TCA785 zero crossing circuit: Should I ground the negative rail for correct operation?

The discussion centers on the correct grounding approach for the TCA785 zero crossing circuit used in a high current power supply design. The original question concerns whether the negative rail should be grounded to zero volts to achieve the expected waveforms, as the user struggles to replicate the datasheet results. Responses clarify that the IC's ground pin (pin 8) is not necessarily connected to a fixed ground in the datasheet examples and may serve as a reference output rather than a ground input. It is suggested that the circuit's isolated 18V DC supply is floating, which is generally undesirable, and grounding can be done by referencing either the AC input or the secondary DC output. Connecting the reference node to the anodes of the trigger SCRs (TH1/TH2) can establish a proper ground reference, enabling more direct control. The importance of using an isolated oscilloscope or isolated AC input for measurements is also noted. The user confirms modifying the circuit to match the datasheet and achieving partial expected behavior, with ongoing adjustments to clarify grounding and waveform issues. The discussion highlights the complexity of grounding in half-wave rectified mains-driven circuits and the need for proper referencing to ensure correct zero crossing detection and SCR triggering.
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