I recently replaced a set of incandescent bulbs from the interior of my old car with some very inexpensive festoon LEDs. Unfortunately the LEDs are not 100% compatible with the lighting system as dimming is accomplished via pulsing the voltage signal. The pulsing effectively dims the incandescent bulb, but causes the LEDs to flicker/blink.

I think a quick solution to mitigate or eliminate the flicker is to solder a ceramic or film capacitor in parallel with the LED power supply terminals. However, I don't have access to a scope to measure the pulse frequency so I can't calculate an ideal capacitor value to use.

The bulbs run at 12V, draw ~250mA of current -- can someone suggest a safe range of capacitor values I can experiment with? Perhaps the frequency needs to be estimated for this question to make any sense.

I may also be able to emulate the resistance of the incandescent bulb with a resistor in series, but I don't want to lose the intensity that the LEDs are currently providing.

  • \$\begingroup\$ A lot of multimeters provide you with Hz measurements, and its likely related to mains power, but in the end your dimmer is switching on the LEDs PSU that often in a second on and off, so better get a compatible system. \$\endgroup\$ – PlasmaHH May 23 '18 at 21:02
  • \$\begingroup\$ Caps will not be a great solution due to the low rate of pulses and high peak currents. Is this a Lada? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 23 '18 at 21:11
  • \$\begingroup\$ @Tony: Not that old :). A 2000 VW. \$\endgroup\$ – Steve Guidi May 23 '18 at 21:12
  • \$\begingroup\$ you could try any capacitor, as long as its voltage rating is higher that 18V or so (an automotive power system runs higher than 12V) .... use a capacitor from a scrap PC power supply if you have one \$\endgroup\$ – jsotola May 24 '18 at 4:52
  • \$\begingroup\$ The solution is not to buy cheap poorly designed LED light bulbs. \$\endgroup\$ – Misunderstood May 30 '18 at 15:27

There's a visual difference between flicker and blink...

Flicker (if you can see it) would be something like 40-60 Hz.

Blinking would be much lower, maybe 10 Hz or less.

The difference is important, because flicker can be solved with a cap, but blinking could indicate that there is a bit of a problem that won't be solved with a cap.

For example, the car's electronics could measure the current drawn by the bulb, then decide that the current is too low (because you installed LEDs instead of incandescents) and then decide the bulb is blown, then sleep for half a second, and then restart the cycle. That would result in blinking.

Or the driver chip in your LED bulbs could be "cheap" (ie, braindead), and being driven by PWM pulses could give it a bit of a seizure.

I've had good success with sets of lights by not replacing them all with LEDs, but leaving at least one incandescent bulb. This obviously works only in the case where you got several lights in parallel, but it seems to fool the electronics into thinking that the original bulbs are present. So try this first (ie, one LED, the rest incandescent).

However if the issue is indeed that the PWM frequency is too low, the problem with a capacitor in parallel with the LED bulb is that the discharged capacitor acts like a short and will draw a high current when the bulb turns on, which may cause problems with the switching MOSFET. In this case, a RC network to filter the voltage, may be a better option, or just use a low quality cap with enough ESR to not cause a current spike.

(for example, using a 0.1R ESR cap in parallel with the lights would cause a 12V / 0.1R = 120A current spike at turn-on... but a cap with a few ohms ESR would be okay)

  • \$\begingroup\$ I had some of the bulbs removed from the circuit(s) in the car when testing, and that's when I observed the blinking (low frequency). After I fitted all the bulbs, the dimming function worked as expected in almost all of the lighting circuits. Some combination of the extra circuit resistance or logic in the lighting controller must have been satisfied and things are pretty much normal now. Only two circuits exhibit blinking now, but it is negligible: very low luminosity and one circuit is rarely used (glove box). \$\endgroup\$ – Steve Guidi Jun 1 '18 at 17:19

It would make more sense to bypass your current incandescent bulb dimming system, as it is looking to power a much heavier load than you are proposing.

Are their enough lights on all the time to achieve a significant power savings/ reduced load on your electrical system?

I would determine if you still want to do this considering the time and probability of laying on your back in a funny position for some time, working under the dashboard to accomplish this. Plus the cost, of course.

If you still want to do it, there are some pre-made switcher (DC-DC) boards which could be purchased inexpensively, that would be a better solution.


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