# How far can I push a 12V bulb?



## J!m (Feb 20, 2008)

I have an idea for a light.

I plan to use a standard 12V (rated) auto bulb and was wondering how far I could push it.

I know that a fully charged battery and good alternator operate at 14.8 volts, so if I was to go that high, no problem. What if I went higher? Say, 18 volts? would this be a problem? That's only 17.7% over the normal 'max' voltage.

I will be running unregulated, so there will be a potential for a full 18V in-rush. POviously, the in-rush will diminsh as the batteries slowly die...

Anyone?


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## adamlau (Feb 20, 2008)

There are no hard and fast rules, suffice to say that most bulb manufacturers allow for +10% with regards to spikes above rated bulb voltages. Review what LuxLuthor has done in Destructive Incan Bulb Tests and contribute accordingly.


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## allburger (Feb 20, 2008)

A while back i made a flood light which mounts to my backpack strap for hunting. It uses a 12v 20w light that you would find in a track lighting system. I am running it off of an 18v drill battery. When this thing is fully charged, the bulb does not flash. 

It really depends on the light itself though, i would suggest purchasing one and trying it. If it blows, then buy another one and install some sort of resistor.


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## J!m (Feb 20, 2008)

Thanks for the input so far- I will have to check that thread.

As to using a resistor, I'd prefer not to. I want to keep things DEAD simple, for reliability, and easy reapir...


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## J!m (Feb 20, 2008)

I have read the entire post, and the problem I have is that I plan to use an 'abnormal' bulb for my project. I haven't even decided which one yet, but I thought there were general guidelines for a maximum overdrive voltage for a 12v (rated) bulb.

I will check over those charts completely for 12v bulbs, and determine an average maximum voltage, and shoot for the mean between the 'max ave' and the 12v rating.

Thanks! Keep the ideas coming!

Now, on another note, are there small, simple, cheap ramp-up regulators for these crazy lights? As you can quickly tell, I am new to this whole thing. I assume (from reading the thread mentioned) that ideally, I want to ramp up power slowly, not use a simple hot contact to rush full voltage to the cold filament. I say cheap, because I may need more than one regulator, dependant on my final configuration/design...

Thanks again!


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## lctorana (Feb 20, 2008)

J!m said:


> ...are there small, simple, cheap ramp-up regulators for these crazy lights? As you can quickly tell, I am new to this whole thing. I assume (from reading the thread mentioned) that ideally, I want to ramp up power slowly, not use a simple hot contact to rush full voltage to the cold filament...


 
Yes!

They are called "NTC"s, which stands for Negative Temperature Coefficient thermistors.

They are rated by their cold resistance.

For example, a 1 ohm NTC will have a resistance of 1 ohm at room temperature, but when hot (e.g. with a large current) the resistance drops to nearly nothing. Takes about 2 seconds, and gives you the perfect soft start.

OK, so let's say you want to run a 12V, 50W bulb from an 18V supply.

Normal current is about 4.2A, and the desired *initial *voltage drop at cold startup is 6V, so as not to stress the bulb. So let's go with a 1.5 ohm NTC, that will drop the voltage all the way to 12V.

2 seconds later, there will be well over 17V at the bulb, circuit resistance permitting.


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## mdocod (Feb 20, 2008)

asking how far a 12V bulb can be overdriven is an empty question, as it depends entirely on the specific bulb. As you already know, automotive bulbs are actually closer to 14V bulbs. So you can't even take the "assumed" label voltage as gospel.

A 12V 50 hour bulb can only be driven to about 14V before it is close to popping. 
A 12V 6000 hour bulb can be driven to about 20V give or take.

I can't recall the exact numbers for typical autimotive bulbs at the moment... if you look into the depths of the spotlight forum, you may be able to dig up some experiments done by -I think it was Newbie, he tested a bunch of bulbs and IIRC was driving around 15+V at the bulb.


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## J!m (Feb 21, 2008)

lctorana said:


> Yes!
> 
> They are called "NTC"s, which stands for Negative Temperature Coefficient thermistors.
> 
> ...


 
That sounds like exactly what I need...

Where would I find a couple of them? I will need at least two for my project; I assume they are not TOO expensive...:thinking:


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## DM51 (Feb 21, 2008)

J!m said:


> That sounds like exactly what I need...
> 
> Where would I find a couple of them? I will need at least two for my project; I assume they are not TOO expensive...:thinking:


There's a related thread here, where you can learn more about them and which one you might need. NTCs aren't high-cost items. jimjones3630 has done quite a lot of research into them.


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## J!m (Feb 21, 2008)

Thanks! I'll be reading that today...


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## rokspydr (Feb 23, 2008)

I think it depends on the bulb. I physics we were experimenting with electrical exuivalent of heat. we had a 1157 bulb in water.Bulb was sealed but glass was submersed in water. anyway we ran it up to 25V and had it puting out just under 97 watts. it is a standard 12V tail light bulb from a car. i was supprised it got to 25V but I am positive if it wasn't in water it would have flashed sooner. it did flas at 26V but the water was boiling already, funny thing is the filament was that it was the post that melted.


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## lctorana (Feb 24, 2008)

Now 102 uses for your flashlight.

#102: As an Electric Jug element


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## J!m (Feb 25, 2008)

It's funny you mention that, as I was considering water-cooling for my bulb...

Anyway, I think I have a handle on what I need to do, for my intended project.

Now I just have to find the time to acomplish it...


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