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fluorescence light Watt meter

Discussion in 'LEDs and Optoelectronics' started by Wsx, Feb 15, 2016.

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  1. Wsx


    Feb 15, 2016
    I put my multimeter on the fuse board to see how every of my electronic devices are doing and surprisingly my fluorescence lights measurements were way off chart. For example my 18w osram tube was showing a 70w i know my multimeter is not the best and it's rms measurements are not accurate but why would it have so high divergence? And could this divergence be on my power company's meter also?
    Last edited: Feb 15, 2016
  2. davenn

    davenn Moderator

    Sep 5, 2009
    and exactly how were you connecting the meter and in what measuring mode ??
  3. Wsx


    Feb 15, 2016
    It's online ofc on 10A ac mode. I am not doing anything wrong. What i'd like to know if someone has measured the current on a fluorescence tube himself has the same diversions as i.And if someone can upload a polygraph's current graph of a fluorescence tube maybe would explain the diversions
  4. Bluejets


    Oct 5, 2014
    18W is the light output.
    70W is the circuit consumption i.e. losses in the ballast + power to light the tube.etc.
    davenn likes this.
  5. Wsx


    Feb 15, 2016
    I'm pretty sure when they write 18w it's the consumption not the light output ;).And ballast shouldn't burn more than 4w.Plus i can tell it's 20 and not 70w just by the heat it emits.
    Last edited: Feb 15, 2016
  6. Tha fios agaibh

    Tha fios agaibh

    Aug 11, 2014
    True, but 18w is the consumption of just the lamp. As Bluejets pointed out, there is other loads as well.
    Your wattmeter is impacted by things like circuit impedance and harmonics.
    Assuming 220v, an ammeter would probably read about .3A total instead of .1A (just18w)
    Bluejets likes this.
  7. duke37


    Jan 9, 2011
    An ordinary fluorescent lamp limits the current with an inductor. This gives a very poor power factor so the current is higher than would be needed if the power factor was one.
    Larger lights often have a capacitor in parallel with the mains to correct the power factor. I do not know if this is due to regulation.

    The meter measures power consumption, taking power factor into account.

    What are you measuring, just current?
    Wsx likes this.
  8. Wsx


    Feb 15, 2016
    Duke37 thank you.
    I did not know the term power factor but that was exactly what i meant to say.
    So if poor power factor tricks my multimeter that i consume more energy that i actually do does it also tricks the meter of my power company meaning i pay more than i should?
    P.S. Yes I only measure current..
    P.S. 2 nevermind u already answered this by saying the meter measuring consumption takes power factor into account.

    Last edited: Feb 16, 2016
  9. Tha fios agaibh

    Tha fios agaibh

    Aug 11, 2014
    Your multimeter is not being tricked.
    It's reading the Rms voltage and Rms current. This voltage and current multiplied together will give the apparent power.
    If you multiply this apparent power (va) by the power factor, you get real power (w).
    Power factor is true power divided by apparent power.
    PF is where voltage and current are out of phase with each other making it more inefficient for power companies to transfer the same power. They need larger transformers and wire to overcome poor PF.

    When you have inductive loads the current lags voltage. The resulting phase angle makes it harder for the utility to transmit and more current is necessary to deliver the same power.

    Here in the states, utility companies can charge a fee for industrial settings, where poor PF are found (like large inductive motors) but not for residential customers.

    But remember, you are not consuming any more power and are not being charged for any more. You are paying by the kwh, not just by the amperage. When an inductive load is switched off the magnetic field collapses and the back emf is returned.
    so, I'd say power factor is really more of a power utility concern than a residential one.
    duke37 likes this.
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