new here have a question about LEDs

[basicconcepts]

so my mom bought some really cool led Christmas lights so i started looking at them wondering what i could do with a set of 30 leds for only 7 bucks and it hit me... underglow... ive wanted to underglow my camaro for a long time but cant see my self spending 200$ on some neon tubes that are extremely fragile

these leds are battery opperated (3 C batteries) ive already "tested" the leds with a car battery and they work but... the resistor thats inside the battery compartment on the lights was glowing brighter than the leds... lol

so my question is what size resistor will i need (do i need a resistor at all) and i want to put an inline fuse in there too what size would be best

 

[audioguru]

Do you want to light the ground under your car? LED Christmas lights are too dim for that.

 

[basicconcepts]

ya but these lights are bright im not trying to get a complete ground lighting just a glow from under the car ive already test fit the lights under there and checked the glow using the batteries and its plenty

 

[audioguru]

Sure they are bright when you feed them with nearly 10 times the correct voltage (14.4V from the car instead of 3.5V to 4.5V from three C cells). Then they will burn out very soon.

 

[basicconcepts]

when i fit them i used the 3 c batts not the car batt anyways thats what i want to know what do i need to keep from blowing them isnt that what resistors are for??? (i took some classes in HS but that was like 7 years ago so my electronic knowledge is a little fuzzy)

 

[audioguru]

The resistors limit the current when the battery voltage is 3.5V to 4.5V. If you use 13.2V to 14.4V from the car then the resistors need to be re-calculated. Maybe two or three strings of LEDs can be connected in series and in series with a new current-limiting resistor.

But we don't have the spec's for the LEDs and don't know what resistor value is used now.

 

[basicconcepts]

how can i get the specs for the leds i know about the rings on the resistors but dont know how to read them

 

[audioguru]

The colour of an LED tells you roughly how much voltage it uses. We assume that the thirty LEDs are matched and are wired in parallel with a single current-limiting resistor.
You can look in Google for Resistor Color codes.

 

[basicconcepts]

well the leds are green and i yes there in parallel and the code on the resistor is brown black black gold

if i read this right it should be 10 ohm with a tolerance of +/- 5%

 

[Reloadron]

Without seeing the circuit itself and how it is configured hard to call but you seem to have a 10 Ohm series resistor in the battery compartment. If you have three C cells in series you have about 4.5 volts. So if we take about 13.5 volts / 4.5 volts we get 3. We could say 3 X 10 = 30 and try about 30 Ohms in series rather than 10 with about a 13.5 volt source. I would start with maybe a 33 Ohm series resistance. That is just winging it without getting into any detail. Simply something to try so the resistor doesn't light brighter than the LEDs.

(Orange Orange Black (Gold or Silver)) Just make sure the resistor can dissipate the needed power.

Ron

 

[audioguru]

The green LEDs might be old 2.2V ones or new very bright 3.3V ones.
Assuming 20mA in each LED then 30 of them draw 600mA. The voltage across 10 ohms with a current of 600mA is 6V which is impossible from a 4.5V battery.

If the LEDs are old 2.2V ones then the 10 ohm resistor will have 4.5V-2.2V= 2.3V across it and its current will be 2.3V/10 ohms= 230mA. Then each LED has a current of only 230mA/30= 7.67mA which is not bright.

A few groups of 30 paralleled LEDs can be connected in series and in series with a current-limiting resistor.
If 4 groups are in series and each uses a voltage of 2.2V then the total is 8.8V. The max voltage of the car battery is 14.4V when charging so there will be 14.4V-8.8V= 5.6V across a single current-limiting resistor. The resistor could be 5.6V/600mA= 9.3 ohms but 10 ohms is the nearest common value. Its power is 5.6V squared/10 ohms= 3.1W so use a 5W resistor which is pretty big.

Maybe 5 groups of LEDs can be in series for a total of 11V. Then the resistor can be (14.4V-11V)/600mA= 5.67 ohms but 5.6 ohms is the nearest common value. Its power is 3.4V squared/5.6 ohms= 2.1W so use a 5W resistor.

 

[basicconcepts]

soooo... from what i think i can see i need a 33 ohm 5 watt resistor???

ill take a pic and show how bright they are and what everything looks like

 

[basicconcepts]

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i dont know if this gives you any idea about brightness but it lights up my whole 3 car garage

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[basicconcepts]

so maybe you guys could give me some more help i cant find a 30 ohm 5 watt resistor anywhere

 

[Dragon Tamer]

It's a good thing Mouser is here to help because I just spent 1.5 minutes on it and got over 20 results for a 30 ohm 5W wire-wound resistor. power resistor

 

[audioguru]

Why not simply connect the groups of LEDs and their 10 ohm resistors into 3 strings in series? 3 x 4.5V= 13.5V which is near the 14.4V max of a car battery. Then you don't need another resistor.

 

[RMMM]

You see that little tag that says "For indoor use only?"

I may not be the brightest bulb on the tree, but cars are usually OUTSIDE.

1 little puddle, and your experiment is over.

You will need to hot glue every bulb base where the wires connect to water-proof them.

Turn this death trap off when pumping gas. You are not listening to what you are being told. You are only hearing what you want to hear.

Let me put it to you straight. You screw this up, they BURN. Burn = Fire. Fire + Pumping gas = a failed experiment because you are being cheap.

10 dollars in LEDs. 3 dollars in shrink wrap. A dollar or two in resistors, And some decent outdoor rated automotive wiring.

For under $20 You can use 5mm LEDs, much brighter, safer, and properly wired.

Take a few more minutes and you can use the car battery and a switch.

Its your car. Do it right.

Use the indoor Christmas lights for Christmas decor.

"Hey son, I just bought some Christmas lights!"
"Cool Mom, Im gonna hack them up and put them on my car!"

 

[basicconcepts]

It's a good thing Mouser is here to help because I just spent 1.5 minutes on it and got over 20 results for a 30 ohm 5W wire-wound resistor. power resistor

well im not to comp smart i pretty much know how to work my facebook and forums thats about it plus you know where to go all i can do is google search and try to figure out this crazy code of words numbers and symbols you guys use

Why not simply connect the groups of LEDs and their 10 ohm resistors into 3 strings in series? 3 x 4.5V= 13.5V which is near the 14.4V max of a car battery. Then you don't need another resistor.

first that resistor is probably done for it got so bright and hot that it melted the plastic case it was in a little and im not quite sure what you mean here 3 strings??? its already 1 string containing 30 bulbs with 1 resistor connected to the power switch and then the + side of the wires to the leds


im going to deff apologize for any confusion im new to all of this and sometimes dont read thing right so if anyone says anything and i miss it im sorry

 

[audioguru]

first that resistor is probably done for it got so bright and hot that it melted the plastic case it was in a little and im not quite sure what you mean here 3 strings??? its already 1 string containing 30 bulbs with 1 resistor

It is too bad that you burnt the 10 ohm resistors. Get new ones.
Yes, one string is 30 LEDs in parallel and they are in series with a 10 ohm resistor. Three strings have three 10 ohm resistors that are a total of 30 ohms that you need.

 

[basicconcepts]

but wont i just fry the new resistors too