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Note: Solved, I posted my reply/answer

I am working on this circuit, where I try to control the fire angle on a Thyristor, using a Diac:

schematic

simulate this circuit – Schematic created using CircuitLab

It was a success on lower AC voltage (20Vrms) but once I tried higher voltages, could not get the results that I wanted. I don't know if my probing is wrong (I really do not know where to plug the GND of the probe, even though my setup is isolated from Mains) or if there is something wrong with my circuit.

I believe it is bad scope probing. Here is why. First, just a look on the pcb/Heater and my probes: (These 3 green pcbs are on-the-go-changing-resistors, Chinese stuff)

Testing setup

  1. I don't give any signal (0V) on MOC3020's input/Anode. This is what I see from the oscilloscope:

0V on Diac oscilloscope

Note: I know in reality that no current flows through the heater/78Ohm resistor, since it does not get hot at all.

When I give no signal to the MOC3020, No matter where I hook up the GND probe, those three always show the full sine wave. (I use 1Mohm in series when connecting the GND probe anywhere on the AC)

  1. When I try to control the fire angle, I see this:

oscilloscope fire angle weird

Which does not make sense to me.

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I found the solution. It was bad probing. I do not know why I was using the correct probing when messing with lower signals (20Vrms) and on the 110V I changed my probing, I guess it was out of fear not to destroy my oscilloscope, even though the transformer was isolated. Here is what my probing looks like now (I just put the GND probes on the opposite side of the AC):

schematic

simulate this circuit – Schematic created using CircuitLab

So probe 2 shows me a full sine wave, since its probes are actually on the output of the transformer.

Probe 1 shows me the part of the AC that my Triac cuts off.

So channels 1 and 3 are probes 1 and 3 respectively:

scope 1 and 3

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