Connect with us

Resistances of different lamps

Discussion in 'General Electronics Discussion' started by electronicsLearner77, Jul 14, 2015.

Scroll to continue with content
  1. electronicsLearner77

    electronicsLearner77

    162
    1
    Jul 2, 2015
    In automotives like cars trucks we have different kinds of lamps like 75W, 21W etc. I wanted to know if their resistance is same or different. Does a 21W has higher resistance compared to 75W? please help. Can it be modeled as fixed resistance?
     
  2. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    464
    105
    Aug 27, 2013
    E = iR
    P = iE
    Therfore:
    P = E^2/R or P = R * i^2

    So..... a 120W Bulb Nominally Rated for 12Vdc
    ==>> 120W/12Vdc = 10A
    ==>> 12V = 10A * R ==> R = 1.2ohms

    A 12W Bulb Nominally Rated for 12Vdc
    ==>> 12W / 12Vdcc = 1A
    ==>> 12V = 1A * R ==> R = 12ohms
    These are laws, not suggestions :)

    Have Fun!

    Fish
     
  3. electronicsLearner77

    electronicsLearner77

    162
    1
    Jul 2, 2015
    Thank you very much for the reply. Few questions
    1. Suppose the lamp is rated at 24V but i give 12V will the lamp still glow. In case if lamp does not work for 12V what component of the lamp will stop this functionality.
    2. Suppose if i give 12V by taking more current can it still work that is by making V*I constant.
    3. Suppose I know only the wattage let us 120W and if i do calculations for different cases
    24V
    120W/24V = 5A
    24V = 5A*R => 4.8Ohms

    12V
    120W/12=10A
    R = 1.2Ohms.
    How for the same load different resistances are possible with different power supplies.

    4. I came to know that in cold conditions the resistance is different for lamps. Is it correct?

    Where am i making mistake in understanding.
     
  4. LvW

    LvW

    604
    146
    Apr 12, 2014
    Of course, it is correct. Remember that the inventor of the WIEN oscillator (Hewlett) has used a tungsten lamp as a PTC resistor for stabilizing the amplitude of the oscillator.
    Each lamp of this type has a positive temperature coefficient which means: It is a PTC type thermistor (the resistance increases with temperature).
    As a consequence, the resistance depends on the current resp. the applied voltage.
     
  5. Martaine2005

    Martaine2005

    3,507
    951
    May 12, 2015
    Yes, but dimly.
    Because V=IR.

    Martin
     
  6. electronicsLearner77

    electronicsLearner77

    162
    1
    Jul 2, 2015
    One final question is if i take a multimeter and measure the resistance across the terminals of the lamp. Will it give me the resistance of the lamp?
     
  7. Martaine2005

    Martaine2005

    3,507
    951
    May 12, 2015
    Yes, I just had to try that.

    Martin
     
  8. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    It will give the resistance at whatever voltage it is using to test it. That will not be the same as the resistance when the bulb is operated at it's rated voltage.

    Bob
     
  9. Martaine2005

    Martaine2005

    3,507
    951
    May 12, 2015
    That's a good point Bob.
    Both the bulbs I just checked wer 240v and 40w , 60w.
    Obviously both had different resistances.
    But the resistance measured was for a 240v 40w bulb.
     
  10. Minder

    Minder

    3,093
    663
    Apr 24, 2015
    But as per the previous answers, you will not be able to calculate the wattage of the lamp from this.
    M.
     
  11. cjdelphi

    cjdelphi

    1,096
    104
    Oct 26, 2011
    You'd need to switch on the bulb until hot, then quickly before it cools down measure the resistance. ..

    the resistance will change as it cools
     
  12. electronicsLearner77

    electronicsLearner77

    162
    1
    Jul 2, 2015
    so finally can I conclude that resistance is changing because of temperature , which follows the formula between temperature and resistance.
     
  13. LvW

    LvW

    604
    146
    Apr 12, 2014
    Yes - however, have you such a formula for the device under discussion?
     
  14. electronicsLearner77

    electronicsLearner77

    162
    1
    Jul 2, 2015
    I thought there is a simple common formula between resistance and temperature for all the conductors.
     
  15. LvW

    LvW

    604
    146
    Apr 12, 2014
    No - of course not. Each conductor has its own temperature coefficient (positive or negative). More than that, in most cases, in particular for lamps, there is specific (strong) non-linearity!.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-