Solid state relay (SSR) is a special type of control device that switches electric circuits by use of semiconductor elements without moving parts or conventional contacts. The greatest feature of a solid state relay is that it does not use switching contacts that will physically wear out. That’s why its working principle is different from an electromechanical relay.
Solid state relays are typically comprised of an optoisolator input such as photocoupler or phototriac. The optoisolator activates a solid-state switching device such as a triac, transistor MOSFET, or thyristor.
Although these are the fastest switching elements when actuation times are compared, the release, or turnoff time, is long. AC control is a normal application for triacs and thyristors because the turnoff time is decreased when the device is switched off during a zero-crossing. Also, their isolation is limited by the leakage currents of the semiconductor devices, and they have a high insertion loss for low-level signals. DC control is a normal application for transistors and MOSFETs.
How does a solid state relay work?
Solid-state relays are similar to electromechanical relays, in that both use a control circuit and a separate circuit for switching the load. But their operating principle is different.
The working principle of a solid relay can be described as follows:
- When voltage is applied to the input of the solid-state relay, the relay is energized by a photocoupler or another electronic device (Diode, LED, resistor, and transistor). The photocoupler changes electric signals into optical signals and relays the signals through space, thus fully isolating the input and output sections while relaying the signals at high speed.
- An electric signal is transferred to the trigger circuit in the output circuits.
- The switching element in the output circuit turns ON.
- When the switching element turns ON, load current flows, and the device connected to the output can be controlled.
- Removal of the input voltage disables the control circuit and the solid-state switch is turned off.
Due to the way of switching, solid state relays can be divided into two basic groups:
- relays switching at zero-crossing,
- random-on relays.
The relay is switched on when the voltage goes through zero, and it switches off when the current reaches zero. This method of switching allows limiting surge currents generated during switching operations. The relays are recommended for applications controlling resistive, capacitive, or slightly inductive loads.
Random-on (instant-on) relays
The relay is activated immediately after the control signal appears (control voltage is applied). In this case, we have a faster turn-on time than when using zero-crossing switching. This type of switching is used for inductive loads in applications where a fast response time is required.
To fully understand the working principle of a solid-state relay, you have to know its technical parameters.
Technical definitions for the input side
Rated voltage is the voltage that serves as the standard value for an input signal voltage.
Operating voltage is the permissible voltage range within which an input signal voltage may fluctuate.
Must operate voltage is the minimum input voltage when the output status changes from OFF to ON.
Must release voltage is the maximum input voltage when the output status changes from ON to OFF.
Input current is the current that flows through the solid state relay when the rated voltage is applied.
Input impedance of the input circuit and the resistance of current-limiting resistors used. In Soldi state relays, which have a wide range of input voltages, the input impedance varies with the input voltage, and that causes the input current to change.
Technical definitions for the output side
Load voltage is the effective power supply voltage at which the load can be switched and the SSR can be continuously used when the SSR is OFF.
Maximum load current is the effective value of the maximum current that can continuously flow into the output terminals under specified cooling conditions (such as the size, materials, and thickness of the heat sink, and the ambient temperature radiating conditions).
Leakage current is the effective value of the current that flows across the output terminals when a specified load voltage is applied to the SSR with the output turned OFF.
Output ON voltage drop is the effective value of the AC voltage across the output terminals when the maximum load current flows through the SSR under specified cooling conditions (such as the size, materials, and thickness of heat sink, and the ambient temperature radiation conditions).
Minimum load current is the minimum load current at which the SSR can operate normally.