Let's say you have a device which is fed by AC line voltage and features an isolated SMPS. The switcher outputs 5 VDC, and that powers a PIC micro. I'm curious how is the Microchip's ZCD (Zero cross detection) module supposed to work here? In the technical brief (TB3138), they give the following diagram:

Microchip's usage example from TB3138

However that assumes the AC and the PIC share the same ground, which is not the case. Probably they assume the PC will be on the non-isolated side, or that the isolation would be a 50/60 Hz transformer. I've searched Microchip's app notes on this topic, and I couldn't find any mention of this scenario.

Any ideas about how to interface the AC to the ZCD pin, is it even feasible?

(And yes, I know those opto-couplers cost pennies).

  • \$\begingroup\$ How much current does the input pull? \$\endgroup\$ – ThreePhaseEel Sep 15 '17 at 18:19
  • \$\begingroup\$ The ZCD pin input current is 300 µA \$\endgroup\$ – anrieff Sep 15 '17 at 18:27

I'd use either a very small 50/60Hz step-down transformer (in the low single digit VA range) as a coupling transformer if space wasn't a major issue, or a Y1 capacitor in the few-nanofarad range as a coupling cap if I was tight on space. Either way, the relatively light load of the input shouldn't pose an issue, and the coupling device will act to limit the current so you won't need the resistors any longer.

  • \$\begingroup\$ Thank you for your answer! Before accepting it I'll want to simulate or test it on real HW though. Space is tight so I'll go with the Y cap, but I'm worried about the ballpark estimation you gave ("few nanofarad"). I believe it has to be more precise than that - I want to support both 110V/230V networks, and ensure max input current stays within the 50 - 300 µA range. \$\endgroup\$ – anrieff Sep 16 '17 at 13:47
  • \$\begingroup\$ @anrieff -- yeah, you'll probably have to figure out the right value (I'm not sure what all your parameters are) \$\endgroup\$ – ThreePhaseEel Sep 16 '17 at 14:38

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