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Re: Determine Resistance of a Halogen Light

Discussion in 'Electrical Engineering' started by Jeff Strickland, Jun 5, 2008.

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  1. Why do you even bother reinventing the wheel, or the light bulb? Use a set
    of aligator clips and connect the lamps yo have to a car battery, if they
    light they will work, if they don't light they won't work.

    Any 120V light will not work in a 12V circuit. I recently had a home owner
    that installed a standard light bulb in a low-voltage system, and called me
    because the light did not come on. Among various breaks in the wires, I
    found the offending bulb. It turns out they (whoever "they" is) make a 12V
    bulb for RV applications that use a standard base. Since the bulb she had
    fit, she put it in.
     
  2. James Sweet

    James Sweet Guest

    It's more complex than this. A cold filament has a MUCH lower resistance
    than a hot filament, so if you measure the lamp with an ohm meter the
    results will be nowhere close to actual behavior.

    Look at the bulb type in your halogen lights, often it's a 55W H7
    halogen lamp, but others are available. If you want to know the exact
    draw, wire one up to 12V and measure the draw with the DC Amps range on
    your multimeter.
     

  3. It's more fundemental than that. The OP stated he has some halogen lamps,
    and INFERS they are household type lamps, but he wants to install them in an
    automotive application.

    Beyond the inability to accurately evaluate the resistance or current draw
    of a cold filiment, one is never able to light a household filiment (120V)
    with an automotive (12V) power supply.
     
  4. MY APOLOGIES.
    I went back to re-read the original post, and there is no such inference.
     
  5. James Sweet

    James Sweet Guest


    You didn't read the original post very well, he said they were offroad
    type lamps, those have been halogen for decades. Headlamps, fog lamps,
    driving lamps, they're all halogen, I didn't see anything suggesting in
    any way that they are household mains voltage lamps. They're very likely
    either 55 or 100W H7 or H1 12V halogen bulbs.
     

  6. I agree. Odds favor the 55W variety, but the choices are 55 or 100. I think
    they will be H1 or H4, but it doesn't really matter. The housing will define
    the lamp type ...
     
  7. Roy

    Roy Guest

    From: (James Sweet)
    Newbie Disclaimer: I am not an electrical engineer, but I am very
    interested in the field, so forgive the naive question.
    I have a set of halogen lights, like off-road lights for a car. I know
    nothing about these lights. I would like to figure out the amperage
    they'll put off when connected to a 12V DC electrical system.
    So I was thinking all I need is to determine the resistance and then I
    could do the I = V/R formula and poof there I go.
    I don't have the lights at home with me, but for a quick proof of
    concept I grabbed a household light bulb, 120V 60W, regular old bulb. So
    I figured out the amperage for that with I = P/V (60W / 120V = 0.5
    amps). So to double check those numbers I grabbed a DMM, switched the
    range to selector to the OHM symbol, and touched the leads to the
    contacts on the light bulb, it finally gets a steady reading of 17.5
    OHM. My calculation says that for the above formula to be correct you
    would need to have a resistance of 240 OHM.
    Is there something that I'm not aware of in all of this? Is a digital
    multimeter not the right tool for this? What is the best way to
    determine the amperage for these lights? I want to choose the correct
    switches, relays and wire gauge for all this.
    Thanks for any advice in advance.
    Nate

    It's more complex than this. A cold filament has a MUCH lower resistance
    than a hot filament, so if you measure the lamp with an ohm meter the
    results will be nowhere close to actual behavior.

    I Don't Think So...Resistance drops as heat increases, hence R rises as
    heat decreases/=>

    Look at the bulb type in your halogen lights, often it's a 55W H7
    halogen lamp, but others are available. If you want to know the exact
    draw, wire one up to 12V and measure the draw with the DC Amps range on
    your multimeter.
    <=\

    Roy Q.T. ~ US/NCU ~ E.E. Technician
    [have tools, will travel]
     
  8. Guest

    | Beyond the inability to accurately evaluate the resistance or current draw
    | of a cold filiment, one is never able to light a household filiment (120V)
    | with an automotive (12V) power supply.

    Never is a long long time. OTOH, if you reverse the situation, it will light
    for a very very short time.
     
  9. Palindrome

    Palindrome Guest

    In this case, "never" is < 7 minutes. Which would be about the time it
    would take to take an inverter to the car, together with a household
    filament table lamp. ;)
     

  10. You sure make it easy for us to "draw" a picture of your capacity and
    aptitude for electrical engineering.

    Ever heard of a precision shunt, dipshit?
     
  11. Roy

    Roy Guest

    If you see responses with long strange email addresses & scary inflamed
    names Don't listen to them ....

    It's just some escaped Monkey, he may sound smart but he is 57 years
    old, neurotic and is only mimcking his human masters.

    Oh' We believe he's a racists too.
     
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