Running a thermistor in parallel to a light bulb?

[james125]

I want to be able to make my headlight bulbs last longer. I know the main reason they burn out is the massive current surge when you turn them on when cold.

I was wondering if you put a thermistor in parallel, then the current would be split through the thermistor reducing the surge. Eventually increasing current to the bulb once the thermistor is hot.

Will this work?

[anonymous-user]

james125 said:

I want to be able to make my headlight bulbs last longer.

er, Why?

[98elise]

james125 said:

I want to be able to make my headlight bulbs last longer. I know the main reason they burn out is the massive current surge when you turn them on when cold.

I was wondering if you put a thermistor in parallel, then the current would be split through the thermistor reducing the surge. Eventually increasing current to the bulb once the thermistor is hot.

Will this work?

No, both in a parallel circuit they will both draw whatever they need.

You will not be able to protect the bulbs this way, they degrade through heat and age regardless.

More to the point why?

[james125]

Max_Torque said:

er, Why?

Why wouldn't you? Headlight bulbs cost money and time to replace

[ewenm]

james125 said:

Why wouldn't you? Headlight bulbs cost money and time to replace

I wouldn't because my headlight bulbs blow so rarely that it's not something I worry about.

[jet_noise]

Dear j,

you need a series NTC thermistor designed for inrush current limiting.
Or you could fit a switch bypass resistor to keep the filaments at red heat.

Both of these techniques can be used to extend filament bulb life. You may spot an obvious disadvantage to the 2nd in a battery powered application

As other posters have suggested though: why bother?

regards,
Jet

[anonymous-user]

I'm not sure that the current surge is why incandescent bulbs burn out? After all, yes, the current is high, but the filament temp is low. High current, in itself, is not a limit. The limit is thermal.

[redstu]

How often are the bulbs blowing?
If they last a few months rather than years then you probably have a fault somewhere.

[Flibble]

Max_Torque said:

I'm not sure that the current surge is why incandescent bulbs burn out? After all, yes, the current is high, but the filament temp is low. High current, in itself, is not a limit. The limit is thermal.

This is true - they burn out due to evaporation of the filament which is due to heat. Thermal shock from switching them on doesn't help either; an already weakened filament will often break on switch on due to uneven thermal expansion.

Not to mention the cost of a suitable thermistor or current limiting circuit would likely outweigh the cost of replacement bulbs anyway.

[james125]

jet_noise said:

Dear j,

you need a series NTC thermistor designed for inrush current limiting.
Or you could fit a switch bypass resistor to keep the filaments at red heat.

Both of these techniques can be used to extend filament bulb life. You may spot an obvious disadvantage to the 2nd in a battery powered application

As other posters have suggested though: why bother?

regards,
Jet

After some research I have found the same results a NTC thermistor is needed. They cost around £2 each. Cheaper than a bulb. But then again I could simply buy an LED bulb which is a similar price to the original. and also a lot less effort to fit!

-EDIT. Apparently LED dipped beams are underpowered and not worth using.

Edited by james125 on Thursday 3rd October 14:02

[Flibble]

james125 said:

After some research I have found the same results a NTC thermistor is needed. They cost around £2 each. Cheaper than a bulb. But then again I could simply buy an LED bulb which is a similar price to the original. and also a lot less effort to fit!

You might want to check the power/current rating on the £2 thermistors, then consider that a headlight bulb is typically rated at 55W so you'll need a thermistor to match... Putting a little £2 thermistor into your headlight circuit will just burn it out (and possibly start a fire).

[dvs_dave]

Headlight bulbs just don't blow regularly, nor are they expensive to replace (assuming non HID?). What has lead you to this odd conclusion? You are either using rubbish bulbs, or there is a problem elsewhere with your vehicle's electrical system. I would be almost certain of the latter. Check your alternator voltage output, as if the regulator has had it, it's pushing out too higher voltage, and this is your issue. The Heath Robinson solution you're looking into is a complete waste of time and will most likely just cause an electrical fire.

[TheEnd]

That's a whole new level of being tight.

[Dr John]

Theatre lamps use preheat ie they powered with ~5-10% normal power when "off" so the filaments are kept warm to reduce inrush current.
Bearing in mind these are 1000 to 2000w lamps and have an expected life of only a few hundred hours these are serious bits of kit which cost in the order of £30-40 each . Non the less there is considerable debate about wether this preheat is a complete waste of time!