Increased voltage increases or decreases current?

  1. I am a little confused on how an increase in voltage would increase current.

    If i have a 1000w fixture with 120vac supplied the current will be 8.333amps
    After increasing the voltage to 240vac, the same 1000w fixture would draw 4.16 amps.

    Please help me understand what I am missing here.
  2. jcsd
  3. Pengwuino

    Pengwuino 6,942
    Gold Member

    Well, tell us what you think should happen.
  4. You seem to have a poor understanding of the relationship among impedance, voltage, current, and power. Try looking up the definitions.
  5. Exactly what happens in my example. But I have read elsewhere that the current is directly proportional to the increase/ decrease of voltage.
  6. read more
  7. Obviously that is what I am doing. A little direction would be helpful.
  8. vk6kro

    vk6kro 4,058
    Science Advisor

    The lamp will not be the same lamp on 240 volts as you used on 120 volts.

    So, if your lamp was 14.4 ohms, the current in it would be 120 volts / 14.4 ohms = 8.33 amps and the power would be 120 volts * 8.3333 amps or 1000 watts, as you said.

    But if you now put a different lamp of 57.69 ohms in, and apply 240 volts, the current will be 4.16 amps.
    So, the power will be 240 volts * 4.16 amps or 1000 watts.
  9. Thanks! I forgot about the fact the bulb must be replaced... although the ballast remains the same.
  10. But now that I realize the bulb doesn't need to be replaced and can run on 120 or 240v.
  11. vk6kro

    vk6kro 4,058
    Science Advisor

    You didn't say anything about a ballast.

    If it was an electronic ballast, you can't really apply Ohm's Law to it. It would just give the lamp the current it requires even if the input voltage changed.
  12. Is that the same for a magnetic ballast?
  13. vk6kro

    vk6kro 4,058
    Science Advisor

    No, I don't think so.

    If you tried to use an iron-cored inductor intended for 120 volts, on 240 volts, I expect it would destroy the lamp and possibly itself with the extra current.

    It definitely would not supply less current to the lamp at the higher voltage.
  14. It is a magnetic ballast that can be used with either 120v or 240v. I don't believe the bulb needs to be changed with a change in supply voltage.

    Maybe the best way to figure it out would to just wire it on 120v, check the amp draw.. then change the supply to 240v and check amp draw because I feel like I'm only confusing myself more.

    Thanks for your help tho.

  15. You are exactly correct if the load is constant power. Most modern ballasts will do exactly what you describe.
  16. Some ballasts you have to open and change a wire(I assume it is for the number of turns that are energized?) to 240(208?) from 120...
  17. NascentOxygen

    Staff: Mentor

    In which case I expect you will find that you need to connect it up differently for the different potentials.

    That is sure to give you some answers. Stand well back from it when you apply the 240v.
  18. sophiecentaur

    sophiecentaur 14,711
    Science Advisor
    Gold Member

    Are you talking about Resistors or Transformers?
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?