[SOLVED] Measuring light bulb (blown or not blown)

Discussion in 'Electronics' started by DRBC43AMG, Mar 13, 2013.

  1. DRBC43AMG

    DRBC43AMG Hardcore MB Enthusiast

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    Please bear with me as electricity and even more so electronics are not my favourite cup of tea...

    I have two bulbs in my w202 c43 dash which are not working, so probably blown. I pulled the dash unit out of the console and removed the bulbs. I have a volt meter which measures the usual stuff and putting it at the lowest ohm setting (20 ohms) it reads zero on blown bulbs. A normal working bulb reads around 10ish ohms. Is this the correct way to check a blown bulb and normal readings?
     
  2. Ted

    Ted MB Club Veteran

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    It should read infinity on a blown bulb not zero.

    That is as good a method of checking as any, but be aware that the cold resistance of a bulb is nothing like the hot resistance (as when lit).
    In theory a bulb that measures 10 ohms would be 14 watt - that would scare you :) .

    Replacing with a known good bulb is probably the easiest method of testing.
     
    Last edited: Mar 13, 2013
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  3. OP
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    DRBC43AMG

    DRBC43AMG Hardcore MB Enthusiast

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    Thanks, I'm getting a couple of new ones to replace and have one or two in reserve.
    Hmm, what is an infinity reading? No reading at all?
     
  4. Ted

    Ted MB Club Veteran

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    on an analogue meter there should be no needle movement. On a DMM if you put the wires together that will show (nominally) zero ohms - prolly shown as 0.00. Infinity is where the display doesn't change (I think it shows something like 0L on mine)
     
  5. OP
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    DRBC43AMG

    DRBC43AMG Hardcore MB Enthusiast

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    OK, understood!
    Thanks:thumb:
     
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  6. markjay

    markjay MB Club Veteran

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    As above - a faulty bulb will have its internal wire broken so upon testing resistance (Ohm) there will be no change on the display.

    But make sure you do not touch the bulb's metal bits or the probes' metal bits with your bare hands, or you may end up measuring your own body's resistance instead of the bulb's resistence....
     
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  7. Ted

    Ted MB Club Veteran

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    A good way to check a bulb -especially the larger wattage ones such as flasher or headlight is to flick the glass.
    You will see a broken filament vibrate, while a good one won't.
     
  8. OP
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    DRBC43AMG

    DRBC43AMG Hardcore MB Enthusiast

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    Exact, keep the probes away from everything except the contacts. First time testing I was getting some weird readings
     
  9. st13phil

    st13phil MB Club Veteran

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    Shouldn't be an issue with a tungsten filament bulb as the cold resistance is a matter of a few ohms vs several hundred kilo-ohms for your own body resistance.
    That would suggest that the bulb you tested was open circuit.
     

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