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.

Table of relay
Group Sub-Group Notes
Contact
(Mechanical Relay)		 Signal Relay
  • low current switching mechanical relays, often below 2A,
    for applications such as signal, circuit, high- frequency control etc.
  • includes-high frequency relays
Power Relay
  • high current switching mechanical relays, above 2A
  • includes general-purpose relays for control panel, high DC current control power relays etc.
Contactless
(Semiconductor Relay)		 Photorelay
(MOSFET output)
  • uses MOSFET as the output device
  • mainly used as signal relay replacement
  • able to handle both AC and DC loads
SSR
(Solid State Relay)
  • uses semiconductor photo triac, photo transistor or photo thyristor as the output device
  • photo triac, photo thyristor output devices are limited to AC loads

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

Comparison five main items between mechanical relay and photorelay
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
  • High speed
  • Low noise
  • Quiet
  • Low speed
  • Spikes at on and off with contact bounce signals
  • Audible mechanical clicking sound
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

Terminology – Photorelays vs mechanical relays
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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