Solid State Relay
Solid-state relays are primarily a class of switches based on semiconductor devices. There are many different types of switches, for example, some use DIN diodes for switching high frequency signals. However, the most commonly used type with similar properties to reed relays are generally FET-based switches. A solid-state FAT switch uses two MOSFETs (NMOSFETs and PMOSFETs) in series, with their gate poles connected together as a driver to control the relay on and off, and differs from a reed relay as follows:
- All solid-state relays have leakage currents, which are determined by the characteristics of the device itself. Therefore, the insulation characteristics of solid-state relays are not as good as those of reed relays and electromagnetic relays, and the insulation resistance and on-resistance are also non-linear, changing with the change of load current.
- There is a compromise between path resistance and static capacitance. The capacitance is higher when the path resistance is low, (some even in the nF class), which limits the bandwidth of the relay, and the channel iodine solution increases; if you try to reduce the channel capacitance, the size of the switch increases, and the channel resistance also becomes larger. The capacitance of a solid-state relay is much greater than that of a reed relay.
- The control circuit of a reed relay is completely isolated from the channel, physically speaking the coil is wrapped around the outside of the glass tube and is completely out of contact with the signaling channel; however, the drive end of a solid state relay is connected to the circuit and is not isolated.
- Solid-state relays have faster switching speeds and higher switching frequencies than reed relays due to their lack of mechanical devices, which are determined solely by the characteristics of the semiconductor that performs the switching action.
- Solid state relays have higher power ratings.
- Generally speaking, reed relays are more like perfect relays than solid state relays because they have mechanical contacts.
