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Schematic

The circuit has 12V SMPS & 3.3V LDO supplies. Does connecting the Schottky diode SD2 to 24V supply is advisable? If so, is there any advantage over connecting to 3.3V supply?

The Zener diode ZD is added as an extra protection. Is this required just in case of a worst scenario?

Edit: Typo Error: 24v. There is only 12v & 3.3v supplies. The left side of Optocoupler is connected to AC Neutral. The 12v & 3.3v supplies are to be considered as labels. Should I connect the SD2 to 12v or 3.3v. The 3.3v is from a LDO connected to 12v.(which is not shown here)

If there is any serious mistake, can someone pls help with a better schematic?

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This circuit is not good.

The very important thing to understand is that it is using an opto-isolator to give complete electrical isolation between the mains on the left and the logic on the right. There must be no connection between the two.

So what's wrong?

enter image description here

  1. The return path is connected to earth. This should never be done. It should be connected to the 230 V AC neutral.
  2. SD2 has no termination. This is a warning that something is odd about the circuit. It might be that SD1 and SD2 are both in the one 3-pin package and that we don't need SD2. It's not clear from the schematic.
  3. The 220k resistor needs to be rated for 230 V AC which is \$ 230\sqrt{2} = 325\; V \; peak \$. A 400 V resistor would be considered wise. If you can't find one then use two series-connected 120k, 1 W resistors.
  4. The 10R resistor is doing nothing.

Does connecting the Schottky diode SD2 to 24V supply is advisable?

No. There is no 24 V supply in your schematic anyway.

If so, is there any advantage over connecting to 3.3V supply?

Never do that.

The Zener diode ZD is added as an extra protection. Is this required just in case of a worst scenario?

The Zener diode limits the voltage on the 100R + 4N35 opto-isolator to 5.1 V. It is required.


It seems to me that you are not yet qualified to do this project.

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  • \$\begingroup\$ Thanks for pointing the errors. 1. It is AC neutral. I haven't marked. 2. SD2 termination is asked. The 2 power supplies are shown for identification only. 3. Is one 220k 2 Watt resistor not sufficient? There is no 24v power supply, typed by mistake instead of 12v. \$\endgroup\$ – John Aug 28 '17 at 14:19
  • \$\begingroup\$ Good answer, this circuit can zap you. Also the Zener diode is, after all, a diode, and will conduct in forward bias and protect the opto's LED against excess reverse bias, so I don't see the use for these two schottky diodes... About 3, Transistor means that components such as resistors have a maximum voltage rating. For example if you put 320V on a 0805 resistor, even if it's a 10 megohm, it'll burn, because it is only rated for something like 80-150V, tops. \$\endgroup\$ – peufeu Aug 28 '17 at 14:36

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