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If possible, what are calculations and precautions involved in making a simple transformer without core? The only purpose of this transformer is to trigger an opto-coupler for zero-crossing detection of mains supply.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Why do you want to avoid a core? For a mains-connected transformer a core will be necessary for a practical design. \$\endgroup\$
    – John D
    May 1 '19 at 14:52
  • \$\begingroup\$ @john-d: space (Lining Box) and cost. \$\endgroup\$
    – M.Youcef
    May 1 '19 at 15:01
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    \$\begingroup\$ This transformer would be big as your house. \$\endgroup\$ May 1 '19 at 15:13
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    \$\begingroup\$ Any mains connected transformer will need to have a primary inductance of a henry or so so, if air-cored, think about how many turns are needed. \$\endgroup\$
    – Andy aka
    May 1 '19 at 15:16
  • \$\begingroup\$ @M.Youcef Does your output need to be isolated? There are other ways to get a zero cross signal. \$\endgroup\$
    – Aaron
    May 1 '19 at 15:19
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Since you said that it didn't need to be isolated, maybe something like this would work:

enter image description here

Output looks like this: enter image description here

If you are on 120Vac, then change the 2.2k to ~1.1k.
Again, beware that the left hand side is not isolated from the mains.

Source: https://circuitdigest.com/electronic-circuits/zero-crossing-detector-circuit-diagram

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The above circuit by Aaron is NOT possible! The sine wave graph is also not correct!!

Reason, simple.

current at 2.2K resistor is 230*√2/2200 =0.1478496 A
Heat produced at 2.2K resistor is 0.1478496^2 * 2200 =48 WATT !!!

The graph of zero cross is not correct either, because diode generally drops 0.75v, at positive cycle of AC zero cross will occur at 0.75v earlier and negative cycle will zero cross 0.75v later.

The time can be calculated as (0.75/((230x√2x2x3.14x50)=7.34 micro second

Since it is about zero crossing, so it does not work. And it's devastating for a novice.

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