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I've been in the process of replacing the ordinary "mint green" lights on the interior of my Intrepid to some red LED's I have laying around (I have 50 blue ones on the way which I think I'll go with).

I've wired up and installed them on my shifter bezel and climate control panel (basic three knob unit). In the house, it all runs perfect off a 9V battery. Take it outside, plug it in, turn on the car, they come on, and suddenly die out never to come on again.

I've NEVER burnt out LED's in my life running them on 12V connections, in fact, these LED's were pulled from a few old Intrepid Clusters that I've been experimenting with.

Any advice boys?
 

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Never bothered as I never had to use drop down regulators, resistors etc in the past, always ran LED's off straight out 12V. Care to share your thoughts?
 

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I believe they are typical 5mm, these ones are red. I have 5mm blue ones on the way.

I've wired both units (climate control and shifter bezel) differently hoping to yield different results. One main power going light to light. The other, all the lights having independent wires going to the power source.
 

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LED's are typically 3.3v, you have to have a resistor or wire them in series to keep the voltage levels correct.

3.3v at 12v means for about a 470ohm resistor.
 

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Oh, I understand, so 12V, I need to either add enough LED's in series to distribute the voltage evenly or add a resistor (470ohm I'm assuming for all situations)?
 

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Yeah, I prefer to use one resistor with one led and not wire them in series. But that's just me.
 

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Bottom line is that LED's need current limiting. If you've used them in the past with 12 volts with no resistor, that means that those particular ones have a built in resistor or some active current limiting circuit. But this time you must have gotten plain LED's with no resistor or whatever.

The calculator that Crypt Keeper linked is probably good. But if you want some insight into designing in LED's, here's a tutorial I posted a few years ago:

As already stated, LED's need current-limiting resistors. You stated it well when you said the LED acted like a fuse. That is good so nothing else was damaged.

Here is information I copied from an earlier post of mine. If a person follows the information below, they should have no problem designing LED's into practically any situation. This should be made into a How-To:

You always need resistors with LED's to limit current when a constant voltage source is powering them (like when replacing regular light bulbs). Some LED's come with built in resistors, but their packaging or spec. sheet will say that if that's the case. Assume it has no resistor built in if it doesn't say.

Decide on how many milliamps you want going thru the LED at full voltage. (mcd = millicandellas is the measure of brightness at a given current).

R = (V(b) - V(LED)) ÷ C
where:
R is in ohms
V(b) is your car's power system voltage (assume 13.8 with it running)
V(LED) is the voltage drop across the LED (assume 1.2 unless the spec. sheet tells you)
C is the current you want in amps. 20 mA = 0.02 amps

So for example: For V(b) = 13.8 volts, V(LED) = 1.2 volts, and desired current is 20 mA (0.020 amps), the resistor would be 630 ohms - find the nearest value to that - 620 is a standard value I think.

Lower resistance will give you more current, more brightness, shorter LED life.

Some of these bulb locations may have lower source voltages, so for a given location, measure the voltage with the brightness turned all the way up with the original bulb installed, and use that number for your V(b).

For multiple LED's in series, use their total voltage for V(LED) in the formula. For example, if one LED voltage drop is 1.2 volts, and you are putting 2 LED's in series, use 2.4 volts for V(LED) in the formula. For 3 LED's, put 3.6 volts in the formula. Do not stack so many LED's that their total voltage gets more than 4 or 5 volts from the source voltage. Because the source voltage varies some, you need to have a resitor in there so that the current (and the LED brightness) is not going to be super-sensitive to those variations. If you use a voltage regulator, you still need a resistor, but you can get a little closer to the source voltage (in that case, the output voltage of the regulator) because the regulator will give less variation in its output voltage than the vehicle's system.

To determine the wattage needed in the resistor, use the formula:
P = C^2 x R
where:
P = power in watts
C = current in amps
R = actual resistor value in ohms

So for our above example of 20 mA (milliamps) and 620 ohms, the resistor would dissipate 0.248 watts. That's right at 1/4W, but you would not use a 1/4W - you want to go up one size which would be 1/2W. Always try to use at least 50% safety margin on the wattage (for one thing, the wattage rating gets de-rated at higher ambient temperatures). You need some safety factor on that so your resistor lasts forever. If your wattage calculation came out to, say, 0.15 watts, you could safely get by with 1/4W. Any more than that, move up to 1/2W. If wattage comes out more than 0.33 or so, use more than 1/2W (or parallel two 1/2 watters of twice the resistance value each for the same net resistance value).
 
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I have ran 5mm Red LEDs ALL OVER my car. However to do this correctly is to put a 470ohm resistor (as previously stated) on the negative lead (the shorter leg) of the LED, as I have found that these cause usually have a common ground and will cause the LED to burn out with out the resistors.
 

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I have ran 5mm Red LEDs ALL OVER my car. However to do this correctly is to put a 470ohm resistor (as previously stated) on the negative lead (the shorter leg) of the LED, as I have found that these cause usually have a common ground and will cause the LED to burn out with out the resistors.
You can't make a general rule for the resistor value. You have to base it on the current that the LED can handle and the source voltage to be sure. If you don't have other LEDs in series, the 470 ohms will give about 25mA (if power source is full vehicle voltage). Fine for some LEDs - too much for others, might be low for others. Best to calculate it based on voltage and current specs. You're right - absolutely the LED will blow without any resistor.

Hopefully your resistors are 1/2 watt or more. They are dissipating more than 1/4 watt each (assuming you have full vehicle voltage and only one LED in each string).
 
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