When I was doing my student apprenticeship, at a firm making thyristor drives, for a reason I can’t remember now the electronics ran off about 15 V d.c (of course), but the whole lot was live to line. It was common practice to have all the 'scopes unearthed, so when the scope’s GND lead was clipped onto the electronics 0 V, the body and the case of the 'scope was live. That’s when I learned to be very careful when working on live gear. And I can’t remember ever getting a shock.
This of course was a long time before the days of health and safety gone stupid.
Simultaneously the waveform is smoothed very slightly, and noise is added.
Adding a 100nF capacitor in parallel with the burden made the output noisier, no perceptible phase shift.
Here’s 47uF in parallel with the burden, much more shifty, different scale too so the amplitude is actually decreasing significantly:
The c.t burden voltage should be 200 mV rms. It actually looks to be about 50 mV. I would suggest the noise is generated inside the 'scope.
I have of course looked at the data sheet. What’s attractive about this c.t. is it operates at 2 mA, therefore the power dissipated by the multiplier resistor is low - ½ W - which can easily be handled by two ½ W resistors in series.
For the multiplier resistor, I’d normally put two equal values in series anyway for the voltage rating. Those are in series with the primary to convert the mains voltage into the current that the c.t. requires, then the burden resistor recreates a voltage for the ADC. The snag of course is that 200 mV is rather too low for an ADC with a 3.3 V reference, hence the suggestion to use an op.amp to generate a more appropriate voltage.
With the phase shift claimed to be less than 20 minutes of arc, I wouldn’t expect to be able to see that on a 'scope at that horizontal resolution.
I agree the opamp as the next stage makes good sense.
I notice you describe this as a c.t and not a v.t.
I’ve been calling it a voltage-sensing-transformer… not sure how important the exact name is. c.t makes sense because of the use of burden resistor I guess.
I’ve checked the noise situation against a reference photo I took of our scope here, happens to be at the same 20mV/div scale. Photo taken with as many things turned off as I could manage at the time. Aye, it’s mostly just scope noise.
for the sake of reference:
That’s because it is. Look at the phasor diagrams of the two side by side and you’ll see that they are almost exactly the inverse of one another. We say that a c.t is the “dual” of the v.t., and vice versa.
Or if you don’t understand those, look at the data sheet.
Voltage transformers work on voltage.
Current transformers work on current.
The clue is in the name.
The module, with multiplier resistor on the front end and burden on the back, works as a voltage transformer; the device encapsulated in blue plastic is most definitely a current transformer.
There was my expectation at a serious explanatory video shattered by the intro music.
An accent different from the typical American… almost refreshing.
I’ve seen a phasor diagram before, in relation to Reactance. To compound the resistive and inductive/capacitive properties, they can be depicted as amplitude, phase shift and reactance in a phasor diagram, if I remember correctly. Seems a very elegant way to convert reactance into phase shift, diagrammatically.
My personal scepticism is out on whether this is a ct or vt, althought I’m sure you’re correct.
