CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

LED headlight conversion

@BillRussell What do you mean by thermal runaway? Only asking because I barley know the basics of electricity never heard that term before.

“Thermal runaway” means the amount of current increases through the component as the temperature increases, which makes it even hotter, and the process continues until it burns out. Incandescent light bulbs don’t burn out quickly in part b/c the amount of current they pass decreases as they get hotter. But some components do the opposite, which makes them subject to thermal runaway.

@GeorgeSanJose Thank you now I understand, I just never heard that term before.

I’ve had some recent experience with this . That’s the reason I know. I replaced the dome light on my Corolla with three LED bulbs I got out of a flashlight gadget, and the first attempt they all burned out! Well, not all at once. One by one as I was trying to debug the problem Finally it occurred to me I was experiencing thermal runaway, so I put a current limiting resistor in series. I must say I really like the LED dome light experience now I finally got it working.

1 Like

@GeorgeSanJose The only experience I have with leds has been with many years with truck lights with never a blowed one yet. One moe question I recently got some for the house & they all work good except for one it worked about a month then started flasing like a strobe.If I take it out for a day it woks normal for 1 or 2 hours then does it again.

Thanks George, beat me to it. I might clarify it as follows:
Diodes, including LEDs, have a sharp voltage versus current curve (small voltage changes cause a large current change), and the voltage decreases with increasing temperature. If you supply the diode with a constant voltage, as it warms up the current increases by a large amount, causing heating, which increases the current still more. This continues till it melts.

If you supply it with a constant current, the reverse happens. When it warms up, the voltage decreases. With the current fixed, this results in less heating, so it stabilizes.

A tungsten light bulb is the opposite, stable with constant voltage supplied. But run it from a constant current supply, and it will go into thermal runaway also.

Renegade: “flasing like a strobe”, that sounds like a thermal problem. It overheats, the circuitry shuts it down, it cools a bit, repeat.

1 Like

ANY LED will burn out without the appropriate resistor!!!

thermal runaway is a fancy word for overheating…nothing else. It’s like when you take your car to a repair place and they tell you that your ECM had a thermal runaway and it cost you $500 bucks to fix it. Another repair shop tells you that you have a burned out (overheated) resistor in your ECM and charge you $20 buck to solder a new one in.

It woks (works) normal for a while then flashes. The light is defective. Putting the light in a different place and if it does the same thing that will confirm it is bad.

lol … true, very true, but as long as we aren’t the one getting stuck with the $500 invoice, worth a chuckle … lol …

Thank y"all for your for your replies. I now know a little more than I did yesterday. As I told to @GeorgeSanJose ththe only experience I have is with 12 volt truck lites have never had one go bad yet. I understand that 110 volts leds are fairely new & may hve different problems.

As far as I know all LED’s operate on 3 volts or less. The 110 volt kind for house-use just have some kind of circuit to reduce the voltage to the LED’s.

Bill Russell’s comment above about thermal runaway is sort of interesting physics. Electricity flows in wires differently than in semiconductors like LEDs. In wires it is like a marbles rolling down a slide, the more marbles there are they start bumping into each other, slowing the flow. So w/ In semiconductors it sort of diffuses through, like if you put a drop of ink in a bucket of water. The hotter the water gets, the quicker the ink diffuses. That’s why semiconductors like LED’s the current flow increases with temperature.

George is correct as usual. LEDs all operate at about 3 volts Vf.

You can see some circuits by searching google for “circuit 120v LED”

1 Like

Plastic headlight covers are fogged from the UV in sunlight (there’s not much of that from an incandescent filament, and the fog is mostly on the outside) and when fogged they not only block light but scatter a fair amount of it in all directions, including oncoming drivers’ eyes. Replacing them is best, but one can try polishing below the surface fog or having a body shop do it which without power tools is arduous.

it’s called resisters that reduce (by resistance) the voltage to the appropriate amount needed. The minimum to tun on a LED is about 1.7 volts. For a decent brightness it’s about 1.9v…to keep it simple

1 Like

The Vf for LEDs varies by their material construction. They vary between around 1.2V to 3.6V+.
It’s the amount of current allowed to flow through them that determines their brightness. Of course you can starve them for voltage and get a similar effect albeit with less range :wink:

Like I said my garage and shop was the next focus to convert to LEDs. Rockler Hardware has a sale on for 2400 lumen shop lights for $30 so I just put in 11 of them in my garage and shop. I had 11 100 watt bulbs and now these things are super bright plus use a fraction of the power. Just like daylight now. So I now have a surplus of used 100 watt bulbs. Sunglass time.

1 Like

:sunglasses:

yep, they are amazingly bright thats why i thought about getting them for my headlights. I put a 32 inch LED light bar on my suburban and thats when i noticed how dim the headlights were.when i turned the light bar off it seemed like the headlights werent even on,they were so dim compared to the LEDs on the light bar. since i started this thread i have replaced the foggy old headlights with new ones and the new ones are brighter but they still dont come anywhere close to matching the crystal clear white light from the light bar.
I`ve read that LEDs only create %30 of the heat that a regular headlight does.

That light bar is almost certainly NOT certified for on-road use, and could cause you some grief with the local constabulary if you catch them in a bad mood (as caused by blinding them with said light bar, for example).