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Cars do not have constant 12V voltage. So car bulbs should also be operable at those unstable voltages. My question - what are the limits for basic H7 halogen (or LED) bulbs used in cars? Could they reliably work at 15V?

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    \$\begingroup\$ Can you provide a link to the manufacturer's datasheet for the bulb you want to use? Without that, no one can give you an accurate answer. People will opine and guess, but you need the data to be sure. \$\endgroup\$ Mar 16, 2022 at 11:33
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    \$\begingroup\$ IIRC, the lifetime of a filament bulb goes roughly as the -14th power of the applied voltage. I used that to drive ordinary 1000 hour filament bulbs as 'photofloods' with a few hours life by over-driving them with rectified, smoothed, mains. And yes, with 1.4 x the rated voltage, they lasted a few hours. From which you can infer that a small increase in voltage over their rated voltage probably won't be immediately noticeable as a reduction in life. Now, what is their rated voltage? If I was making auto bulbs, I'd allow for at least 14 V, perhaps more. \$\endgroup\$
    – Neil_UK
    Mar 16, 2022 at 11:42
  • \$\begingroup\$ The common test voltage for lifetime specifications is, afaik, 13.2 V. Surely they will work above that, but the lifetime will decrease from what the manufacturer specifies from there. \$\endgroup\$
    – Klas-Kenny
    Mar 16, 2022 at 15:50
  • \$\begingroup\$ Yours truly made a PWM circuit for soft starting and regulating the output voltage to a halogen bulb for flashlights back before LEDs were around. Without soft start, about 18 V is the limit for a 5000 hours rated 12 V bulb. With soft start, about 21.5 V is possible. You’ll get almost 40 lm/W at 3400 K but runtime is a few hours. \$\endgroup\$
    – winny
    Mar 16, 2022 at 17:36
  • \$\begingroup\$ Yes, bulbs reliably work at 15V, my car does charge at up to 15.3V and the bulbs work fine. \$\endgroup\$
    – Solar Mike
    Mar 16, 2022 at 19:09

3 Answers 3

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There is no precise "maximum voltage" for incandescent bulbs as there might be for a semiconductor- there is a continuous and steep reduction in life as voltage increases. There is a corresponding decrease in efficiency and increase in life as voltage decreases (along with some other undesirable things that happen when halogen bulbs in particular are run too cool, inhibiting the halogen cycle).

To state the perhaps obvious, the normal operating mode of headlights in an automobile is with the engine running, so the normal supply rail voltage will be around 14V and the bulbs are designed to have a reasonable life when operated in that voltage range (perhaps minus some volts for the cheap thin wires used in cars). They will produce light at 12V or 10V but that's not how they are normally operated for most of their life. If the voltage increases to (say) 15V, cooking the battery etc. (say, because of a faulty voltage regulator) the life of the bulbs will be significantly truncated.

Unfortunately, even official-looking datasheets for halogen bulbs (eg. H4) from manufacturers such as Osram do not show the normal operating voltage, and instead show consumer-friendly numbers such as "12V/55W" for example. The standards will contain the proper voltages such as 13.2V for '12V' systems and 28V for '24V' systems.

enter image description here The Tc/B3 life time specifications of 400/800h & 170/350h (for each of the two filaments) in the linked 24V H4 bulb datasheet apparently mean the time for 63.2% of bulbs to fail vs. 3% to fail.

Note that the standard voltage for '12V' system bulbs is 13.2V while the voltage for '24V' system bulbs is 28V which implies less voltage drop in the truck systems (the current will obviously be about half for the same power but manufacturers would tend to use thinner wires where practical for cost and weight reasons). This is speculation, maybe there is some other reason, but it would appear to me that they assume that the truck systems have heavier duty wiring.

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Typical 12 V automotive alternators had fixed voltage regulator set points to maintain battery charging voltage between, for example, 13.8 V and 14.2 V.

Nowadays, with automobile ECUs having dynamic control of alternator output, the upper limit of voltage could be in excess of 15 V.

The higher voltage would have necessarily been taken into consideration in auto bulb design.

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    \$\begingroup\$ No 14.2V is not the upper limit, cars have charging systems now that are designed to run at up to 15.3V and yes, I have one and I have the workshop manual showing that data. \$\endgroup\$
    – Solar Mike
    Mar 16, 2022 at 19:08
  • \$\begingroup\$ @Solar Mike - Hi, Many thanks for pointing it out. My answer has been duly edited. \$\endgroup\$
    – vu2nan
    Mar 17, 2022 at 5:02
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Could they reliably work at 15V?

Sure. Connect 15 volts and they will reliably light up.

The question is, for how long? I would guess something on the order of hours or maybe days. They'll get a lot hotter, dissipating about 50% more power, but glass and metal are pretty good at this sort of situation. The issue will be how long the filament will last. Halogens work by recycling the tungsten which evaporates from a hot filament, but it's anybody's guess how well this will work in this particular case.

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