Small DC relay refers to a small electronic control device that uses DC current to supply power together. It has a control system (also known as an input loop) and a controlled system (also known as an output loop), and is usually used in automatic control circuits. It is equivalent to "automatic switch", which plays the role of automatic adjustment, safety protection, and conversion circuit in the circuit.

Small DC relays are widely used in automatic control circuits because of their small pickup current and small size. Common domestic small relay HG series are HG4088, HG4098, HG4099, HG4100, and their parameters are shown in Table 1.
In Table 1, the dielectric strength of HG4088 and 4098 is: AC 500V/1min between the contacts will not break down (that is, the contacts can be connected to 220V AC).
The DC resistance of each relay coil is 25Ω for a rated voltage of 3V, 100Ω for 6V, 220Ω for 9V, 400Ω for 12V, and 1600Ω for 24V.
Similar to the above domestic relay HG4100 (the same size), there are HRS1-S type produced by HKE company, JRC-21F type produced by XING HUO company, etc.