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I simulated the circuit shown below using MultiSim and obtained some interesting results.

The simulation suggests that you can accurately reproduce the AC line voltage from the input to the output. Given that optocouplers are typically not linear, this outcome is surprising to me.

Has anyone else conducted a similar experiment or observed this on an oscilloscope? I looked at the Spice model and saw various sub. models used to make the opto model. To me, it looked like a normal diode transistor modeling. Could the result be due to the simulation model of the optocoupler used by the simulator (SPICE)?

Simulation setup

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  • \$\begingroup\$ Have you considered to measure it on the AC side with an ADC and only send the digital data over an optical or otherwise isolated interface? \$\endgroup\$ Commented Jun 13, 2024 at 7:11
  • \$\begingroup\$ Yes I did and am aware of the various solution. I am looking for the most simple straight forward implementation, and it doesn't have to be a 1% measurement 3% is good enough. I just want to know if this simulation has anything to do with reality. It looks in my simulation too good to be true. \$\endgroup\$ Commented Jun 13, 2024 at 23:00
  • \$\begingroup\$ How do you compensate for CTR to get to 3 % accuracy? \$\endgroup\$ Commented Jun 14, 2024 at 5:02
  • \$\begingroup\$ winny Commented Jun 14 at 5:02 By adjusting R3 and if need R1 \$\endgroup\$ Commented Jun 19, 2024 at 1:49

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The relevant specification is Current Transfer Ratio (CTR).

V1 is large and R3 is large, so the diode current will closely match the input voltage.

But the transistor current to diode current ratio (CTR) is not linear, nor is it calibrated. Several figures in the specification demonstrate this.

The 5V line in figure 11 appears to be fairly straight, but this is a log graph, slight deviations are worse than they appear on the graph.

enter image description here

The 5V line in figure 13 should be horizontal, the slope corresponds to non-linearity.

enter image description here

The opto model may not accurately simulate the non-linearity. Or, the non-linearity may be small enough so it is difficult to see on your plots.

Try passing a sine wave by biasing the input with DC. Then you can measure the harmonic distortion on the output.

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  • \$\begingroup\$ Correct, the log CTC curve looks straight and in the area where I use it it looks even more strait according to my simulation. My worry is that my simulation is a case of too good to be true, and that is why I raised the question if the simulation result which looks to me wonderful has anything to do with real life electronics. I hope it does, but I am not sure, Your idea to simulate it with a DC signal is a good idea, I'm gone to try that. \$\endgroup\$ Commented Jun 13, 2024 at 23:06

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