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.
Best Answer
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)