I have recently bought a new car and replaced the number plate bulbs with leds. The Leds work; however when the lights are switched off, the car does it's own electrical check of all the bulbs by sending a pulse down the particular line.

With normal bulbs, this doesn't do anything but the leds flash.

I have came across a video where someone used a 12 V relay to fool the car when it's doing its self check, but he had to leave the bulbs connected to this circuit which he then fixed behind some interior trim.

I don't want to be driving around with bulbs knocking around behind trim panels, so was going to use a resistor in their place.

I'm having trouble with deciding on the value of the resistor.

The bulbs are 12 V, 5 W and gave a resistance of 3.5 Ω. There are 2 of them wired in series.

So would I need a 7 Ω 10 W 12 V Resistor?

Or a 7 Ω 5 W 12 V Resistor?

Or would I be better with some sort of diode?

Or am I way off with it all?


The vehicle diagnostic does its check on cold bulb only. I want a Resistor that will mimic the cold bulb. Once the lights are switched on and they get a full 12 volts the relay will come into play and divert power to the leds which work no problem. Issue only occurs when lights are off

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    \$\begingroup\$ I think you are better off having a mechanic do this work for you. It is not a shock hazard per se, but a car battery can source hundred of amps of current. A miss-wire that shorts the battery will likely melt a few wires. \$\endgroup\$ – user105652 Jan 8 '18 at 22:19
  • \$\begingroup\$ Thanks for the advice, its all actually quite basic Im just no good when it comes to resistors and can't decide. I dont think this us anything a mechanic would take on. Its only for the number plate lights. \$\endgroup\$ – Schad Jan 8 '18 at 22:26
  • \$\begingroup\$ Sometimes, when a project turns out to be more involved than anticipated, but you're already committed and don't want to give up, it might be a good idea to have a nice cup of tea and reevaluate the cost/benefit ratio of the whole thing. Incandescent bulbs have worked for ages, and your car is obviously designed for them. This project might not be worth the effort. Of course that's ultimately your decision. I'm just saying. \$\endgroup\$ – Dampmaskin Jan 8 '18 at 22:37
  • \$\begingroup\$ A very simple circuit to supress 12v pulses from illuminating some LEDs might have the answer for you. \$\endgroup\$ – Andrew Morton Jan 8 '18 at 22:53
  • \$\begingroup\$ You have been measuring the resistance of cold bulbs. A 12V 5W bulb has a resistance about 12^2/5 = nearly 30 ohms when hot, not 3.5 ohms. ( 2 in series would have some intermediate value of resistance as they wouldn't get so hot; and you'd have to measure the current that took). Your 7 ohm resistor would take nearly 2 Amps and dissipate about 25 Watts, probably starting a fire behind the trim. \$\endgroup\$ – Brian Drummond Jan 8 '18 at 22:56

it depends how tight the tolerance for bulb power rating is assumed in the car lamp fail detect circuit.

However tungsten lamps are 10x lower R when cold so the current surges then decays to 10% when hot.

14V 5W R=V²/W=14²/5 ~ 40 Ω hot and 4 Ω cold times 2 in series = 8 Ω which would draw 50 watts with 14V with a fixed resistor when LEDs are turned on.

The threshold depends on the duration of the pulse. I would start with a 1000uF capacitor and a 1k 1/2W shunt resistor and see if that reduces the flashing and warning.

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  • \$\begingroup\$ Thanks for the advice, im not going to lie.. Im confused!! The plan was to put a 12volt relay in so when the car does its little check which it only does when the power to the lights is off it wouldnt see the Leds but would see the old galogrn lamps (in my case what I want a Resistor) so I dont think it would matter about the bulbs being hot as the check is only performed when cold. Then when 12 volts are applied when the lights are closed the relay comes into play and turns on the leds \$\endgroup\$ – Schad Jan 8 '18 at 22:35
  • \$\begingroup\$ That was supposed to say halogen! With my above comments in mind would you still try the 1000uF capacitor and a 1k 1/2W shunt resistor first? \$\endgroup\$ – Schad Jan 8 '18 at 22:43
  • \$\begingroup\$ It depends on how short the pulse duration is, and if cap is low ESR, but yes. i welcome any critical comments from colleagues \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 8 '18 at 23:05
  • \$\begingroup\$ Thanks again appreciate your time. So im guessing id have to try and measure the pulse then before deciding on what resistors or capacitors to use \$\endgroup\$ – Schad Jan 8 '18 at 23:19

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