So I made a basic transformer based probe to do some investigation on my TIG welder which doesnt work and has a "low input voltage" error.
It's not perfect but should help me do some basic testing, relatively safely. [I am aware of the risks and do decharge capacitors before going anywhere near them, i have probed mains equipment with multimeters for many years, now adding osciloscope to see wave forms]

I reveived some helpful advice on this probe in a previous post: https://forum.allaboutcircuits.com/threads/measuring-mains-in-an-oscilloscope.203885/

Now I see some strange readings/behaviour which I don't understand at all. My steps and observations are below;
(I live in England btw)

-measure mAins voltage with multimeter at 245v rms
- use transformer probe to measure same mains as : +/= 14v peak, 10v rms. Multimeter and scope give similar readings
-i therefore "calibrate" my probe; Vreal = ~24 * Vmeasured , all simple enough so far...
-now i move to the TIG welder. The board in the photo, i believe is a filter for the mains in, before the power goes to rectifier, inverters etc. There are two sets of wires coming out of the board, each reading ~120v with DMM. so i guess the welder splits the voltage to use lower power/cheaper components downstream..

-First question: i would expect the red, black and yellow to be"live" "neutral" and "earth", but measuring red and black with DMM gives me 0v. Red/yellow of Black/yellow gives 120v. confused why this is the case, seems more than just a different colour convention?
-And here's the funny part: When i add the transformer probe to red/yellow the measure voltage reads;
~25v RMS with DMM and ~1.2v RMS with scope (my calibration factor *24 =~25)
as soon as i disconnect the transformer probe, the DMM reading jumps to 120v! i dont get it at all....( i get the same result even if the transformer probe is not connected to the scope)
the waveform has some kinks from the capacitors, but otherwise looks reasonable.

any insight on this would be appreciated!
i already see the filter circuit is not the problem with the TIG just using the DMM, but ideally i can understand the behaviour of this probe for next steps of the troubleshooting.

 

Guess there isnt enough info to go on here?

i did have a thought about whats going on: possibly the drop in voltage is exactly the problem that i was trying to find in the welder?!!

I still cant figure out why the dmm doesnt cause the drop in voltage when connected, but the wall-wart-transformer-oscilloscope-probe does (even when not connected to oscilloscope)?! (Not like its drawing any current or anything…)

What next? - get a 100x 600v probe to repeat tests. I was hoping to avoid having to spend on this, finding a suitable mains probe that doesnt cost a ton is a job in itself… and i wont use it much after this project..

i might also connect a load (fan/light etc) to the 120v filter board output and see if the voltage drops again..

any thought appreciated (even they are ‘give up and go home’

 

Why do you need a transformer to read mains voltage? You have a good four channel oscilloscope. Look up how to make safe differential measurements using two regular 10/1 probes (ch1 - ch2).
Before you make any assumptions about the power wiring, make some resistance measurements between the various coloured wires and the power ground input pin, with the power disconnected of course! You will then know which is a safe ground reference to use for your differential measurements.

 

Why do you need a transformer to read mains voltage? You have a good four channel oscilloscope. Look up how to make safe differential measurements using two regular 10/1 probes (ch1 - ch2).
Before you make any assumptions about the power wiring, make some resistance measurements between the various coloured wires and the power ground input pin, with the power disconnected of course! You will then know which is a safe ground reference to use for your differential measurements.

i read about mains s ope measurements and there were lots of warnings so I went with the transformer approach, will read up on diff readings as you suggest, I get the concept will make sure I'm confident before I go ahead and maybe get a pair of 100x probes too..

I did some further tests and when hooking up any load, the. Voltage drops. So either I found the problem with the welder or its a feature (eg for current limiting), looking for a cir uit diagram to. Confirm...

resistance test, of course!

Thanks for the tips!

 

Either way, it shows the limitations of the transformer probe, at least in this case…