very hot/smoking resistors with LED

Thread Starter


Joined Jan 30, 2009
i have brought these 10mm LEDs and i am trying to put them onto my car but i dont think i am using the right resistors

led specs:

  • Reverse Voltage:5.0 V
  • DC Forward Voltage: Typical: 3.0 V Max: 3.8V
  • DC Forward Current:100mA
the resistors i am using are smoking and turning black. The LED's light up and seem like they are working.

The picture shows how i installed the leds and the ohms and what watt resistors also . .



Joined Jul 17, 2007
I'm afraid you made a bad assumption about the voltage in your car. :(

An automotive system may be as low as 11.4v (not running, fully discharged battery) and as high as 15v (running, charging a very discharged battery) but nominally is around 13.8v.

You need to use a voltage regulator to get the voltage constant first, and then go from that reduced voltage level.

Since your resistors have gotten burned up, you have very likely stressed the dickens out of your LEDs as well - their life will be much shorter and they will be less bright due to the overcurrent.
The calculator should've recommended 1/2 Watt resistors since even with 12v, the power dissipation would have been 200mW. You always double the power dissipation, and then use the next higher wattage.

If your charging system was putting out a nominal 13.8v, your LEDs would be getting about 178mA current and your resistors would be dissipating about 691mW each. :eek:

Also, you gave the calculator 3.3v for a typical Vf, where the specifications call out 3.0v. This means that the current may have been even higher.

Without measurements, it's difficult to tell.
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Thread Starter


Joined Jan 30, 2009
i was afraid of that sgt . . .i am running power from the side bulb's wire .... i checked the voltage and on the multimeter it said high 11 something to just about 12v ...

did you see the picture that i linked up to the thread?

i am pretty novice at this but learning from mistakes that i try to avoid. Now i should get 1/2watt resistors with the same ohms that the calculator is telling me @ 3v for the led?
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Joined Jan 14, 2009
The resistors's impediance can vary +/- 5% or even 10% depending on type. This often happens with low impediance range especially. so, rough calculations by hand, you get power dissipation of 235-240mW. Which is at the limit. Always try to get at least 80% of rating of any element, if you are planning to run it for long time. I do it this way at least..

And short solution, just try to use 1/2W or even 1W resistors..See what happens. Maybe there is some other problem.

All the best,



Joined Mar 24, 2008
A simple way around is something like a LM317 as a current regulator, which I'm surprised Wookie didn't mention (he usually does). It'll get hot too, you'll probably need to heat sink the LM317.

It simplifies all the calulations, as well as automatically compensating for voltage fluctionation in a car.

Check this article I've been writing out...


Joined Aug 21, 2008
Good so far. One small bit of fine tuning. Don't run resistors beyond about 60% of their ratings if you are at all concerned about reliability.

LEDs shoudl also be run below their ratings for the same reason, though I don't know what an appropriate derating factor would be. In lieu of other guidance, I would not run more than 80% of the rated current throught the LED, and given that it could be in a hot ambient, even less would be better.

Keep in mind that you should be designing for the worst case condition, of 13.8 volts if taken across the battery.

Quick calc: 13.9V - 9V = 4.9V
4.9V / 80 ma = 61.2 Ohms
(4.9V * 4.9V)/(620.64 Ohms * 60%) = 0.64W. Use 1 watt resistors.

The LM317 current source is a great solution because the LEDs will run at constant current, just remember to derate the LED current appropriately and make sure the LM317 has adequate heatskinking and protection from transients from the automobile's electrical system.

So many details...

Thread Starter


Joined Jan 30, 2009
ok just got the lm317t's today ... hooked them up and everything .. while car was off i got just about 12v .. when it was started it was 13v and some

with the regulators, it kept it just below 12v when the car was on ... i used 33ohm 1/2watt 5% resistors and it seemed to work pretty well for me. . . . no burning . . or smoking ... resistors were a bit hot though ... just just enough to touch for a second . ... . i kept the car running, and driving for about an hour and some ... then checked the resistors .. still looked about the same ...

thanks guys ... if you guys suggest me do something else to the led's ... please let me know if i have to fix anything
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Joined Feb 4, 2008
You can use 5W resistors, provided they have the correct resistance, but it is not necessary because you waste money and space.


Joined Mar 24, 2008
Depends. If you're using the LM317's as current regulators. If their voltage regulators then calculate the voltage they will be dropping, then plug it into this equation...

Watts = Vr²/R

The resistor should be double the wattage needed, at least. Most times a ¼W will work.