Resistors for city lights LED's that won't burn holes in/set fire to my headlight hou

[Gunnarrrrr]

One of my LED's were flickering so I checked it out and found this



And the other one that was still functioning



I used some cheap eBay resistors and they actually worked great​ for almost a year or so. What are you guys using/are they safe?

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[b1337]

run longer wire to move them somewhere all metal and easy to swap?

[Gin+]

Not worth the trouble IMO but if I were to venture down this road, I'd consider buying high quality resistors from a known electronic parts supplier i.e. digikey, mouser, etc. Additionally, the heat generated under the hood of the car can adversely effect the resistor and its performance so I'd consider mounting them in front of the radiator or something of the sort.

[Spike00513]

what if you revert back to traditional H6W halogen bulb,
or LED ones that have the resistor built into them, rather than separate heat sinks

wonder if other solutions exist that I am not yet aware of.

[DiertyEuroSpec]

Resistors for city lights? Maybe NA headlights are different but i run LED non-resistored wedge bulbs in my E-codes. No warning light but they are just tiny 194 style 5W wedge bulbs with a cree LED in them

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[Gunnarrrrr]

Halogen bulbs don't match the headlights, and if I don't run resistors they'll start flickering within a month

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[CyberPMG]

Keep it simple and go with BAX9S CANBUS LED bulbs. No need for the additional resistor/heat. The only thing I don't like about them is that they are super fragile and sometimes break apart when putting into the bulb socket. Once they are in, they last.

[Kevin C]

Two ways to regulate the LEDs current. One would be a drive circuit with some type of solid state junction to control the current and the other is to just use a dropping resistor. The heat generated the the resistor is the square of the current times the value of the resistor. From the LED data sheet determine how much current you want to run through the LED. The lower you go the longer the LED will last. Underrating the current reduces die temperature. Part of that is how well heat sunk the LED itself is.

Voltage dropped across the resistor: System voltage-junction voltage=voltage across LED.

Example: 14.7-1.2=13.5V
If you want to run 200 ma: V=I*R 13.5= .2*X
13.5/.2= 66 ohms Round up to the next common value)
.2*.2*66=2.64W
Get a resistor that's rated for 10 watts and mount it on a heat sink.
This assume that you're running a bare LED with no control circuit.

System voltage should be peak voltage with alternator charging when cold.
Underrating the LED by a good factor helps with life and most applications are not well heat sunk and data sheets give optimistic maximum currents.

[ErikG]

Kevin, you are correct on the calculations but i would increase the current from 200mA to at least an order of magnitude more. 2 Amps is not unheard of for LED headlights, i imagine these to be a bit more. Hey Gunnarrr, can you give us the wattage of the headlights? They are probably close to 50 watts and with Current = Power / Voltage we get 4 amps. Anyways that resistor you are using looks like its rated for 25 and maybe up to 50. I would think that if it were a 25W resistor it would have failed with 2x the amount of power its rated for running through it, but that also is why its so hot. If it were a 50W resistor , then the longevity of the resistor is fine even though we are close to its limit of power dissipation which is also why it gets so hot. If you mounted the resistor to a small computer heat sink, and then mounted that heat sink to the car some where, your melting issues go away. The active system would be the way to go but since you already have these you probably dont want to go that route.

[Kevin C]

Kevin, you are correct on the calculations but i would increase the current from 200mA to at least an order of magnitude more. 2 Amps is not unheard of for LED headlights, i imagine these to be a bit more. Hey Gunnarrr, can you give us the wattage of the headlights? They are probably close to 50 watts and with Current = Power / Voltage we get 4 amps. Anyways that resistor you are using looks like its rated for 25 and maybe up to 50. I would think that if it were a 25W resistor it would have failed with 2x the amount of power its rated for running through it, but that also is why its so hot. If it were a 50W resistor , then the longevity of the resistor is fine even though we are close to its limit of power dissipation which is also why it gets so hot. If you mounted the resistor to a small computer heat sink, and then mounted that heat sink to the car some where, your melting issues go away. The active system would be the way to go but since you already have these you probably don't want to go that route.

To be clear, the term example was to illustrate how to calculate a correct LED dropping resistor and not a recommendation on what current to run . The takeaway is to read the data sheet. Good to know I'm good on my calculations (I used to build and test LEDs and LED drive circuits for a living as well as laser drive circuits). Also, the suggestion on dropping resistor rating is based on an expected suboptimal heat sinking of the resistor and allows for ambient conditions under the hood of a car. Over spec'ing is a very low cost way of adding safety margin. Resistor are typically derated for hot environments (it's on the parts spec sheet). Good practice is to have a significant safety factor on a power resistor.


Pre packaged LED driver, looks interesting.
http://www.ledsupply.com/led-drivers...-dc-led-driver