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I live in Egypt where the AC voltage is 220v. Yesterday i bought two fluorescent lamps (60cm and 120cm).

I toke the following photos for the what is written on those lamps:

enter image description here

Please have a look to the lamp at the top it claims 19w and 0.35A although 0.35 * 220 = 77w

Then have a look to the lamp at the bottom it claims 38w and 0.41A although 0.41 * 220 = 90.2w

My question is: are those two lamps 19w and 38w or 77w and 90.2w ?

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    \$\begingroup\$ Have you considered that those current calues are maximum ie include starting current and are not the current when running? \$\endgroup\$
    – Solar Mike
    Commented Apr 24, 2020 at 16:18
  • \$\begingroup\$ I thought since they are not a motor like loads, this starting current thing doesn't apply to them. \$\endgroup\$ Commented Apr 24, 2020 at 16:24
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    \$\begingroup\$ Possible duplicate Rated vs. actual power usage of a linear fluorescent lamp, with a good answer. \$\endgroup\$ Commented Apr 24, 2020 at 16:26
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    \$\begingroup\$ You are "conflating" computed input VARs with rated Output Watt tubes. I would return everything and get 4500K or 5000K tri-phosphor 120 cm T8 tubes...True daylight 50khr rating with 4 independant outputs that you can extend from ballast. \$\endgroup\$ Commented Apr 24, 2020 at 17:03

2 Answers 2

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Fluorescents work differently.

You are familiar with incandescent bulbs. A lot of work was done to find a filament that would be coarsely resistor-like when lit. Your country's bulbs have filaments carefully sized so they do the right thing when mains voltage is applied.

So you think "Lamps work on 220V. Got it."

As you know, LEDs are not like that. They are creatures of current, with variable voltage depending on conditions, and you need an external driver to keep them at spec current. They don't "work on 220V" and have particular voltages based on their array length and current draw etc.

Fluorescents are even more extremely so. They are arc-discharge lights. Once they "strike", they are effectively a dead short. You need a ballast to limit current - much like an LED. It's called a ballast instead of a driver, because up until recently, they were effectively a current transformer with some added tricks to help strike the arc. Now, yeah, they're basically LED drivers with some tricks added. Obviously, transformers change voltage. So the fluorescent tube is not operating on mains voltage, but on its internal voltage (whatever that may be) for the spec current.

Stick a voltmeter on it in-service and see for yourself.

This for sure, don't connect a raw tube to mains power, or you'll get all the light at once :)

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With a conventional inductive ballast the power factor is very low and results in the difference between the VA product and the real power.

Modern electronic ballasts have a much better power factor that is close to unity.

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  • \$\begingroup\$ So are the two lamps 19w and 38w or 77w and 90.2w \$\endgroup\$ Commented Apr 24, 2020 at 16:26
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    \$\begingroup\$ @MostafaMahmoudIbrahimMohame Your 90.2 should be 90.2VA not W. It is Apparent Power you are calculatind. \$\endgroup\$ Commented Apr 24, 2020 at 16:28
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    \$\begingroup\$ The lamps are not resistive loads. So they do not consume power that is V*I. They are 19W and 38W. \$\endgroup\$
    – Justme
    Commented Apr 24, 2020 at 16:35

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