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Testing H4 car bulb with multimeter



 
 
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  #1  
Old January 15th 07, 12:47 AM posted to uk.rec.cars.maintenance,uk.rec.cars.misc,uk.d-i-y
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Default Testing H4 car bulb with multimeter

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

Thank you for any info.
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  #2  
Old January 15th 07, 01:00 AM posted to uk.rec.cars.maintenance,uk.rec.cars.misc,uk.d-i-y
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Default Testing H4 car bulb with multimeter

"Stan" wrote in message
...
Using a multimeter, what resistances would I expect to get from a
working H4 car headlight bulb?

Thank you for any info.


If this is a "normal" halogen bulb like this
http://www.bestrestproducts.com/cele...4-HP_240-1.jpg

then 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...


  #3  
Old January 15th 07, 01:59 AM posted to uk.d-i-y,uk.rec.cars.maintenance,uk.rec.cars.misc
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Default Testing H4 car bulb with multimeter

On Mon, 15 Jan 2007 01:00:21 -0000, Sparks wrote:

then no resistance would mean it is dead.


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).

--
Cheers
Dave. pam is missing e-mail



  #4  
Old January 15th 07, 02:51 AM posted to uk.d-i-y,uk.rec.cars.maintenance,uk.rec.cars.misc
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Default Testing H4 car bulb with multimeter

"Dave Liquorice" wrote in message
ll.com...
On Mon, 15 Jan 2007 01:00:21 -0000, Sparks wrote:

then no resistance would mean it is dead.


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).

--
Cheers
Dave. pam is missing e-mail


Lol, Arse!

That's what I actually meant of course!


  #5  
Old January 15th 07, 08:51 AM posted to uk.rec.cars.maintenance,uk.rec.cars.misc,uk.d-i-y
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Default Testing H4 car bulb with multimeter

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


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
--
Andy
  #6  
Old January 15th 07, 09:04 AM posted to uk.rec.cars.maintenance,uk.rec.cars.misc,uk.d-i-y
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Default Testing H4 car bulb with multimeter


"Stan" wrote in message
...
Using a multimeter, what resistances would I expect to get from a
working H4 car headlight bulb?

Thank you for any info.


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

mrcheerful


  #7  
Old January 15th 07, 10:20 AM posted to uk.rec.cars.maintenance,uk.rec.cars.misc,uk.d-i-y
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Default Testing H4 car bulb with multimeter

In article ,
Stan wrote:
Using a multimeter, what resistances would I expect to get from a
working H4 car headlight bulb?


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

--
*I have plenty of talent and vision. I just don't care.

Dave Plowman London SW
To e-mail, change noise into sound.
  #8  
Old January 15th 07, 10:45 AM posted to uk.rec.cars.maintenance,uk.rec.cars.misc,uk.d-i-y
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Default Testing H4 car bulb with multimeter

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


Thank you for any info.


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.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk


  #9  
Old January 15th 07, 12:18 PM posted to uk.rec.cars.maintenance,uk.rec.cars.misc,uk.d-i-y
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Default Testing H4 car bulb with multimeter


Stan wrote:

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


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.

  #10  
Old January 15th 07, 10:01 PM posted to uk.rec.cars.maintenance,uk.rec.cars.misc,uk.d-i-y
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Default Testing H4 car bulb with multimeter

"Stan" wrote in message
...
Using a multimeter, what resistances would I expect to get from a
working H4 car headlight bulb?


See responses below but why would you want to?


 




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