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Resistance of a lightbulb?

  1. Oct 18, 2007 #1
    [SOLVED] resistance of a lightbulb???

    hey dudes
    so i've a lightbulb that says 60w 230v
    so i figure it draws 60/230 amps
    so i figure its resistance is 60w/((60/230)^2), which gives me 882 ohms

    so i checked this light bulb with a multimeter and it reads 65 ohms!!!!!

    so i figure that at its "operating temprature" the resistance increases to around 880 ohms? is this correct

    also i've noticed that when a lightbulb blows its ususally just as you switch it on,
    is this because it has lower resistance initially which allows it to draw more current and it ends up overheating and melting??

    thanks for your help, (or should i go buy a new multimeter, this one only cost me €6!) (thats not 6 factorial, its just six):)
  2. jcsd
  3. Oct 18, 2007 #2


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    Exactly correct on both accounts! A common tungsten light bulb filament runs at 2800K, for a resistance ratio of about 15:1 according to
    This is close to the ratio you observe.

    I've never heard of such a cheap multimeter, must be used, yes? Figure its accuracy is inversely proportional to its price divided by the new price of a good one... :eek:)
  4. Oct 18, 2007 #3


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    Determining the resistance of a bulb with a meter is inaccurate. The filament has a positive temp. coefficient. This means its resistance rises with temperature. When cold you'll get a few ohms of resistance, but at operating temperature, expect it to rise dramatically. In case you're wondering the lamp at 60Hz can be considered a purely resistive load.

    Thats a cheap meter! Get one with a high sensitivity.
  5. Oct 19, 2007 #4
    thanks guys,

    the meter is new, i bought it to use with my final year project (mechanical engineering) last year. its a "catagory II".#
    i checked it against expensive ones in the electronics and control lab in college,
    and for the comparisons i did it worked pretty well, i cant remember any figures but i tested it using about 5 or 6 resistors and, and it was measuring mV accractly aswell!

    found one on the internet just now http://www.multimeterwarehouse.com/dt830bf.htm
    for8 dollars. i bought mine in a hardware and tools shop, it was the only mm in the whole store (i think they must have ordered a batch of them about 5 years ago and were just waiting to be rid of em!! it was in a type of bargain box which just had a big mix of stuff from spririt levels, allen keys, drill bits, and 1 multimeter!)
  6. Oct 19, 2007 #5


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    The DT830B is a pretty good meter. I use to use it quite extensively in the lab and still use it when I need to do measurements. You'll only need to worry about meter sensitivity when dealing with analog meters. Its important for your meter to have a high input impedance when measuring voltage so it doesnt introduce a loading effect into the circuit. Sensitivity comes into the play because the product of sensitivity and your full scale voltage setting gives you your meter's input impedance.

    But you won't need to worry about sensitivity if you have a digital meter, since these meters have high input impedances in the order of mega ohms. Generally you'll run into trouble when measuring the voltage across a component that has a resistance that is equal to or greater than your meter's input impedance. A nice ratio (component resistance: meter resistance) would be like 1:10 or even higher! You will have to pay attention to this if you want reliable voltage readouts.
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