The four-pin relay is similar to the five-pin relay, and there is a structural schematic diagram on the side. The two terminals are pins 85 and 86. You can see from the side structure diagram that these two terminals are used to control the electromagnetic pull-in and disconnection, so one end is usually connected to the positive pole of the power supply, and the other end is connected to the low-side drive output lead. foot.

Pins 30 and 87, of which pin 30 is the normally closed end, and pin 87 is the normally open end. Therefore, pin 30 is also connected to the positive pole of the power supply, pin 87 is connected to one side of the load, and the other side of the load is grounded.
There are many types of electromagnetic relays, which can be divided into three categories, namely voltage relays, current relays and intermediate relays. Among them, voltage relays can be divided into overvoltage relays, undervoltage relays, time relays, speed relays and so on. Current relays can be divided into overcurrent and low current relays and so on. Intermediate relays are divided into high-current intermediate relays similar to contactors, ordinary intermediate relays, and sealed intermediate relays.