Testing H4 car bulb with multimeter

[Stan]

Using a multimeter, what resistances would I expect to get from a working H4 car headlight bulb?

Thank you for any info.

[Sparks]

If this is a "normal" halogen bulb like this

no resistance would mean it is dead.

I think the pinouts are, looking at the bottom of the bulb with a terminal on the left, right and top

Left terminal is common Top terminal is low beam Right terminal is high beam

So you need to measure between the left pin and the top, then the left and the right.

Sparks...

[Dave Liquorice]

Quick get a patent on it for room temperature super conductivity. B-)

Bulbs are either a few ohms (working) or lots of ohms as in open circuit (bust).

[Sparks]

That's what I actually meant of course!

[Andy Wade]

Very low indeed, and too low to measure with any accuracy with a normal multimeter.

Say it's 55 watts at 13.8 volts. Resistance is voltage squared over power, so 13.8^2 / 55 i.e. about 3.5 ohms when hot. When measuring cold you can divide that by about 15, so about 0.2 ohm.

HTH

[mrcheerful]

provided there is contact between any two pins it will work (because the neutral is common)

mrcheerful

[Dave Plowman (News)]

Just measured one - 0.4 ohm. I'd guess it will vary somewhat between makes and depending on ambient temperature.

[Harry Bloomfield]

Stan pretended :

Usually you can check them without a meter - just give the lamp a sharp tap with your finger tip and see if the filament vibrates excessively, as if broken.

If you really want to test it...

One of the three pins is common - connected to both filaments, the other two are the main and dip beam connections. With a multimeter and assuming the lamp is good, you should get a value of a few ohms between any two pins. The two pins which have the higher value between them, means that the third pin is the common.

With care you could test the lamp across a 12v battery. Similar to above, the two pins which produce the dimmest light out, means that the third unconnected pin is the common.

[Andy Dingley]

Zero resistance (short circuit) or as close to it as you're likely to be able to measure with a cheap meter and simple test leads.

You might be able to see the diference when you have two filaments in series, but you're unlikely to measure any difference between the high and dipped beams, even though they're of different powers.

Resistance of them goes up when they're hot, but a meter won't deliver enough power to notice this. Working resistance of a bulb is v^2/w, so about an ohm or two. They'll be under an ohm when cold.

[Vim Fuego]

See responses below but why would you want to?