What is a photorelay?
Relays are electrically operated switches and can be broadly classified into contact (mechanical) and contactless (semiconductor) types. Photorelays are semiconductor relays consisting of an LED optically coupled to a MOSFET and are used mainly as signal relay replacements. Having no movable contacts, photorelays are known to have better long-term reliability than mechanical relays.
| Group | Sub-Group | Notes |
|---|---|---|
| Contact (Mechanical Relay) |
Signal Relay |
|
| Power Relay |
|
|
| Contactless (Semiconductor Relay) |
Photorelay (MOSFET output) |
|
| SSR (Solid State Relay) |
|
How do photorelays compare with mechanical relays?
Five main items (mounting area, reliability, power consumption, switching characteristics, and Hot Switch) of photorelays are compared with those of mechanical relays.
General
| Characteristic | Photorelays | Mechanical Relays |
|---|---|---|
| Size | Generally smaller than mechanical relays | Package size increases with higher-power products |
| Reliability | Long lifetime | Limited lifetime due to movable contacts |
| Input power consumption | Low. Battery operation possible | High. Also requires additional components in the circuit for operation |
| Switching |
|
|
| Hot switch capability | Good | Reduction in lifetime due to stresses from instantaneous current flow at contact and arcing at contact off. |
Terminology – Photorelays vs mechanical relays
In this section, terminology used for mechanical relays is mapped to that for photorelays
| Mechanical Relay Characteristic | Explanation | Photorelay Characteristic Equivalent | |
|---|---|---|---|
| Rated Coil Voltage and (Coil) Nominal Operating Current | Voltage, intended by design, applied to the coil for operation and the resulting value of current flow in the coil | Input Current (IF), Input Voltage (VF) Recommended Input Current (IF) | |
| Contact Form | Contact mechanism and the number of contacts in the circuit Eg: Normally Open × 1 Contact (1a) Normally Close × 1 Contact (1b) Change-over contacts × 1 Contact (1c) |
Contact Form Eg: Normally Open × 1 contact (1a) Normally Close × 1 contact (1b) |
|
| Contact Resistance | Total resistance when the contacts meet | ON Resistance (RON) | |
| Contact Capacity | Voltage and current that the part can handle in the ON state | OFF-State Output Terminal Voltage (VOFF), ON-State Current (ION, IONP) | |
| Maximum Allowable Contact Power | Upper limit of power within which the part can be turned on and off properly | Output power dissipation (PO) | |
| Maximum Allowable Contact Voltage | Maximum open circuit voltage Requires derating according to operation load and current |
OFF-State Output Terminal Voltage (VOFF) | |
| Maximum Allowable Contact Current | Maximum current that the contacts can handle Requires derating according to operation load and voltage |
ON-State Current (ION, IONP) | |
| Switching (Time) Characteristics |
Operation Time | Time from which power is applied to the coil until the closure of the contact. (Bounce time not included) | Turn ON Time (tON) |
| Release Time | Time from which power is removed from the coil until the return of the contact to its initial position. (Bounce time not included) | Turn OFF Time (tOFF) | |
| Lifetime | Mechanical Life | Minimum number of operation cycles the relay can undergo with no load on the contacts. | LED Lifetime Data |
| Electrical Life | Minimum number of operation cycles the relay can undergo with a specified load on the contacts. | LED Lifetime Data | |
| Operating Temperature | Ambient temperature of the environment at which the relay is operated. | Operating Temperature (Topr) | |
Photorelays (MOSFET Output)
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