In the following OptoMos datasheet the OptoMos is defined as Normally closed, when there is no input the output is closed.
But in other OptoMos like the SFH618A-2 i can't find if the OptoMos is NO or NC.

  • \$\begingroup\$ OptoMos is totaly different device compared with optocoupler. \$\endgroup\$ Dec 1, 2015 at 10:46

1 Answer 1


The SFH 618A is turned on by LED current - the symbol of the device is a phototransistor and this can only be turned on by light. A good clue is that this device talks about CTR i.e. current transfer ratio i.e. more LED current means more collector current through the phototransistor.

The OptoMos device appears to be a solid state relay but designed so that light turns the device off.

  • \$\begingroup\$ So the SFH 618A optocoupler is normally open ? \$\endgroup\$
    – Pakito
    Dec 1, 2015 at 11:40
  • \$\begingroup\$ Yes it is normally open. \$\endgroup\$
    – Andy aka
    Dec 1, 2015 at 12:18

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