Photorelays and Their Operation
A relay is an electrically operated switch that consists of a set of input terminals for one or multiple control signals and a set of operating contact terminals. Relays are classified into two groups: contact relays (electromechanical) and contactless relays (semiconductor). Within these groups are a variety of subgroups. Subgroups of contact relays include signal and power relays, while those of contactless relays include solid-state and photorelays. Solid-state relays typically utilize semiconductor photo triacs, phototransistors, or photo thyristors as the output device and are limited to AC loads alone. Adversely, photorelays use MOSFETs as the output device and are capable of handling both AC and DC loads. Photorelays are mainly used as replacements for signal relays. This blog will discuss photorelays in greater detail.
Photorelays are widely available in two main packages: the frame type in an SO6 package, and the substrate type in the S-VSON package. Both of these configurations utilize a PDA chip and MOSFET chip enclosed in epoxy resin for a hermetic seal. As its name suggests, a photorelay uses an LED to emit light as a current passes through the diode. This light crosses the isolation boundary onto the PDA chip’s sensor. In turn, the sensor powers and drives the gate of the MOSFET, turning the MOSFET on and allowing AC and DC current to flow through the power terminals of the MOSFET.
Compared to signal relays, the smaller size of photorelay mounting areas offer significant space-saving advantages. Furthermore, as photorelays have no moving parts to fail, they are more reliable than many of the mechanical relays they replace. The basic operation of a photorelay involves an LED light triggering a photodiode array that drives the MOSFET. On the other hand, mechanical relays suffer from wear and tear induced degradation. The drive circuits of photorelays are much more simple than those of mechanical relays, which require a buffer transistor to supplement the microcomputer output. Additionally, because photorelays do not have contacts, they are maintenance-free. Designers only need to consider the lifespan of the LED itself.
Mechanical relays also suffer from chattering or bouncing, a phenomenon wherein contacts connect and disconnect rapidly before ultimately settling in place. In high-speed devices, this bouncing can cause misreading of the relay status. Mechanical relays also require an additional diode to handle the high voltage generation from back electromotive forces (EMF). Photorelays do not experience bouncing or back EMF. Photorelays also have a longer lifespan than mechanical relays, unless the mechanical relay is connected to the inactive side of a circuit. This is because mechanical relays arc when their contacts open while connected to high voltages. The lifespan of a photorelay is not affected by being connected to the active or inactive side of a circuit.
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