Classification of relays include two main groups—contact type or electromechanical relays and contactless type or semiconductor relays. While sub-groups of the mechanical type include signal relays and power relays, those of the contactless type include the solid-state relays and photorelays.
Solid-state relays generally use semiconductor photo triacs, phototransistors, or photo thyristors as the output device, and such relays are limited to AC loads alone. On the other hand, photorelays preferably use MOSFETs as the output device that is capable of handling both AC and DC loads. Photorelays are mainly used as replacements for signal relays.
Photorelays are available mainly in two packages—the frame type in an SO6 package, and the substrate type in an S-VSON package. Both packages use a PDA chip and a MOSFET chip encased in epoxy resin for a hermetic seal.
As evinced by the name, a photorelay contains an LED to emit light when current passes through the diode. The emitted light crosses the isolation boundary to fall on the light sensor of the PDA chip, which in turn, powers and drives the gate of the MOSFET. This turns the MOSFET on, and allows AC/DC current flow through the power terminals of the MOSFET.
Compared to the electromechanical signal relays that the photorelays replace, the miniaturization of the mounting area offers a huge advantage in real-estate recovery. For instance, Toshiba is replacing large size packages such as SOP, SSOP, and USOP with miniature packages such as the VSON and S-VSON types. Replacing with photorelays contributes greatly to the miniaturization of the device.
As photorelays have no moving parts to fail, they are more reliable than the mechanical relays they replace. The basic operation of the photorelay involves LED light triggering the photodiode array, which then drives the MOSFET. Mechanical relays, on the other hand, suffer from wear and tear induced degradation. Photorelays are maintenance free, as they do not have contacts.
Since an LED drives the photorelay, the drive circuit can be relatively simple when compared with the drive circuit that a mechanical relay requires—a buffer transistor to boost the microcomputer output. The output pin of a microcomputer can easily drive a photorelay, as this is equivalent to driving an LED by the microcomputer, requiring very low currents of 3 to 5 mA maximum. Designers only need to consider the LED lifetime.
Mechanical relays suffer from chattering or bouncing—contacts connecting and disconnecting rapidly before finally settling down. In high-speed electronic devices, this chattering can cause misreading of the relay status. Moreover, every mechanical relay requires an additional diode to take care of high voltage generation from back electromotive forces. Photorelays do not suffer from chattering or back EMF forces.
Unless connected to the cold side of a circuit, mechanical relays have a shorter lifespan, as they arc when their contacts open when connected to a high voltage. On the other hand, it does not matter for the photorelay whether it connects to the hot or the cold side.
However, unlike the mechanical relay, photorelays cannot offer normally closed contacts without power being applied to the LED.