comparison of LED T8 linear lamp with a fluorescent lamp

Hi
I want to compare two kinds of lamps, 1- a 20w LED t8 linear lamp and 2-a 40w t8 fluorescent
both work with AC power supply, I calculate the current of each, ampere meter shows 400mA for fluorescent and a clamp ampere meter shows 70mA for LED, both supply voltage be equal (voltmeter shows 230V), now, how can I write the power formulation for this two types of lamp?
Regards

The current measured this way is meaningless because these lamps are more likely electronic equipment rather than a typical incandescent lamp which acts like a pure resistive load at a steady temperature. Perhaps a portable power meter would give you a more accurate result.

Thanks for reply
cant I use power formulation here? ( P=VIcos(fi) ) ?
in other hand I want to calculate the real power consumption of a t8 linear 20W LED lamp and a 40W fluorscent lamp too, and I just have ampere meter and volt meter, Isnt possible to calculate the power consumption of them?

If the fluorescent lamp is operated with a choke ballast (400 mA suggests you do), the current is roughly sinusoidal, but with low power factor due to reactive current. Electronic lamps (LED or fluorescent) have little reactive current but some have power factor down to 0.5 due to waveform distortion. In both cases, an electronic power meter can show the consumed real power.

It's not possible to measure real power of a reactive or non-linear load with separate voltage and current meters. For reactive loads, you have to determine the phase angle, for non-sinusoidal loads, instantaneous voltage and current values must be multiplied with an analog multiplier (or a digital circuit).

The PHI factor is supposed to be with linear loads, but some of these electronic lamps have a strongly non linear behavior due to the diode rectifiers. Anyway, if you have already the information about their power comsumption, what is the purpose of measuring that ?

Power (watts) = V*I*pf.

for reactive loads, you are correct, power factor it is cos(phi).

for electronic loads, it is related to harmonic distortion: pf = 1/sqrt(1 + THD^2)

EDIT;
last but not least.....the real comparison test is how much light you obtain per unit of power. In other words, lumens per watt.

Lumens is very difficult to measure as you require to integrate light output across a spherical surface.
A simpler solution is to measure luxes. You put your fluorescent ballast at a certain height and position and measure the luxes at the work bench below. Then you replace it with a LED ballast, maintaining the same height and position, and maintaining the luxometer at the same spot in the work bench, measure again.

In this scenario, you must ensure that there are no other light sources; i.e. nearby lamps or an exterior window.

I dont have the information about their power, ofcourse on the tubes of them typed 20 and 40 watt, but I dont know are they real or no?
P=VIcos(fi) is just for linear loads? Is there any formula for non-linear loads (reactive load) ?
this is my setup:



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schmitt trigger said:

Power (watts) = V*I*pf.

for reactive loads, you are correct, power factor it is cos(phi).

for electronic loads, it is related to harmonic distortion: pf = 1/sqrt(1 + THD^2)

Click to expand...

for reactive is cos(phi) or for active??
then for LED lamps or fluorscent we can type the formula for calculation the power as : P=VI(1/sqrt(1+THD^2) ??

electronicman26 said:

for reactive is cos(phi) or for active??
then for LED lamps or fluorscent we can type the formula for calculation the power as : P=VI(1/sqrt(1+THD^2) ??

Click to expand...

reactive is the phi angle
active is the THD value.

Unfortunately, you won't be able to measure neither without a power analyzer. Which will also provide a watt reading directly, you don't need to calculate anything.

schmitt trigger said:

reactive is the phi angle
active is the THD value.

Unfortunately, you won't be able to measure neither without a power analyzer. Which will also provide a watt reading directly, you don't need to calculate anything.

Click to expand...

1-now LED lamps are reactive or active? (is reactive equal to non-linear and active equal to linear or no? )
2- then for measure the power consumption I need an electronic power meter ? (how this equip measure the power? dont use voltage and currnt? )
3- on the lamp tube the factory types 20W, is it the value of power measurment with power analyzer?

A T8 retrofit LED tube as shown in the photo will be operated with the standard choke ballast and most likely show a reactive power factor despite of built-in electronic circuits. But it's relative useless to guess about, you better measure displacement and distortion power factor.

All manufacturer specifications will refer to the consumed real power, measured with a power meter.

how this equipment measure the power? don't use voltage and current?

Click to expand...

It does use voltage and current, but also their relation. As said, by multiplying instantaneous voltage and current values. A classical electricity meter determines real power respectively energy (it's integral) by electromechanical means as explained here https://en.wikipedia.org/wiki/Electricity_meter#Electromechanical_meters

See also https://en.wikipedia.org/wiki/Power_factor

excuse me, in this type of LED tube, the ac voltage first convert to dc and then use or no?
if dont convert , then input voltage is sinusoid and the current waveform is non-sinusoid
but if first input voltage convert to dc, then input voltage of led after convert is DC and the current is DC too ?????

FvM said:

A T8 retrofit LED tube as shown in the photo will be operated with the standard choke ballast

Click to expand...

Is it really true? Do they also need a starter?

A conventional fluorescent tube is a gas discharge tube that needs a relatively high striking voltage and a constant current (limited) afterwards. As run from a AC source, the tube goes off twice every cycle.

c_mitra said:

Is it really true? Do they also need a starter?

A conventional fluorescent tube is a gas discharge tube that needs a relatively high striking voltage and a constant current (limited) afterwards. As run from a AC source, the tube goes off twice every cycle.

Click to expand...

I read in a catalog from manufacturer that this type of lamp (LED t8) has driver and dont need ballast (choke) and starter

Is it really true? Do they also need a starter?

Click to expand...

I was talking about retrofit LED tubes. There are different types to be used in existing lamps either with electromagnetic or electronic ballasts. The types designed for electromagnetic ballast may even use a special "starter" replacement device, of course not a real starter.

Some retrofit devices can be alternatively operated with direct 230 V supply. Review the product documentation.

There are other considerations to think about, besides just how much light output versus power you get.

LED lights are ultimately fed from dc which gives a continuous light.
Fluorescents pulse completely on and off at twice mains frequency.
That may not matter, but for some applications such as illuminating moving machinery, the strobing effect of fluorescents can be either annoying or dangerous.

Another effect is that LEDs usually form a very bright and sharp point source of light, which tends to produce extremely sharp high contrast shadows.
Unless you have multiple well displaced light sources, which then produce multiple overlapping shadows.
Fluorescents because of their length, (at least in one plane) produce a much more diffused light source, which is subjectively much more natural.

There are ways around all these problems, but it is something to think about if you are planning to replace existing lighting with LED lights.
The results may range from highly satisfactory, to truly awful, depending on the particular activity you are trying to illuminate.
One thing is for sure, LED light can be subjectively very different to most other source of light.

thanks for all replies
now I have some basic questions,
1-for LED lamp that fed from DC voltage, current waveforms is DC too? or it is a non-sinusoidal?
2-what is the role of electronic ballast in LED t8 lamp?