There are two types of optical-coupled MOSFETs (OCMOS FET, Solid State Relay, SSR): The "make" type ("a" contact: Normally open) and "break" type ("b" contact: Normally closed). The differences between these two types by operation are explained below.

"Make" Type ("a" Contact: Normally Open)

The "make" type contains a couple of enhancement-type MOSFETs. They shut off by application of no bias, and conduct by application of forward bias.

The initial state is "OFF".

ON

Optical-Coupled MOSFET - Make Type On

OFF

Optical-Coupled MOSFET - Make Type Off

"Break" Type ("b" Contact: Normally Closed)

The "break" type contains a couple of depletion-type MOSFETs. They conduct by application of no bias, and shut off by application of reverse bias.

The initial state is "GO".

STOP

Optical-Coupled MOSFET - Break Type Stop

GO

Optical-Coupled MOSFET - Break Type Go