We are in the beginners section and therefore, it is normal that you make mistakes.
Let us put a little order in the ideas:
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There are 2 ways to control the primary of a transformer:
- or by phase control
- or by applying the full waves and controlling the time where the voltage is applied to the transformer
You should first select the right option.
The phase control is of general use and allows to vary the effective voltage applied to the transformer from 0 to 100%
There are some conditions to respect:
- The control symmetry must be perfect so as not to create a continuous voltage that could saturate the transformer.
- Precautions must be taken to ensure that the triac or thyristors (pulse train or wide pulse) will conduct and stay conducting. (they may be turn off if current is to low and that gate signal is not applied)
The triac or scr's must have a large dV / dt to avoid re-conducting after zeroing the current and avoid loss of control
I recommend choosing either 2 scr's in anti-parallel (complicated solution) or a triac "snubberless" and provide a rc even on the "snubberless" triacs.
Do not forget that opto-triac is a weak point at dV / dt point of vew and that it needs a very efficient RC.
- the control by "train of sine waves" of variable length.
This is only used in electric heating applications where the load has a high inertia
What there is to know:
- the initial firing of the triac must not take place at the moment of the 0V passage of the voltage but at the moment of the passage of current by zero, which is between 45 and 80 degrees lagging on the passage of the voltage by 0.
This phase shift must be adjusted for the minimum inrush current during turn on.
If possible, there must be a logical order between the polarity of the turn on and turn off of the triac, the polarity of the next turn on must be the reverse polarity of the last turn off.
I see you want use this controle for a spot welder....With or without current controle ?
You can apply full voltage to transformer and only controle time or controle applied voltage and time...What did you choosed ?