Simple problem

Discussion in 'General Electronics Chat' started by dustin63933, Jul 20, 2011.

  1. dustin63933

    Thread Starter New Member

    Jul 20, 2011
    I'm using an Innova 3320 auto ranging multimeter. I know how to use ohms law pretty good now. I found this old 24v 10w lightbulb. I wanted to test my general ohms law knowledge that I've just been studying.

    So I measured the ohms of the lightbulb and got 5.7ohms.

    I divided 24v by 5.7ohms and got 4.2amps.
    Multiplied 24v by 4.2amps and got 100.8watts! So I thought no way. Measured the ohms again and got the same result. I kept thinking and decided to apply some voltage (just a 12v 5AH LA battery) just to heat it up to increase the resistance when I measure the ohms again in hopes of getting a higher ohm readout and trying to get my equation to equal 10w.

    It only read 5.8ohms. The filament did glow but not brightly at all. Do I just need to apply 24volts to the filament to get my resistance readout. I need 57.6ohms to make 10watts.

    Any ideas? Is my multimeter broken? I just bought it and I tested with some resistors and got the right numbers.I have no way of producing 24volts at the moment. Need a variable power supply!
  2. Bill B

    Active Member

    Nov 29, 2009
    1.What setting is your ohmmeter on. It sounds like you are reading your meter wrong. Instead of 5.7 ohms you are most likely getting 57 ohms.

    2. Is the power rating 10W or 100W? If its 100W, then you are at 5.7 ohms.
  3. Adjuster

    Late Member

    Dec 26, 2010
    Tungsten filament lamps undergo a very large increase in resistance (possibly over ten fold) between ambient temperature and operating temperature. The "cold" resistance is therefore much less than what you might expect from the wattage and operating voltage.

    As a result, lamps draw a significant surge of current when initially switched on. This can be a considerable nuisance in big installations, and also plays a part in the fact that many lamp failures occur at switch-on.
    Last edited: Jul 20, 2011
  4. PeeSeeBee

    Active Member

    Jun 17, 2011
    As you suspected, you are reading the bulbs resistance when it is cold. The resistance will be much lower when the bulb is cold compared to when it is at it's working temperature.

    You cannot pre-heat the bulb first, before trying to measure it, as it cools down extremely quickly.

    Your meter sounds like it is working fine.

    Edit...posted exactly the same time as Adjuster!
  5. dustin63933

    Thread Starter New Member

    Jul 20, 2011
    I see. So from ambient to 3500degrees causes a difference in resistance. I guess that's where my problem is.

    and adjuster my meter was measuring just ohms, no K's, M, plain ohms and the lightbulb is definitely 10watts.
    Alright, thanks almighty knowledgeable forum.
  6. KJ6EAD

    AAC Fanatic!

    Apr 30, 2011
    Powering a 24V bulb with 12V will not make the filament incandesce and until it does at 3k°F - 5k°F the full resistance (≈15X the cold resistance) will not be seen. It's funny when everybody posts at the same time. It's like paper/scissors/rock.
  7. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    You have sufficient data available to calculate the operating resistance of your light bulb. It is rated for 10W. It runs on 24V. We don't know the operating current nor resistance. But those can be calculated.

    P=VI or I=P/V
    I = (10/24) = .4166

    Now that we know V and I, we can use Ohm's Law to calculate R.

    V=IR OR R=V/I
    R = (24/.4166) = 57.6 Ω
  8. kubeek


    Sep 20, 2005
    You can´t measure resistance when you have voltage in the circuit, but you can measure the current and voltage and go from there.
  9. someonesdad

    Senior Member

    Jul 7, 2009
    If you have the ability to control the current through the bulb, you can measure the current versus voltage characteristics of the bulb. I've attached a graph of this relationship for a 3 volt flashlight battery. You can immediately see the difference between the cold resistance and the resistance when the filament is hot. In this case, the relationship is well approximated by two lines whose slopes are the cold and hot resistances. This isn't the case for other bulbs I've measured.