This is a DC Input DC Output SSR Solid State Relay. It was made for Printing or Photo Processing Equipment. It has an optical isolation and can be used for inductive loads.
DC Load DC Control SSR
The R1 Limits current and Z1 and D1 protects. The 2N3904 limits current in the Opto-Led. When the LED in CNY17-3 turns on, the Light passes thru a electrically insulating area and falls on the photo-transistor in the opto-coupler. This transistor conducts biasing Q2 PNP power transistor.
Then Q2 drives Q3 to saturation, turning it on. Q3 is a High-Voltage and High-Power rugged device. This can derive a solenoid, DC motor or Lamp.