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Resistor and LED Help

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Blazer_Kydd

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I am trying to put some ultraviolet LEDs in my truck to light up my custom gauges at night. The bulbs I have are a 3.3v forward voltage with a 4v max, with a 20mamp forward current. I am running a 12v system off a standard car battery with 6 LEDs total. I would like to know how much resistance needs to be used and how to run it. I don't necessarily need a schematic but I wouldn't turn it down. I have no clue what I'm doing and the only experience I have is the half of Electronics For Dummies that I've read. I tried to figure it out, but couldn't do it. Thank you for your help.
 

I would use a circuit like this. Two LEDs in series will drop between 6.6 and 8 volts, so with a 12 volt supply, there's between 4 and 5.4 volts across each resistor.

If you use 470 Ohm resistors, the current through each LED will be between 8.5 and 11.5 mA. The current will be higher when the battery is charging and the voltage is higher, but will still be below the 20mA maximum.

The resistor power rating isn't important; 1/4 Watt would be fine, but you can use ones rated for higher power if they're easier to find or cheaper.

 
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    IanP

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Yes, UV light causes fluorescent objects to glow. btw: Anything marked with a highlighter pen will glow under UV light too, so if anything's not glowing that should be glowing, just mark it with a highlighter.

[ot]
Some other things that glow under UV light are Gin and tonic, scorpions, and most laundry detergents.:smile:
[/ot]
 

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