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I am trying to make a trailing-edge dimmer based on the FL5150 IC, that I can control with a microcontroller.

After doing some research I stumbled upon this forum post which seems to be exactly what I would need, however I do have some questions about the footnotes he left

This assumes that you use a dual optoisolator, and that the CTR is the same. To ensure they ARE equally matched, you can buy the more expensive IL300 or LOC110 - built especially for this purpose.

Is the author trying to indicate that you can/should use a dual optoisolator, or perhaps that you should not use mismatching optos, or could it be something else?

As well as I noticed in the schematic all the grounds are the same. However shouldn't on the FL5150 side the ground be of the isolated PSU, the same one that is used for powering the FL5150?

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The author is referring at matched optocouplers, these optocouplers are used for galvanically isolated analog transmitters. They are exclusively made for this purpose, many different manufacturers.

The grounds are the same because of the way the SPICE simulation software works. You always need a reference potential i.e ground. In real life, those grounds are separated, still they have some real potential near ground even if its floating. Those optos are just an example, what SPICE offers with default library, however your double matched opto will behave in similar manner.

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  • \$\begingroup\$ Ah, okay, that makes a lot of sense. Thank you. So then if I understand correctly - a proper implementation of the design that was provided would look something like this ? Also, are there any kind of special details I should take into consideration when designing the PCB ? \$\endgroup\$
    – Arthur Isnt original
    Commented Oct 10 at 18:47
  • \$\begingroup\$ @ArthurIsntoriginal Yes, but it's an overkill if you really intend to drive a dimmer. A simple optocoupler and PWM would give you better and cheaper control. \$\endgroup\$
    – Marko Buršič
    Commented Oct 10 at 19:44

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