What are Solid State Relays?
An electrical switching device called a solid-state relay (SSR) turns on or off when an external voltage (AC or DC) is applied across its control terminals. They perform the same task as an electromechanical relay but are more durable since solid-state electronics don't have any moving parts.
For DC loads, an SSR based on a single MOSFET or many MOSFETs in a paralleled array can be effective. A single MOSFET cannot block current in both directions because it has an intrinsic substrate diode that conducts in the opposite direction. Two MOSFETs are positioned back-to-back with their source pins connected for AC (bi-directional) operation. Both sides of the output are linked to their drain pins. When the relay is off, the substrate diodes are alternately reverse biased to stop current flow. When the relay is turned on, the common source always rides on the instantaneous signal level, and the photo-diode biases both gates positively with respect to the source.
Characteristics of solid relays
When compared to electromechanical devices, most of the relative characteristics of solid-state relays over electromechanical relays are shared by all solid-state devices.
- Totally noiseless operations.
- SSRs switch stronger than electromechanical relays; the switching period of a typical optically coupled SSR is based on the time required to power the LED on and off - on the order of microseconds to milliseconds.
- Enhanced lifetime, even if it is used many times, as there are fixed parts to wear and no contacts to pit or build up carbon.
- Clean, bounce-less functions.
Types of solid relays
A coaxial relay is frequently used as a TR (transmit-receive) relay in situations where radio transmitters and receivers share a single antenna to move the antenna from the reception to the transmitter. This shields the receiver from the strong transmitter signal. These relays are frequently used in transceivers, which are devices that combine a transmitter and a receiver. The relay connections are made to offer extremely high isolation between the receiver and transmitter terminals and not to reflect any radio frequency power back toward the source.
A heavy-duty relay with greater current ratings, known as a contactor, is used to switch electrical motors and lighting loads. Common contactors have continuous current ratings ranging from 10 amps to several hundred amps. With silver-containing alloys, high-current connections are created.
A latching relay, without applying power to the coil, also known as an impulse, bistable, keep, stay, or simply latch, keeps either contact position permanently. One coil uses electricity just briefly when the relay is being switched, and the contacts of the relay maintain this position even during a power loss. With a latching relay, building illumination may be controlled remotely without the hum that a continuously (AC) powered coil could create.