Thrilling LED bulb replacement action!

LED lamps for standard low-power automotive sockets - things like interior lights, number-plate illuminators and brake lights - are now widely available and dirt cheap.

So I bought one, to see if it works any better than the standard interior light in my car.

There was nothing wrong with the standard interior light, but like a lot of low-power automotive bulbs, it's offensively inefficient.

The bulbs used in cars for things like interior lighting and instrument panel illumination have as their two chief design goals cheapness and durability. Both of these goals push manufacturers towards very low-efficiency devices. And the standard "dome" light in the middle of the ceiling of most cars generally doesn't even have much of a reflector behind its bulb, so something approaching half of the light just goes into warming up the light fitting.

So the dome light in my car looked like a fine candidate for LED improvement to me. Particularly now that one lamp will only cost you $AU8.98 delivered from Hong Kong.

(I got mine from this eBay seller.)

My car's interior light uses the small 31mm size of "festoon" bulb, the kind that look like a glass fuse but with points on the metal caps on each end.

The 31mm form factor doesn't give a lot of room for modern super-LEDs. You can now get 31mm lamps with a single allegedly-one-watt white LED in them...

LED bulb

...or you can go for the type I got, with no fewer than six surface-mount sub-1-watt super-LEDs.

There are also replacement bulbs that use a cluster of standard 5mm LEDs. They may be OK for things like instrument panel lighting, but you shouldn't expect as much light as you'll get from a single 1W LED unless there are at least a dozen 5mm LEDs in there. Even then, it's doubtful.

LED bulb detail

If you don't see a lot of yellow phosphor looking back at you, you're probably not looking at a very bright lamp.

I gave the new bulb a whirl on my bench power supply to see how much power it consumed. Then I tried the same thing with the (rather old) stock bulb.

The LED lamp drew only about 55 milliamps (mA) at twelve volts, for a power rating of only about 0.66 watts. Raising the supply voltage to 13.8V - which is what you'll get when the car's running and the alternator's turning - raised the current draw to about 105mA, for 1.45 watts.

The stock bulb has a nominal ten watt rating. From 12V it drew around 0.725A - that's 8.7W. From 13.8V it drew only a little more, about 0.785A (this is because the resistance of light bulb filaments rises with their temperature), giving 10.83 watts.

I expected the LED lamp to deliver much more light per watt than the incandescent bulb, and it also gets a big effectiveness boost from only throwing light out one side, wasting none of it by shooting it uselessly into the dome light fitting. But this was still a pretty huge power difference. At 13.8V, the old bulb draws 7.5 times as much power as the LED lamp; at 12V it draws more than thirteen times as much.

It was pretty easy to install the new lamp, although it did turn out to be a bit longer than it was supposed to be, making it a bit of a tight fit and also making it impossible to install it perfectly level. It ended up tilted a bit toward the left seat, though not enough to make a huge difference to the illumination on the two sides.

To cancel out any side bias, I tested the brightness of the two lamps with my somewhat accurate light meter sitting at the base of the gearshift (and with the standard plastic diffuser in place, too).

The light meter is calibrated in lux, a unit that's weighted to match human brightness perception. This gives the LED lamp another advantage, because the long-life low-temperature incandescent bulb gives very yellow light, while the LED lamp gives the characteristic blue-white of "white" LEDs. The blue-white has more energy around the green frequencies where human vision works best, so a given raw energy level of yellow-white light will appear dimmer, and read lower on a luxmeter, than the same energy level of blue-white.

Anyway, the stock bulb gave a reading of about six lux with the engine off (12V), and about nine lux with the engine running. Not a bad illumination level, given that it was being measured quite a bit lower than the place where you'd typically be, say, holding a map you were trying to read.

Swapping in the LED lamp gave... exactly the same readings!

My light meter isn't terribly accurate down in the single-digit lux, so I won't swear to you that there wasn't actually a bit of a difference one way or the other. But there clearly isn't a huge difference. And the new lamp, subjectively, lit up the cabin of the car just fine. Despite drawing around a tenth as much power.

This sort of thing can make a big difference in certain circumstances. If, for instance, you have a typical small car battery with about 25 amp-hour capacity before it starts getting very unhappy, a ten-watt interior light will drain it in thirty hours. Swap to a one-watt LED lamp and you'll probably still be able to start the car even if you leave the light on for ten days.

This doesn't matter much for normal automotive interior lighting, but if you've got a caravan or motor home or something that has a lot of friendly yellow incandescent bulbs in it, it could be a very good idea to swap them for the new cheap LEDs.

20 Responses to “Thrilling LED bulb replacement action!”

  1. furrfu Says:

    In my experience, these LED lights do burn out an awful lot quicker than incandescent ones, though.

    I got two to replace the licence plate lights on the back of the car, which otherwise get hot and, in winter, cause crud (salty water from road gritting) to dry up on them. I got through two pairs before giving up and going back to incandescent ones.

  2. jdanforth Says:

    My elderly neighbor drives about twice a week. I saw her car being towed yesterday from her house. I asked her about it and it turns out that she had left one of the interior lights on and, sure enough, the battery was flat as a pancake.

    I decided it was best not to rock her world by telling her that they had NO reason to tow the car just to swap out the battery. Crooks everywhere. Sheesh.

  3. Coderer Says:

    #1: You'd think that the nominal life of these things would be in the high thousands (ten-thousands?) of hours -- I guess yours died from exposure? Maybe the lesson here is that they're good for *internal* replacement.

    Now, they just need to get cracking on LED-array headlights =-)

  4. swalve Says:

    On a whim, I picked up a pair of the inverse cone type bulbs from the local Autozone. They were purportedly compatible with my sidemarkers and my in-dash idiot light and ilum. bulbs. They WERE compatible in that they fit in and lit up. They were NOT in that the brightness was terrible. Not scientifically measured, but clearly, obviously less bright.

    So be careful out there- they're not all good yet.

    furrfu- there might be some other issue causing burnouts.

    random rant- have you been seeing the newer cars with LED tail lights direct from the factory? They are super bright and instant on like you want. But it seems like they are using some kind of PWM scheme to modulate brightness. There's some kind flicker I see out of the corner of my eye. I find it distracting. Anyone else?

  5. nynexman4464 Says:

    swalve, I've noticed this too at night, when watching a car go by or darting your eyes across the road while driving, I see like a trail of where the lights were -- very distracting. Although I think the LED bulbs are much better (they light up right away instead of half a second after hitting the brake, and are much brighter and easier to see, etc) I wish they'd up the frequency or use filtering caps or something to make it less noticeable.

  6. Stark Says:

    I've been running LED tail/brake and indicator lights in my motorcycle for about 2 years now. I love em. The tail light is about 40% brighter than an incandescent as measured by a light meter at 1 meter from the diffuser and the brake light is nearly 60% brighter. Excellent things if you have to ride much at night, like I do. They are not quite as efficient as Dan's dome light but they still only draw about 40% as much power as the incandescents... which leaves me more power for nice accesories like grip heaters and auxillary headlamps - both of whcich are, in my opinion, neccesities for winter riding in northern California. Now, If I could just get a headlamp replacemet that was 40-60% brighter for 40% less power I'd be thrilled.

    Furrfu, sounds to me like you got either defective units, they really weren't designed for direct exposure to the elements, or you've got some serious elctrical issues. LED's should basically, for all intents and purposes, never burn out under normal use at their designed load. Well, not before your car rusts away anyhow. If they are, for some reason, being serioulsy over driven then they can fail rather quickly - sometimes with a loud bang (accidentaly have your bench power supply set to 48 volts when you hook up a single 5mm led to see what I mean). I'd hook up a good volt meter to your license plate lights and check to see what going on there.

  7. jaws_au Says:

    A great example of the duty-cycled LED tail lights is on the Top Gear episode where they take three supercars to southern France. The brake lights on Clarkson's Ford GT get a great strobe effect going on, I guess due to the frequency of the filming gear.

  8. Eschatonic Says:

    I used a Dick Smith courtesy light replacement LED in a project as a tell-tale on 24/7 (just because it looked cool in an acrylic fuse holder). This was a six LED festoon which I expected to last a seriously long time but within just few weeks the LEDs were very dull and now after 9 months three are black. I guess in an automotive application they would be fine but longevity seems to be a significant problem.

  9. KD Says:

    Might the different spectrum balance of the LED dome lamp make it any less safe to use while driving (say, a passenger reading a map for you, not that the driver is reading)? I'm just wondering. I have no knowledge of a reason this might be. It's just a thought that popped into my mind while reading.

  10. kamikrae-z Says:

    Speaking of strobing lights - does anyone notice the strobing effect from the small orange/yellow lights on the side of freight trucks? I've noticed this for a while - I assume this means they must also use LED's. Makes a lot of sense I guess...

  11. emrikol Says:

    I swear that light fixture looks exactly like my b13 sentra/sunny! Can it be?!

  12. Stark Says:

    Eschatonic, Once again, unless something has gone horribly wrong LED's should outlast incandescent bulbs (even underdriven ones like automotive bulbs) by quite long time. Years long. What you've described is a textbook case of overdriven/under-cooled LEDs. The quick diming and eventual failure is typically what happens when an LED (or an array of them) is fed too much power. I assumed in Furrfu's case that it was probably an eletrcial system issue but it seems much more likely to be a design issue of the LED units.

    Not sure how you could test this though... any bright ideas Dan?

  13. Daniel Rutter Says:

    My car's a Nissan Pulsar, which is closely related to the Sentra and Sunny (to the extent of being essentially the exact same car, for many year models).

  14. Daniel Rutter Says:

    Regarding unreliable cheapo LED lamps, I think these problems can be put down almost entirely to two factors.

    1: Some very poor designs. Say, a circuit that ran the first kind of LEDs they put in it at full rated power and worked OK, but was then changed to use newer LEDs with lower voltage drop without changing anything else, resulting in horrible overdriving. Bonus points will be awarded if the circuit works such that the first LED to die increases the current flowing through the unfortunate survivors.

    2: Nonexistent quality control. Particularly applicable to the bottom-feeding eBay dealers we all know and love. So lights with dry solder joints, lamps that aren't quite glued together, et cetera, head out into the market along with the ones that're properly made.

  15. Daniel Rutter Says:

    Regarding strobing LEDs on vehicles: Despite persistent rumours, you don't get a preceived-brightness or LED-life benefit from flashing LEDs rather than running them at a visually-equivalent constant brightness.

    Pulse-width modulation brightness control, however, is an easy and inexpensive way (in this world of supercheap ICs) to efficiently control the perceived brightness of LEDs. So if there's some regulation that says that trailer-truck side marker lights have to be between brightness X and brightness Y, I can see them using a PWM controller to dial the brightness of LED markers down to match the regulation.

    (If there's no such regulation, I don't see why you wouldn't make simple little resistor-fed modules that plug into a standard 12V socket instead and run the LEDs at whatever brightness you happen to get. A bit less efficient, but the difference is zero from a truck's point of view.)

    I've noticed that cars with LED brake lights often seem to run them at full, no-flicker brightness when they're actually braking, and in flickery PWM half-brightness mode when the lights are just on as driving lights.

    I did not, of course, test this by waving my head around like an idiot while driving home the other day, endangering myself and others. I've no idea where you heard such nonsense.

  16. furrfu Says:

    #3 - I think Dan hit the nail on the head in #14 (hey Dan, can we have threaded comments? :-) -- the bulbs are allegedly meant to be usable in any location that takes them, in- or outside the car, but they're rather cheaply made and there's no overcurrent protection.

    I should also add that I drive a Volvo, so the lights are on all the time...

  17. frasera Says:

    "On a whim, I picked up a pair of the inverse cone type bulbs from the local Autozone"

    yea places like autozone probably carry old generation led type lighting. they aren't really a cutting edge type of place, and generally neither are their consumers. you can tell by the cheesy led flashlights they sell..5mm cluster garbage and all that. it'll take a long time for good stuff to trickle down to autozone/kragen type places

  18. Klingon Says:

    As I understand it, truck and trailer running lights are universal, I.E. they'll run on 12 or 24 volt systems. The way that the manufacturer(s) achieve this is through PWM, regulating the current to/through the diodes. What this means is that running on 24 volts, the flicker should be more noticeable on 24 volts, due to the shorter on time.

    I, too, wonder about the frequency selection. the timing resistor remains the same, but reducing the capacitor to a smaller, hence cheaper, value would satisfy both flicker-reduction and lower per-unit cost.


  19. roland Says:

    I am an artist who builds architectonic sculptures that are very detailed and whimsical. My general philosophy about lighting is that we should have more VERY low-watt options instead of all the over-illuminated environments at present. I've built some art pieces and incorporated 7.5 watt bulbs in them, and run an in-line rheostat to make them even dimmer. But I would like some alternatives to the clunky sockets and bulky cords. Ideally, even a battery operated lamp.

    If anyone has suggestions for inexpensive assemblies that are complete (with the exception of a standard battery) I would be most appreciative. I would also consider a very simple small bulb and socket assembly with standard electric cord.

    If possible, please respond before February 15, 2010. But even after that date i would still welcome responses.


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