Look up LED Calculator for online widgets to figure out the resistors needed.

Rexrich (author)  caitlinsdad2 years ago

Ok checking it out will keep you informed. Thanks

Rexrich (author)  Rexrich2 years ago

Ok was able to order the 3 volt Blues from Evan Designs ModelTrainSoftware.

Wizard recommends a 1/8W or greater 150 ohm resistor.

Thanks again for the help.

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Do you have experience in simple audio circuits. I need to make a couple 1st Order 6Db slope @ 100hz High-Pass Filters. The tricky part is the filter need be on the Line-Level side. I know the Circuit but cant figure the Cap and Resistor values required. I had a good friend do all of these sort of thing for me but he died and things changed so I have to figure it out for myself. Anyhow can I ask you or is there a better area? :)

Rexrich (author)  Rexrich2 years ago

"Do you have experience in simple audio circuits."

I think I figured it out. Image below..

Thanks for the advice and any suggestions for informed are always welcomed.

Line Level Circuit Design.JPG