Optocoupler isolation necessary?

Question

The circuit diagram below shows two possible options for a circuit:

^{simulate this circuit – Schematic created using CircuitLab}

The difference between both circuits is the use of the optocoupler, the first one has a common ground between the controller circuit and the mains supply. The second one however has both supplies isolated, they don't share the same ground. Instead they are indirectly connected via an optocoupler. So the question is whether or not is it really necessary to use an optocoupler?

I'm aware that option B is a lot more safer should there be a fault, but that doesn't mean that option A doesn't function correctly, right? The reason I'm asking this is due to the amount of circuits that I've seen that isolate high voltages (such as mains) from controller circuits. I've come to the conclusion that it is just a safety precaution in the event of an accident, such as a live wire touching the controller, albeit not something that is required assuming that everything goes according to plan.

Thanks

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figures 1 and 2 redrawn for clarity. (GND symbols point to ground.)

I'm aware that option B is a lot more safer should there be a fault, but that doesn't mean that option A doesn't function correctly, right?

Option A is dangerous before a fault. The circuit ground is live every second half-cycle. That means the controller is live and the whole circuit must be insulated to protect against contact.

I've come to the conclusion that it is just a safety precaution in the event of an accident, such as a live wire touching the controller, albeit not something that is required assuming that everything goes according to plan.

Incorrect. Very incorrect. In the top-right schematic I've removed two of the diodes to show current flow on the negative mains half-cycle. The cathode of D8 will be 0.7 V below ground (because N is earthed back at the electricity company's transformer) and the circuit ground will be 0.7 V above the L wire voltage due to D5. So on a 230 V AC supply the peak voltage on your circuit ground will be -230√2 V.

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Tips on schematic drawings:

• Draw your ground symbols pointing to the ground.
• Draw your voltage sources with positive on top.
• Draw current flow top to bottom.

Answer 2

It is also a precaution in case your load shorts and due to that, the mosfet blows, and due to that, the 220V appears at the "Controller 5V" and will likely blow the "Controller 5V" and might kill someone who thought 5V would be safe. So, better not say it is just a precaution, it's a serious precaution, for you, for someone else and for your electronics.

But you're right. The schematic will function without optocoupler, but with optocoupler it will a serious protection in case of a fault.