Temperature difference in a resistor

[Bago]

Hey,
I'm an absolute newbie and have just this one question:
When I installed LED map lights, they would flicker unless the door was open. After someone advised me that I should solder resistors (which I did) the problem was fixed, BUT: I would like to know what is the difference in temp as I down want to start a fire in my car.

Resistors are 2w 330ohm, I used T4W LEDs for which I guess are about 0.5w, and for voltage I think it's 12v.

This is a guide from another forum:

With the lamps flickering, connecting a 100nF capacitor across them stops the flickering, proving this was AC fast pulsing in nature, with effectively a low voltage - but of course no DC current flows through the capacitor . Connecting a resistor across the the lamp causes them to operate at full brightness due to the DC current draw - i.e. the controller senses the load correctly and turns on the power. I think that the problem may be that the LED lamps do not take much current until there is at least about 3V across each LED and the sensing does not reach this level. Adding the resistor allows the sensing circuit to detect a current draw at a lower voltage, so the light comes on .

I tried a few different values of resistor and could get it to work properly with 500 ohms or less. I chose a value of 330 Ohms, so as not to draw too much power. 12V with 330 Ohms draws 36mA, so not much on top of the festoon lamp's 120mA. Worst case dissipation might be 14.5V^2 / 330 = 0.64Watts. To keep the temperature low I used a 2Watt resistor. This is very conservatively rated.

Here are a few photos of the light unit:

 

[mfb]

Difference in temperature between what and what, and in which conditions?

Connected to 12V, the 330Ohm-resistor (resistorS?) draws something like .4W.

12V is not the operating voltage of typical LEDs, however.

 

[davenn]

Difference in temperature between what and what, and in which conditions?

Connected to 12V, the 330Ohm-resistor (resistorS?) draws something like .4W.

12V is not the operating voltage of typical LEDs, however.

However, these LED's (T4W ) are designed for 12V car systems as a direct replacement for the
"dome" lights etc in a car cabin
They have their own built in current limiting resistor and heatsink

The external resistor mod to stop flickering is an ineresting one :)

Dave

 

[Bago]

I'd like to know how hot can resistors get, as I don't want them to melt any plastic and cause a fire.

 

[mfb]

That depends on the power they get and the temperature and heat conductivity of the environment.