Is it dangerous to change xenon bulbs in my car's headlights?

[Vilx-]

At the risk of a little hypocrisy I'll ask this question:

One of my car's Xenon lightbulbs gave out yesterday. After learning how much my dealer charges to replace them ($60-$80 for work, and it's supposed to take 1.5h), I'm getting this urge to try myself. It's a bit tricky to get to the bulb (you have to remove the whole headlight assembly first), but I think I can manage that.

The thing I'm worried about is the warning I keep seeing everywhere - Xenon headlights use high voltages (around 80'000V), which could be lethal, and I should not do it myself. It's even printed in highly noticeable letters under my hood.

On the other hand, there are many stories around the Internet from people who have changed their bulbs without a problem. The technology description also says that the high voltage is only required to start the lamp. Afterwards it only needs around 80V to continue operating.

So - if I don't play dumb and don't try to change a glowing bulb - will I still be at risk? Can some capacitor still have an unpleasant charge in store for me? Or are these warnings just for people who don't realize that it's not smart to try and poke around beneath the hood while the engine is running?

[Left]

I changed out my headlights about a month or two ago. It wasn't hard, hardest part was getting head piece out, I had to pull harder than I thought. I was being more gentle with it than I needed to. But it was a normal bulb and not xenon... does it matter?

for the high voltage part, just disconnecting the battery to car would take care of the problem I think.

[Spork]

I changed out my headlights about a month or two ago. It wasn't hard, hardest part was getting head piece out, I had to pull harder than I thought. I was being more gentle with it than I needed to. But it was a normal bulb and not xenon... does it matter?

for the high voltage part, just disconnecting the battery to car would take care of the problem I think.

I don't think so.  If you have a 12 volt system driving a 15k volt light, there is a capacitor somewhere in the line.  Capacitors will generally hold their charge (as that's what they're designed to do.)

I am going to guess here (so don't count on it!) that with the high voltage comes low amperage.  Remember: it's the amps that kill you, not the volts.  I've touched my share of 10k volt electric fences.  They hurt like hell.  But they're safe to touch.

That said: I'd do it and be careful. 

[_JT]

I would do it. Disconnect the battery, and then when you have the headlight assembly out, short across the terminals or exposed wire to discharge any stored energy from a start capacitor. Make sure you're holding the handle of the tool when you do this and not the shaft, duh. I'd also wear safety glasses.

[Vilx-]

Yes, and it's the sudden stop that kills you, not the fall. :P

I know my electric fundamentals. The classical Ohm's law is - U=IR. Or, if you flip it around, I=U/R. So, the higher the voltage, the more amps you're going to get - provided that the power source has it in itself to begin with. Sure, it's the current that kills you. But it's the voltage that makes sure that the current can get where it needs to. The battery of my car can release a stupid amount of current all at once. But since it's only 12V, I can hold both ends in my hands all day long without feeling a thing.

Anyways, that's what I want to find out from someone who knows what he's talking about. My logic says - disconnecting the battery will make little difference. If the bulb is off to begin with and it doesn't waste my battery, that means that there is no current flowing through it. Which means that at the very least the voltage between the bulb wires is less than the high starting voltage. In fact, I don't see any reason why there should be any voltage at all. The whole thing should be disconnected from the power source. Theoretically.

Which leaves only a capacitor that might store the lethal charge. Obviously it's only used to start the bulb, and it's either not at full charge, or is disconnected from the bulb itself (otherwise the bulb would flash and discharge the capacitor). So - can I touch it by accident? Is it charged at all when not in use? How long can it hold it's charge?

Also - any other gotcha's? I used to think that it doesn't matter which terminal of the car's battery you disconnect first (when changing a battery) until I found out that there is a reason after all. Sure - as long as everything happens as planned, all is fine. But disconnecting the negative terminal first prevents an accident (the wrench slips) from becoming catastrophic (positive terminal gets connected to the car's frame via the wrench and basically welds it in place under a nanosecond). There could be similar tricks here.

[_JT]

I know my electric fundamentals. The classical Ohm's law is - U=IR. Or, if you flip it around, I=U/R. So, the higher the voltage, the more amps you're going to get - provided that the power source has it in itself to begin with. Sure, it's the current that kills you. But it's the voltage that makes sure that the current can get where it needs to. The battery of my car can release a stupid amount of current all at once. But since it's only 12V, I can hold both ends in my hands all day long without feeling a thing.

Bolding mine, because these two statements don't jive. The reason the excessive current available in your car battery doesn't kill you if you touch the terminals is not just because it's a 12V battery. That 12 volts is plenty to send out hundreds of amps to your starter when asked to. The reason it's ok is because your body resistance is sufficiently high enough to keep the current really low. The current in every circuit, no matter what voltage, power source, loads, or configuration, is determined by the load. That's a fundamental.

Here's a prove-it situation: take a small 9v battery, and touch the terminals across the back of your hand. Now touch them across your tongue. Quick and easy demonstration of how resistance affects the flow of current from the exact same power supply.

-------------------

As far as your headlight is concerned, it sounds like you're on the right track there. Exercise caution and the knowledge you already have and I don't see the problem.

[Vilx-]

That's pretty much what I wanted to say. I just didn't touch on the R parameter in the formula in order to simplify it like Spork did. But you're right. It can't be avoided.

[Spork]

If it makes you feel any better:  spark plugs operate in the 12k-45k volt range too.  In the old days, all that was stored in the condenser.  Yes, it hurts when you touch it.  But: not overly dangerous.

[Vilx-]

Well, alright then. It seems that all the evidence points to it being safe. :) To add to it - I didn't find tons of "DON'T DO IT YOURSELF" written all over the Internet - which it would be, if it was truly dangerous.

[Greg]

A xenon bulb is not much different than a halogen bulb.  Different gas in the glass capsule.  You may be thinking of an HID which uses an arc rather than a filament. 

In either case, don't touch the bulb with bare fingers, the oils of your skin will cause a hot spot on the bulb that will shorten its life, possibly to a few moments.

And in either case, unplug the wires to the bulb/lens assembly before removing the bulb.  In the case of the HID system, the ballast in usually separate from the lens, and this is where the capacitor would be.  Unplug it and you should be fine.

[gimp]

Wear rubber gloves.

[Posthumane]

I believe your logic is correct. There is likely a capacitor in the ballast which will store a charge, but unless the headlight is turned on it remains disconnected from the bulb. It is true that disconnecting the battery or even removing the ballast from the car will not necessarily result in a safe (discharged) capacitor, so you wouldn't want to take the ballast apart, but changing the bulb would be fine. That being said, there is still a reason to disconnect the battery from the car - it will prevent someone from accidentally turning on the headlights and activating the ballast while you're poking around at it's high voltage end. This can happen quite easily if you have another person around helping or watching, or even if you have a car alarm that you could accidentally trigger.

[fodder69]

Disconnect the battery and and wear rubber gloves. You can get the cloth coated in rubber kind from Dollar Tree. Those are my favorites since they are comfortable, durable and cheap. If you are really worried disconnect the battery the night before, capacitors are not meant to hold their charge for a really long time so would be discharged after a few minutes likely as not.

[Half-Borg]

A xenon bulb is not much different than a halogen bulb.  Different gas in the glass capsule.  You may be thinking of an HID which uses an arc rather than a filament. 

That is so not true...
Halogen is like your regular bulb, with a piece of wolfram where the current flows through.
Xenon is more like a neon tube with an arc that has to be ignited with thousands of volts.

The Xeon headlight has a starter which provides the spark for the inital arc. The 80 KV are only around for a short time.
But has there are capacitors involved which might hold the voltage for a period of time.

I know my electric fundamentals. The classical Ohm's law is - U=IR. Or, if you flip it around, I=U/R. So, the higher the voltage, the more amps you're going to get - provided that the power source has it in itself to begin with

That does not apply here, simply because the capacitor can not hold up a lot of ampere for even a fraction of a second. But it can still kill you.

That said, I would still not hesitate to do it. Unplug the battery, not only to protect yourself but also the electronics in your car, it is very easy to slip with your screwdriver and short circuit something.
If you wait a couple of minutes to give the capacitor time to discharge, you are going to be fine.

I don't know about your car, but my headlights have to aligned after a bulb has been changed. That is something you can not properly do yourself, but it should take a mechanic like 5 minutes and cost 10 bucks.

[Spork]

I don't know about your car, but my headlights have to aligned after a bulb has been changed. That is something you can not properly do yourself, but it should take a mechanic like 5 minutes and cost 10 bucks.

I've not seen a headlight where the bulb changes the position of the bucket... but if it does:

Drive up to a brick wall.  Look where the other (properly adjusted) light is pointing.  Adjust the new one to match.  5 minutes.  Zero bucks.

There was a time many many years ago that state inspections included a "check headlight adjustment".  It was basically a way for the inspectors to fail you and charge you an extra $10 for the adjustment.  I always self adjusted.  I always passed.

[PantsOnFire]

A xenon bulb is not much different than a halogen bulb.  Different gas in the glass capsule.  You may be thinking of an HID which uses an arc rather than a filament. 

That is so not true...
Halogen is like your regular bulb, with a piece of wolfram where the current flows through.
Xenon is more like a neon tube with an arc that has to be ignited with thousands of volts.

To make things even more complicated, there *are* xenon bulbs that are essentially like halogens--tungsten filament and all.  You can plug them in in lieu of an incandescent or halogen bulb.  For a while they were sold as plug-and-play upgrades for car headlamps--basically the next step up from a halogen.  You still see them in kitchen lighting (puck lighting, range hoods, etc.) and there are 12V versions and 120V versions. 

And then there are *xenon arc* bulbs which require a ballast and different reflector positioning. 

Nowadays most people mean "xenon arc" when they say "xenon" in the context of car headlights, but a few years ago before HIDs were prevalent "xenon" often just meant a brighter, longer lasting version of the halogen bulb. 

[Half-Borg]

I don't know about your car, but my headlights have to aligned after a bulb has been changed. That is something you can not properly do yourself, but it should take a mechanic like 5 minutes and cost 10 bucks.

I've not seen a headlight where the bulb changes the position of the bucket... but if it does:

It's less a matter of the bulb and more that I have to disassemble half of the car to get them out in the first place.

To make things even more complicated, there *are* xenon bulbs that are essentially like halogens--tungsten filament and all.  You can plug them in in lieu of an incandescent or halogen bulb.  For a while they were sold as plug-and-play upgrades for car headlamps--basically the next step up from a halogen.  You still see them in kitchen lighting (puck lighting, range hoods, etc.) and there are 12V versions and 120V versions. 

I didn't know that they are long lasting, I always assumed they are for some kids who want their car to look cool, so I assumed someone who owns xenon-halogen-bulbs does know how to change them.

[Spork]

It's less a matter of the bulb and more that I have to disassemble half of the car to get them out in the first place.

It is this sort of thing that makes you want to hit the automotive engineer responsible with a baseball bat.  I've seen a few cars like that.  Changing light bulbs should not be that hard.

[gimp]

capacitors are not meant to hold their charge for a really long time so would be discharged after a few minutes likely as not.

This is such bullshit. (Electrical engineer here.) Capacitors, depending on the type, can hold charge nearly indefinitely.

People have been killed digging around junkyards, stumbling on old tube TVs (which had been sitting in the junkyard for years or decades), and touching the internal capacitors. Parallel plate caps at 400V will ruin your day, and they can do it today or they can do it ten years from now.

Having said that, modern electronics is designed with this issue well understood. Therefore, there are often bleed-off resistors across the capacitor terminals that allow it to slowly discharge, over a period of whatever timeframe makes sense. Perhaps this is what you are referring to?

[ritchie70]

It's less a matter of the bulb and more that I have to disassemble half of the car to get them out in the first place.

It is this sort of thing that makes you want to hit the automotive engineer responsible with a baseball bat.  I've seen a few cars like that.  Changing light bulbs should not be that hard.

Too true, but make sure you really need to disassemble everything.

My 2002 VW supposedly took an hour to do it. It was about removing the battery and a bunch of other stuff that was in the way. I did it in about five minutes because I have small hands and could snake in there and just change it.

[_JT]

capacitors are not meant to hold their charge for a really long time so would be discharged after a few minutes likely as not.

This is such bullshit. (Electrical engineer here.) Capacitors, depending on the type, can hold charge nearly indefinitely.

People have been killed digging around junkyards, stumbling on old tube TVs (which had been sitting in the junkyard for years or decades), and touching the internal capacitors. Parallel plate caps at 400V will ruin your day, and they can do it today or they can do it ten years from now.

Having said that, modern electronics is designed with this issue well understood. Therefore, there are often bleed-off resistors across the capacitor terminals that allow it to slowly discharge, over a period of whatever timeframe makes sense. Perhaps this is what you are referring to?

Agreed (electrical engineer as well).

Also,

Disconnect the battery and and wear rubber gloves. You can get the cloth coated in rubber kind from Dollar Tree. Those are my favorites since they are comfortable, durable and cheap. If you are really worried disconnect the battery the night before, capacitors are not meant to hold their charge for a really long time so would be discharged after a few minutes likely as not.

What do you think wearing rubber gloves from dollar tree is doing to protect you?

[fodder69]

Quote from: fodder69 on December 12, 2013, 07:10:10 am
capacitors are not meant to hold their charge for a really long time so would be discharged after a few minutes likely as not.

This is such bullshit. (Electrical engineer here.) Capacitors, depending on the type, can hold charge nearly indefinitely.

You are definitely correct there, but they are not *meant* to hold said charge and capacitors meant for this use will almost certainly have a resistor either internal or external to drain the charge after a period of time. I can't imagine a modern car system not having that basic safety feature.

What do you think wearing rubber gloves from dollar tree is doing to protect you?

Well, for me, it mainly protects my fingers from all the nicks and cuts I managed to acquire every time I work on a car. But the rubber does provide more insulation than your bare skin also. The cloth gloves that have the fingers and palm dipped are really comfortable, easy for me to work in and keep your hands clean for less than the $30 mechanix gloves.