High 3rd harmonic on the neutral

Well LEDs below 25watts per fitting can have a power factor as low as 50%, (40% below 5 watts)  (above 25 watts it is a more sensible 90%), and laptop power supplies have a similar stepped rating, but the step is at 75 watts input power, above which the current waveform has to be properly corrected.

I'd be surprised if it was that bad just from a few laptop supplies, but if you have LED lights as well. 

Very roughly you can relate PFC to harmonic content as

1/(1+(THD)^2)


Where THD is the total harmonic distortion,  THD= sqrt(  I 2F^2 + I3F^2 ..)  /I fundamental.


So if you had basically a bridge rectifier feeding a capacitor gives the 'bursty' current waveforms

some images to help, I hope.  

Supply voltage black, current green, heavy black the 'DC' on the smoothing capacitor.



On the AC side this becomes more like the following - Voltage is red, current is blue ; some waveforms are sine, this one really isn't.




  regards Mike

Well LEDs below 25watts per fitting can have a power factor as low as 50%, (40% below 5 watts)  (above 25 watts it is a more sensible 90%), and laptop power supplies have a similar stepped rating, but the step is at 75 watts input power, above which the current waveform has to be properly corrected.

I'd be surprised if it was that bad just from a few laptop supplies, but if you have LED lights as well. 

Very roughly you can relate PFC to harmonic content as

1/(1+(THD)^2)


Where THD is the total harmonic distortion,  THD= sqrt(  I 2F^2 + I3F^2 ..)  /I fundamental.


So if you had basically a bridge rectifier feeding a capacitor gives the 'bursty' current waveforms

some images to help, I hope.  

Supply voltage black, current green, heavy black the 'DC' on the smoothing capacitor.



On the AC side this becomes more like the following - Voltage is red, current is blue ; some waveforms are sine, this one really isn't.




  regards Mike