DIY license plate lamp resistor
01-01-2021, 02:14 PM
#26
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It’s amazing how we all choose to modify.
I’d never think to redo the lights - however - I am happy it makes you happy!
On the flip side - I am currently working (extremely slowly) on interior illumination with piping and LEDs in the dials, cup holder, etc. to freshen the interior! Ha!
I’d never think to redo the lights - however - I am happy it makes you happy!
On the flip side - I am currently working (extremely slowly) on interior illumination with piping and LEDs in the dials, cup holder, etc. to freshen the interior! Ha!
01-01-2021, 09:42 PM
#27
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That is an interesting mod. All the newer cars (W205 etc) have LED theatre style lights but it is rather difficult to wire anything into a W203 cleanly without drilling a bunch of holes. Can you please post some pics? 
Personally I was looking at maybe adding 4000K lights to the footwell areas, but they would have to be tied to the dome circuit.
Personally I was looking at maybe adding 4000K lights to the footwell areas, but they would have to be tied to the dome circuit.
Last edited by slammer111; 01-02-2021 at 07:27 PM.
11-29-2021, 12:20 AM
#28
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For those who are interested, I also converted my reverse lights to LED. Same overall procedure, but different installation setup.
06-16-2022, 03:16 PM
#29
Jumping in here because I am tired of getting pulled over by the po-po for
my license plate lights... Great write up. You sound like a scientist-very thorough! I love this.
I am new to to resisters
and noticed that you said to not pay attention to the values in the pictures-got it.
My question is you say for the left license plate bulb you need 38mm 150 ohm... I got the bulbs on Amazon but
hit a stopping point on the resisters. Did you mean 1 or two
of those 150 ohms on the left side? And are they 2W also? Same with right -do you mean one or two 200 ohms resisters for that side? I am sort of amazed at how many different kinds of resisters there are! If anyone has a place to buy them or an NTE product number?
my license plate lights... Great write up. You sound like a scientist-very thorough! I love this.
I am new to to resisters
and noticed that you said to not pay attention to the values in the pictures-got it.
My question is you say for the left license plate bulb you need 38mm 150 ohm... I got the bulbs on Amazon but
hit a stopping point on the resisters. Did you mean 1 or two
of those 150 ohms on the left side? And are they 2W also? Same with right -do you mean one or two 200 ohms resisters for that side? I am sort of amazed at how many different kinds of resisters there are! If anyone has a place to buy them or an NTE product number?
06-16-2022, 08:06 PM
#30
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You can't go wrong with 2W resistors for the plate lights. The rule of thumb is to use a resistor that is rated at twice the wattage of the expected power draw through the resistor under "normal" operating conditions. In other words, for this exercise you want each 2W resistor to draw no more than 1W when the car is running. This is not a hard and fast rule, but obeying it means the resistor won't get too hot and melt something. Resistors can handle quite a bit of heat without failing. The bigger worry is with melting the bulb or enclosure.
I used 14V for my calculations. For gasoline cars the electrical voltage from the battery is listed as 12V nominal, but in reality the number is more like 12.5 to 14.4V, depending on battery and alternator condition. If you want to be safest, use 14.4V.
I used 2 resistors to spread out the load on each bulb. The numbers I gave above are the total resistance as a result of using both resistors in parallel. See link for the math involved. The other reason for using 2 was that I had a bunch of resistors of different values and it was easier to mix and match to get the resistance I needed. But I could've used 1 resistor if need be. The only real constraint is space inside the bulb enclosure. A 2W resistor isn't much bigger than a 1W resistor.
One thing mentioned earlier that you have to keep in mind is that LEDs draw different amounts of current depending on the junction temperature, which is affected by ambient. They draw less current when it's cold outside. I found while experimenting that sometimes I would have a resistor combo that worked until it was winter or night time. So it's better to draw a bit more current (0.2W or so) than the minimum. Also I found that adjusting one circuit would often affect another circuit, as the circuits aren't fully isolated from each other. This is why it took a while for me to figure out the final numbers listed above.
Tinkering with the electronics on cars (including "complex" German cars) is not as difficult as many people make it out to be, but it does require an understanding of circuit theory as well as Ohm's law. I have a background in this stuff which helps quite a bit.
Any brand of resistor will work. Just pick up whatever is available at your local electronics store. And yes, there are thousands of different resistors out there. Different resistances, wattages, and types.
Good luck.
I used 14V for my calculations. For gasoline cars the electrical voltage from the battery is listed as 12V nominal, but in reality the number is more like 12.5 to 14.4V, depending on battery and alternator condition. If you want to be safest, use 14.4V.
I used 2 resistors to spread out the load on each bulb. The numbers I gave above are the total resistance as a result of using both resistors in parallel. See link for the math involved. The other reason for using 2 was that I had a bunch of resistors of different values and it was easier to mix and match to get the resistance I needed. But I could've used 1 resistor if need be. The only real constraint is space inside the bulb enclosure. A 2W resistor isn't much bigger than a 1W resistor.
One thing mentioned earlier that you have to keep in mind is that LEDs draw different amounts of current depending on the junction temperature, which is affected by ambient. They draw less current when it's cold outside. I found while experimenting that sometimes I would have a resistor combo that worked until it was winter or night time. So it's better to draw a bit more current (0.2W or so) than the minimum. Also I found that adjusting one circuit would often affect another circuit, as the circuits aren't fully isolated from each other. This is why it took a while for me to figure out the final numbers listed above.
Tinkering with the electronics on cars (including "complex" German cars) is not as difficult as many people make it out to be, but it does require an understanding of circuit theory as well as Ohm's law. I have a background in this stuff which helps quite a bit.
Any brand of resistor will work. Just pick up whatever is available at your local electronics store. And yes, there are thousands of different resistors out there. Different resistances, wattages, and types.
Good luck.
Last edited by slammer111; 06-16-2022 at 08:28 PM.
08-15-2022, 01:20 AM
#31
You can't go wrong with 2W resistors for the plate lights. The rule of thumb is to use a resistor that is rated at twice the wattage of the expected power draw through the resistor under "normal" operating conditions. In other words, for this exercise you want each 2W resistor to draw no more than 1W when the car is running. This is not a hard and fast rule, but obeying it means the resistor won't get too hot and melt something. Resistors can handle quite a bit of heat without failing. The bigger worry is with melting the bulb or enclosure.
I used 14V for my calculations. For gasoline cars the electrical voltage from the battery is listed as 12V nominal, but in reality the number is more like 12.5 to 14.4V, depending on battery and alternator condition. If you want to be safest, use 14.4V.
I used 2 resistors to spread out the load on each bulb. The numbers I gave above are the total resistance as a result of using both resistors in parallel. See link for the math involved. The other reason for using 2 was that I had a bunch of resistors of different values and it was easier to mix and match to get the resistance I needed. But I could've used 1 resistor if need be. The only real constraint is space inside the bulb enclosure. A 2W resistor isn't much bigger than a 1W resistor.
One thing mentioned earlier that you have to keep in mind is that LEDs draw different amounts of current depending on the junction temperature, which is affected by ambient. They draw less current when it's cold outside. I found while experimenting that sometimes I would have a resistor combo that worked until it was winter or night time. So it's better to draw a bit more current (0.2W or so) than the minimum. Also I found that adjusting one circuit would often affect another circuit, as the circuits aren't fully isolated from each other. This is why it took a while for me to figure out the final numbers listed above.
Tinkering with the electronics on cars (including "complex" German cars) is not as difficult as many people make it out to be, but it does require an understanding of circuit theory as well as Ohm's law. I have a background in this stuff which helps quite a bit.
Any brand of resistor will work. Just pick up whatever is available at your local electronics store. And yes, there are thousands of different resistors out there. Different resistances, wattages, and types.
Good luck.
I used 14V for my calculations. For gasoline cars the electrical voltage from the battery is listed as 12V nominal, but in reality the number is more like 12.5 to 14.4V, depending on battery and alternator condition. If you want to be safest, use 14.4V.
I used 2 resistors to spread out the load on each bulb. The numbers I gave above are the total resistance as a result of using both resistors in parallel. See link for the math involved. The other reason for using 2 was that I had a bunch of resistors of different values and it was easier to mix and match to get the resistance I needed. But I could've used 1 resistor if need be. The only real constraint is space inside the bulb enclosure. A 2W resistor isn't much bigger than a 1W resistor.
One thing mentioned earlier that you have to keep in mind is that LEDs draw different amounts of current depending on the junction temperature, which is affected by ambient. They draw less current when it's cold outside. I found while experimenting that sometimes I would have a resistor combo that worked until it was winter or night time. So it's better to draw a bit more current (0.2W or so) than the minimum. Also I found that adjusting one circuit would often affect another circuit, as the circuits aren't fully isolated from each other. This is why it took a while for me to figure out the final numbers listed above.
Tinkering with the electronics on cars (including "complex" German cars) is not as difficult as many people make it out to be, but it does require an understanding of circuit theory as well as Ohm's law. I have a background in this stuff which helps quite a bit.
Any brand of resistor will work. Just pick up whatever is available at your local electronics store. And yes, there are thousands of different resistors out there. Different resistances, wattages, and types.
Good luck.
08-15-2022, 01:48 AM
#32
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Thanks for the kind words. Glad it worked out for you. 
FYI the resistance values I listed were the final number. In other words, they are the final equivalent resistances for each circuit, not including the LED bulbs. You can probably get away with removing 1 resistor from each side, which would reduce the power usage and hopefully still work. A bit of experimentation would be required, as we are most likely using different model LED bulbs.
Just looked back at my photos. The resistors in "opposite directions" was just a coincidence (ie whatever orientation I happened to install). Resistors don't have a direction so to speak.
Personally I found the resistors got rather hot even at 100 ohms, so using higher value resistors is better. The main advantage of using multiple resistors is that you are spreading the heat out.
Right now your L side has an equivalent of 75 ohms and your R side has 100 ohms. Using the L side as an example, you can use 2x 300 ohm resistors to get the equivalent of 150 ohms, which would run way cooler. But you can probably get away with 1x 150 ohms. For the R side, I would try 1x 200 ohms (ie just remove one of the 2 resistors as well), or if you want to play it safe, 2x 400 ohm resistors.
FYI the resistance values I listed were the final number. In other words, they are the final equivalent resistances for each circuit, not including the LED bulbs. You can probably get away with removing 1 resistor from each side, which would reduce the power usage and hopefully still work. A bit of experimentation would be required, as we are most likely using different model LED bulbs.
Just looked back at my photos. The resistors in "opposite directions" was just a coincidence (ie whatever orientation I happened to install). Resistors don't have a direction so to speak.
Personally I found the resistors got rather hot even at 100 ohms, so using higher value resistors is better. The main advantage of using multiple resistors is that you are spreading the heat out.
Right now your L side has an equivalent of 75 ohms and your R side has 100 ohms. Using the L side as an example, you can use 2x 300 ohm resistors to get the equivalent of 150 ohms, which would run way cooler. But you can probably get away with 1x 150 ohms. For the R side, I would try 1x 200 ohms (ie just remove one of the 2 resistors as well), or if you want to play it safe, 2x 400 ohm resistors.
Last edited by slammer111; 08-17-2022 at 03:18 PM.
08-15-2022, 09:28 AM
#33
Member
Its a real shame we cant re-program the ECU/ CAN bus to NOT do the stupid filament light test every few seconds!
That way we could change to LEDS without having to worry about strobing LED flashing or warnings on the dash about failed filament bulbs!
That way we could change to LEDS without having to worry about strobing LED flashing or warnings on the dash about failed filament bulbs!
08-15-2022, 09:54 AM
#34
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The bulb out feature is actually pretty nice to have. I do wish the cutoffs values were set lower though. With that said, some LED bulbs do draw enough juice to not require additional resistors, but it would depend on the individual product. For example, I was able to find some nice and bright reverse bulbs that were a direct swap and drew enough juice to keep the car happy.
Last edited by slammer111; 08-15-2022 at 10:30 AM.
