Low-energy lamp power consumption and current waveform

Intrigued as to the power consumption of some low power lamps (compact fluorescents, CFL), particularly as they seemed to give more than their rated power in heat, I measured a couple of cycles of voltage and current with an oscilloscope.

Some plots of the time-waveforms and (low) harmonic spectrum were made, and a calculation of true power consumption was made: both were very close to the rated power. My hand-based heat detection seems to have been wrong.

In the following, the time-graphs show voltage (blue) and current (red). The bar-graphs show the low harmonic spectrum, by an FFT ("fast fourier transform").

Test:  mains_L2
	P    : 7.46W
	Vrms : 232V		|V|mean : 210V		Vpk/sqrt(2): 236	 form: 1.11
	Irms : 0.052A		|I|mean : 0.030A	Ipk/sqrt(2): 0.15	 form: 1.71

pics/L2_t.png pics/L2_f.png

Test:  mains_L3
	P    : 18.7W
	Vrms : 232V		|V|mean : 210V		Vpk/sqrt(2): 240	 form: 1.105
	Irms : 0.131A		|I|mean : 0.0688A	Ipk/sqrt(2): 0.306	 form: 1.91

pics/L3_t.png pics/L3_f.png

The huge harmonic content in both is noteworthy: can the input stage be as simple as a bridge rectifier and capacitor? (Addendum, years later: of course it is, you innocent fool: they're cheap, cheap things, and in spite of their quickly growing "penetration", no one seems to care about the horrible waveform!).

The original data is here, just in case you want it! Each file is CSV data with the first column being time and the second the v or i measurement. v is through a x10 probe (i.e. multiply it by 10 to get Volts), i is the voltage across a 3.33 Ohm series resistance that carried the lamp current (i.e. divide i by 3.33 to get the current in Amps). In case it's of use to know this, the voltage measurement included the drop across the shunt.

The (matlab) script used to read and plot the data is here.

Page started: 2005-03-13
Last change: 2014-02-22