benzoc55

hey guys,

Ibought a pair of l.e.d's for my license plate lights. the problem is they blink on and off when inserted...i tried alot of things but no luck..anyone know or has done this ? wut resistor should i use or what..NEED HELP?????? thanks

newton22

Yes resistors. I used the 100 ohm (I think???) ant sized ones, but those still burned out after a few weeks.

jgsx

Search in the c-class forum. The topic has been discussed to death.

rlee02135

i burned out my rear SAM with those...
luckily, it was replaced under warranty.

m444

LEDs should be driven by a current source, not a voltage source, which would be the reason for adding a resistor. The resistor value would be chosen based on the current requirement of the LED and the voltage drop of the LED. Assume a nominal battery voltage of around 13.2V.

Also, one should know how much current current the stock bulbs require and how much current the LEDs require, before plugging in an LED.

m444

Also, if they are blinking on off, because they are drawing too much current, then use a a very low current relay (with reverse diode accross the coil, to kill the inductive kickback) to isolate the load from the MB driver circuit.

Some current limiting circuits will pulse on/off, if overloaded, which means it is being overstressed.

benzoc55

thanks for the help everyone

Moviela

The lamps blink because the rear SAM is attempting to measure the current flow. The rear SAM measures the voltage across an internal low ohm resistor in the SAM. The license plate lamps are about 5W. While a series resistor is needed to limit the current in LED's, it is somtimes built into the festoon lamp holder that houses the LED's. It is not always built in, and would need to be mounted externally to the assembly. An additonal resistor is needed to properly load the SAM lamp out sensor circuit. Since many LED's are about 1W or less, a resistor is needed that will draw about 3 to 4 watts in parallel with the voltage supplied to the LED. A 35 to 50 ohm 10 watt resistor is suitable for this task.

Here is a link for a resistor that can be mounted to the car frame to eliminate the heat.

http://www.newark.com/jsp/search/pro...sp?sku=65K1854

benzoc55

so if i buy the resistor from the link you provided it should work properly?...by the way thanks cause this has been eating me up that it wont work!!!

m444

The value resistor to use will depend on the particular LED you have. Forward voltage drop can be anywhere from 1.7V to 3.5V, depending on the kind of LED. Typical current draw, for a small LED is about 20mA, but there are some new ones that draw 300mA or more.

The voltage drop, depends on the materials and doping used for the LED semiconductor, since it depends on the band gap voltages. Infrared and red (1.6-2.1v) have the lowest voltage, ultraviolet and blue have the highest voltage (3-3.5V).

There is no universal resistor size for an LED.