Measuring the output current of a ballast

[Panovski]

Hello everybody

I have a Tektronix TDS 210 oscilloscope and I want to use it for measuring the output current of an electronic ballast. I don't have a current probe, actually the only method available to me is by using a shunt. Now, I don't really have a shunt either so I tried using 3.5 m long, 0.25 mm diameter,copper wire ( if I calculated correctly it should have about 1.4 ohms). I connected my shunt (the wire) in series with the neutral line and with the luminaire (the ballast). A wall outlet (231 V line to ground voltage) is my power source. I used two 10x probes for both channels and the math function Ch1-Ch2. The signal a got was: a peak, a few milliseconds zero then a peak but with an opposite polarity. Now this seems logical, but I want to see how far from a sinusoidal form is the current signal and this is far from a clean signal. It should look like a slightly misshaped sine wave. So my questions are:
- Can I use this wire as a shunt or do I have to find longer one (considering the signal I'm getting seems to be that way because the voltage difference is too low)?
- Am I doing something else wrong in the measuring?
- Is it even possible to measure the current with this method (and with this oscilloscope)?

Any help is appreciated, thank you in advance, and of course if something is unclear about my problem I will try to explain additionally.

 

[FvM]

If I understand your setup right, you are measuring the input rather than output current of an "electronical ballast" (a CFL inverter). The output current has to measured at the fluorescent lamp.

For typical input current waveforms of CFL inverters, you can review this previous thread: https://www.edaboard.com/threads/140030/

Your measurement setup is involving a problem of shunt inductance and also limited common mode suppression of the differential voltage measurement, but should give a rough estimation of the real waveform.

 

[Panovski]

https://obrazki.elektroda.pl/57_1229876723.gif
I saw this picture and I believe that the current wave form on this picture (green line), is the closest to what I'm getting. But I can't see it this clearly. Basically I only see the peaks. What could I do to see the signal better? More ohms on the shunt maybe? Oh, and I only care for the shape not the values.
P.S. I actually want to see what is the ballast returning in the power grid so I think I' measuring on the right place. I could be wrong tough...

 

[FvM]

You should use regular low ohmic resistors or resistance wire, but not above about 5 ohm, otherwise you'll possibly reduce the peak current.

The shown waveforms are acquired with a current probe, actually a low sensitive, MHz bandwidth high current probe. It uses a "high resolution" feature of the oscilloscope to get a clean image anyway.

Regarding terminology, I don't see a clear preference if we should say harmonic currents are "consumed" from or "returned" to the grid. In both cases, they are increasing THD and causing a below unity distortion power factor. And of course, the discussion is about input current of the inverter.