12

I decided to install LED headlamp bulbs in my car in the hope that I can avoid replacing bulbs every year. I was quite surprised to find that I can't fit replacement bulbs in my car due to the bulbs having a huge heatsink and, indeed, a fan to keep them cool. I don't have sufficient space in my lamp housing to incorporate the extra hardware.

Looking on Wikipedia at the relative efficiencies of halogen and LED I see that they are both hugely less efficient than I anticipated:

Halogen 3.5% (typical)

LED 14.9% (worst case)

My usual halogen bulbs use 55 watts, and higher-output LEDs use 20 watts. Therefore, the halogen bulbs are creating:

96.5% * 55 watts = 53 watts of heat

The LEDs:

85.1% * 20 watts = 17 watts of heat.

Why do the LEDs need active cooling when the halogen bulbs don't - yet they create over 3 times the waste heat?

Peter Mortensen
  • 1,676
  • 3
  • 17
  • 23
Lefty
  • 311
  • 3
  • 8
  • 10
    In short: The halogen bulbs can handle higher temperatures than the LEDs can. – Hearth Dec 14 '19 at 20:33
  • 2
    the semiconductor used in LEDs is damaged by a lower temperatures than the silica and tungsten used in halogen lamps, – Jasen Слава Україні Dec 14 '19 at 20:38
  • 1
    I have seen LED lamps with passive cooling (in the form of thick copper braids like an engine ground strap) – Jasen Слава Україні Dec 14 '19 at 20:40
  • 5
    Not only can the filament bulbs tolerate higher temperature, more heat can be passively shed at higher temperature. Specifically they can tolerate running at their equilibrium temperature (where heat lost equals that created) while the LEDs apparently cannot tolerate running at their lower one, so need a heatsink and fan to lower their equilibrium temperature still further. – Chris Stratton Dec 14 '19 at 20:41
  • It is all so obvious when explained properly - thank you very much! So the lamps are probably going to have a lifespan of at least 10 years - but it will probably be the cooling motor that fails first. I will have a look out for the passive-cooled version and see if that works for my car. – Lefty Dec 14 '19 at 20:51
  • Please, pretty please, don't do that! Depending on where you live, it may be illegal and it may or may not be enforced. In either case, you are likely to be sweared at by all oncoming drivers and get only an impression of seeing better (you will not!). – fraxinus Dec 15 '19 at 16:25
  • @fraxinus I live in the UK. As it happens I looked at the Halfords web site this morning and was totally put-off the idea of installing them when I saw their general disclaimer "LED headlights should only be used off-road". They are clearly just doing this to cover themselves legally, but it has persuaded me to abandon the idea. To be fair, I'm not really interested in the (supposed) extra brightness, just increased life. Replacing them on my car is a total pain in the - bottom - and I am sick of doing it every year! – Lefty Dec 15 '19 at 16:47

3 Answers3

20

Here's an interesting video comparing a 25/25W LED headlight to a 90/100W halogen.

Philips Luxeon H4 Headlight Temperature Output vs Halogen

The passive heatsink at the rear of the LED lamp got up to 74 °C, while the halogen lamp reached 99 °C at the bulb holder. Considering the difference in power consumption, the temperature difference doesn't seem that much!

The halogen lamp's quartz bulb has to reach a surface temperature of 250 °C in order to maintain the halogen cycle, and the filament inside runs at ~3000 °C, so why doesn't the outside of the lamp get a lot hotter? One answer can be guessed from the measurements at the front of the lamps. The LED lamp only got up to 33 °C, but the Halogen measured 60 °C.

An LED only emits visible light, so most of the heat produced has to be dissipated by the heatsink. A halogen lamp dissipates a lot of heat too, but it also emits infrared light directly out the front along with the visible light. Some of that infrared is absorbed by the front glass, but a lot of it passes through and so does not heat up the lamp.

In the graph below we see that a halogen lamp emits much more infrared than visible light. If that light was useful to us the lamp would be considered far more efficient.

Halogen Lamp Spectrum enter image description here

The other difference is that halogen lamps are designed to run hot (which they must to work properly), but LEDs are less efficient at high temperature and may be damaged by going over 150 °C, so they need better heat sinking.

Bruce Abbott
  • 55,540
  • 1
  • 47
  • 89
  • 1
    _"An LED only emits visible light, so most of the heat produced has to be dissipated by the heatsink"_ — how does the part before "so" imply the part after it? And why is it relevant in the comparison of radiative cooling of the halogen bulb (which works in a completely different way regarding the heat, than does the LED)? – Ruslan Dec 15 '19 at 18:15
  • 3
    @Ruslan An LED emits only visible light, not infrared light. Any energy that isn't directly emitted will heat up the lamp body/heatsink. That 'waste' heat will eventually be radiated/conducted/convected away, but still makes the lamp hot. As you say, a halogen bulb works on a different principle. The bulb itself runs hot, but much of the 'waste' heat is actually radiated as a beam of infrared light, not transferred to the lamp body. The bulb also conducts some heat of course, and the glass absorbs longer wavelengths, so _some_ of it does heat up the lamp. – Bruce Abbott Dec 15 '19 at 19:59
  • 1
    Worth mentioning that radiative heat transfer scales with `T^4` (absolute Kelvin temperature), so getting hotter can radiate away a *lot* more power. (But conduction / convection both scale linearly with delta-T between hot and cold. Still, an extra 25 degrees above the engine compartment temp is significant. – Peter Cordes Dec 15 '19 at 20:08
18

Because incandescents love heat, and LEDs hate it.

An incandescent in a 250°C environment is one happy incandescent. That's why they work so well in oven lights.

An LED in a 250°C environment is dead.

So you need to keep the junction temp down to something like 115°C at the absolute max, though 85°C would be better. That fits hand to glove with the car's cooling system.

Harper - Reinstate Monica
  • 21,639
  • 27
  • 68
  • Tip of the day: You can use HTML entities `Ω`, `μ`, `°`, `×`, etc. as well as `...` and `...` in the posts (but they don't work in the comments). – Transistor Dec 14 '19 at 21:43
  • @transistor You can also just enter a ° manually in the answer or comments. – Hearth Dec 14 '19 at 22:26
  • @Hearth: Not on a laptop keyboard. I use ALT+0176 when I have a full keyboard. Dell laptops used to have a "blue" keypad using 789, UIO, JKL, M in conjunction with the Fn key. I don't know why they dropped it. I used to use CharMap and cut and paste the characters but was delighted (sad life?) when I discovered that the HTML entities worked. – Transistor Dec 14 '19 at 22:38
  • @Transistor I always recommend wincompose for stuff like this. Super easy way to insert arbitrary characters with custom key sequences. – Hearth Dec 14 '19 at 22:43
  • Incandescent...250C, that's the temperature of the _envelope_. The temperature of the actual emitter (i.e., the tungsten filament) is a whole huge heckofa lot hotter than that! – Solomon Slow Dec 15 '19 at 06:24
  • @SolomonSlow good point. Edited. – Harper - Reinstate Monica Dec 15 '19 at 06:25
  • Yes they love heat because it is the heat that radiates at 2500'C and up and they are pretty dim below 900'C yet Tungsten is a very tough metal – Tony Stewart EE75 Dec 16 '19 at 16:12
2

Why do LED headlamp bulbs need active cooling?

1. Because they will quickly fail otherwise. They might have the same lifetime as a cheap 100W bulb at 150'C

If you add thermal insulation to raise the temperature so that the mean life or MTBF= 1000 Hrs and then reduce the temperature, your MTBF will double for every 10°C drop towards room temp. This is a Law of Chemistry called Arrhenius Law.

It just happens that Tungsten filaments operate around 3000 ~ 4000 °C and Semiconductors don't like to operate more than 200°C and preferably < 85°C junction temp .

Therefore due to Arrhenius Law Semiconductor LED's with very high power density require active cooling like CPU's.

For a test on a 100W light bulb, rated for 3000h, put a glass globe over the bulb and trap the convection heat exchange necessary for cooling and expect it to last 3 mos. ( If I recall from memory)

Don't ask how everyone thought LED's should last 50kh=MTBF that was when Mfg process quality was very poorly controlled and no one did any real reliability analysis.

The 1000 and 5000 hour high temp life test follows this Law for all electrolytic capacitors. You then derate your design stress to estimate thermal rise relative to design rise to estimate MTBF.

Tony Stewart EE75
  • 1
  • 3
  • 54
  • 182
  • 1
    Tip of the day: You can use HTML entities `Ω`, `μ`, `°`, `×`, etc. as well as `...` and `...` in the posts (but they don't work in the comments). No need for apostrophes in place of °. – Transistor Dec 14 '19 at 21:44
  • Guess I better start using HTML ☺☻♥­ TY @Transistor \$˜ } ˜\$ – Tony Stewart EE75 Dec 14 '19 at 21:48
  • Interest only: In about 2008-2009 people were claiming 100,000 hours for typical small lighting LEDS. I was designing LED portable lights - being manufactured in China. We were using Nichia LEDS - according to their measurements they were the **ONLY** LEDs on the market at that stage that exceeded 10,000 hours at rated performance. – Russell McMahon Dec 16 '19 at 11:19
  • Cree modern lighting class LEDs are specified at 105C die temperature (or were last time I looked.) – Russell McMahon Dec 16 '19 at 11:20
  • Russ EOI in TW had White Chips verified that I purchased in large qty as LEDs with verified MTBF’s of 50khrs in 2014 when I start with them. They also sold their Red wafers to Nichia as a silent trade agreement when others were being sued – Tony Stewart EE75 Dec 16 '19 at 14:20
  • In 2014 Cree’s XHP family was a 10% brightness reduction or L90 is project at 50kh MTBF and rated to 105’C for Tj but Cree had not stated L90 and 105’C are both rated at 50kh, which is by 85’C is preferred unless you need a 85’C heatsink is comparative lifetime costs to Halogen Headlights for a contract. 50kh = 5.7 yrs but if used 4hr / day implies MTBF 6x as long – Tony Stewart EE75 Dec 16 '19 at 14:36
  • @TonyStewartSunnyskyguyEE75 Yes. Big changes between 2008 and 2014. But the major difference, apart from the silicon, was/is in lenses. "Back then" the LEDs were almost all T1.3/4s and all used epoxy lenses. All (essentially) modern long life LEDs use silicone based lenses. Degradation of the epoxy C-C bonds due to the short wavelength light from the LED sets a sensible upper limit on lens life without any outside help. The bond energy of Si-Si is not vastly higher but enough so to allow vastly longer lifetimes. – Russell McMahon Dec 16 '19 at 15:46
  • There once was a problem from lack of adhesion for Si Lens at high temp.. I mistyped 2014 I started to source LEDs in 50k PO's in 2004 (5mm) when there were 100 LED factories in TW and for 12 yrs sold over 1million to one customer in NZ , 3i... all with my custom specs to meet their needs, 2 wk leadtime – Tony Stewart EE75 Dec 16 '19 at 15:49
  • 3i was the inventor of wireless LED fixtures and still the only one I know doing this for Road traffic lighting http://3iinnovation.com/onroadled/tunnels/#!home – Tony Stewart EE75 Dec 16 '19 at 16:00