dome light LEDs

[leitnin]

i was attempting to change my dome lights (located on the rear view mirror) to blue, and the simplist way i could think was to wire three blue leds in series for the 12v total, because the dome lights are too hot to color with plastic. well, it worked for a while, but then as i closed the door a few days later...the lights started to dim as usual, but then began to flicker. soon, they didn't work at all, or would occasionally flash. i thought i fried the leds but i was takin em out and it was flashing...so.....any ideas? whats the deal? better way to get em blue? any advise would be greatly appreciated

[laser92awd]

Blue/White LEDs usually have around 3.6V Vfwd max. So three would be pretty close to 12V. Fine, but what about when the engine is running and the voltage gets to near 14V? Might not seem much, but it will reduce (possibly, greatly) the longetivity of the diodes. You want to use a reisistor to limit current to however many milliamps the diodes are rated.

Even at 12V you're forcing around 4V across each diode which will probably draw more current than they're rated for. YMMV.

[leitnin]

well normally id agree with you but they are 4.5 volt max leds i believe and the fact of the matter is....after not turning on for quite some time...one randomly just worked after several months and then just died again.. i think LEDs aren't the way to go for those lights. i don't turn em on when the engines runnin relly either, but the dimming of the lights when the door closes they probably don't like. how would you recommend coloring the lights.

[laser92awd]

LED's don't mind dimming at all (unlike fluorescent/discharge tubes)...

I bet you are feeding too much current through though, it's not voltage that counts (though it's directly correlated), it's current that flows through the LED is what kills them.

[leitnin]

so your saying at too high voltage, the resistance of the LEDs isn't enough so it overcurrenting the LEDs and killing them

i'll look into it

[laser92awd]

Try a 270 ohm resistor and the three LEDs in series. This should be enough to safely power the LEDs while still emitting about full brightness. That is if these are the typical 10mA LEDs. If they're 15mA LEDs, try 220 or 180 ohms. Don't go below 150 ohms.

The other way is to give each LED its own resistor, a 1K resistor per LED should be sufficient to get 10mA flowing. You can drop it down to 820 ohms to get more current/brightness if your LED can handle it, but don't go below 470 for a single LED (I don't think there are any single chip LEDs that can handle more than 20mA).

Hopefully this will make your LEDs last much longer.

[leitnin]

well, the LEDs are 20mA so, i'll just see about putting a resistor and solving the whole issue, makes sense.

[MemphisEclipse]

I replaced mine with Varad "Hyper" LED's (one per light). They are really bright and you don't have to worry about adding resistors. Very easy to install.

[leitnin]

i am assuming they are set up to be 12 volt and all that.....how did you wire em in?

[97GS-Toy]

so your saying at too high voltage, the resistance of the LEDs isn't enough so it overcurrenting the LEDs and killing them

Close, but not quite. I would guess that overcurrent is killing them, but it's not correct to regard a LED as having resistance.

A resistor has a voltage / current relationship that is linear:

V = I * R

So, for a given resistor, if you increase the voltage by 10%, you increase the current by 10%.

This is not true for LEDs, or any diode for that matter. LEDs have an exponential voltage/current relationship, so a small change in voltage can cause a huge increase in current. This is why you must always use a resistor to limit current.

To figure out the value, start with the 'on' voltage of the LEDs, in this case 4.5V. If that's max, I would use a value of 3.5V, just to be well within the safe region. Multipy this voltage by the number of LEDs you have, so:

3.5V * 3 = 10.5V

Now subtract the result from your maximum voltage. I'll use 14V here, so:

14V - 10.5V = 3.5V

The fact that this matches the LED voltage is a coincidence for this example. This result represents the voltage that must be 'made up' if you want three 3.5V diodes in a 14V system.

Now, use this value, along with your LED current rating, to get the appropriate resistor value:

3.5V / 0.020A = 175 ohms

Find a resistor value close to this. You won't likely find a perfect match. When in doubt, round up. More resistance = less current.

Finally, the resistor must have sufficient power capability, ot could burn up. Power is voltage * current (by definition), so:

3.5V * 0.020A = 0.070W

For headroom, the general rule is to double this, so you need a resistor capable of at least 0.140W. Resistors come in a standard value of 1/4W (0.25W).

Remember, if you are not happy with the intensity, you have a bit of room to play with because you underestimated forward voltage. Just make sure to recalculate everything before trying something new. It only takes a few minutes and could prevent a fire.

One more note, about the Hyper LEDs. If you're running 12V across these, they're either bulbs with incorrect names, or they have resistors built in. No respectable company (or it's engineers) would tell you to put 12V across a LED. I'm not putting you down, MemphisEclipse. I'm just stating this so people don't make incorrect assumptions about the product.

Sorry this was long. I hope it helps.

[CleanGS97]

!....
Ynot just put light condoms on the stock bulbs?

They sell barrel type light condoms @ AutoZone..

[leitnin]

i don't know if that would work. those lights are freakin hot!!! if it works...then thats easy. anyone try it with those lights? melted plastic? (they are daaaaaaaaaaaaaaamn hot)

[laser92awd]

I've seen those 'color condoms" before, I think my Jeep might have a few to 'soften' the light (and they came stock!) But the biggest issue is that it does reduce light intensity. Then again LEDs arent that bright to begin with (unless you have lots of them) so it might be a wash. So hopefully the guys who made those things made sure it could handle the heat...

[CleanGS97]

Well I bought CF altezzas ('nuff laughing) and I put lights in the center piece... I ended up buying a barrel type fuse holder to mount the bulbs, then put parrel type light bulbs (like in the map lights) back there and put red light condoms over them.. they MATCH the parking lights @ night intensity-wise and the condoms havent melted yet...

Just bringing this up as a possible option....
-Aaron

[MemphisEclipse]

leitnin, the Varad LED's are set up for 12V. To install them, (1) remove the factory bulbs (2) use a multimeter to find out which side of the bulb holder is + and which is - (3) remove mirror from windsheild (4) remove bulb holders and reflectors (5) solder on the LED's + and - wires (6) tuck them inside the mirror (make sure tip of LED is pointing down and reinstall reflectors (7) reinstall mirror.

[leitnin]

while i was lookin for some dash socket holders for the #74 gauge lights, i found some blue dome light bulbs that fit....so problem solved. but the led info will help with my gauge install, makin my own. thanks

[leitnin]

the blue on the dome light bulbs melted off really fast...lame crap. make some new led ones with a resistor.

[TalonESi]

I have blue light condoms on all my non-standard lights in the car that don't have specality bulbs made in blue, I have my whole interior lighting blue, dash, gauges, etc...) including on the dome lights, and I've had them on for a few months and the color is still the same as it was when I put them on. They are intended to "fuse" to the lights with the special heat resistant gel inside of the bulb condoms, so be sure not to push all of it off when putting it on the bulb. Also you can trim them with scissors, if it doesn't cover all over the ends it will dissipate heat better but be sure not to cut it so far that the yellow light beneith leaks out.