Tag Archives: Electromechanical Devices

What Are PhotoRelays?

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.

What are Optically Isolated Relays?

Popularly, relays are known to be electromechanical devices. However, engineers today have access to solid-state relays that operate without any electromagnetic or moving parts. Where reliability and performance is paramount, engineers prefer to use solid-state relays to their electromagnetic versions. However, solid-state relays are more expensive.

While traditional relays have several mechanical failure modes associated with moving parts, solid-state relays offer several advantages in performance and design. These include low power consumption, low leakage current, stable on-resistance, high reliability, extremely long life, small size, fast switching speeds, high vibration and shock resistance, and no switching noise from contact bounce.

Another important feature of solid-state relays is they are optically isolated. That means the relays use an LED or light emitting diode on their input side, MOSFETs or metal oxide semiconductor field effect transistors on their output side, and an array of photo sensors isolating the two.

The design and packaging affect the relay’s performance crucially. Translucent resin molds the electronic and optical components – the LED, photo array, and the MOSFETs – allowing light to pass through, while applying a dielectric barrier between the input and the output.

That means you only need to drive a switchable voltage directly to the input pin of the solid-state relay through a resistor to limit the current through the LED and control the relay. The value of the resistor has to be selected carefully, so the LED can reach its full intensity without being overdriven.

Optically isolated relays are increasingly used in sophisticated test and measurement systems. However, these systems require solid state relays to have characteristics such as low capacitance, low on-resistance, physical isolation, and high linearity. As data acquisition devices become faster and more precise, the above characteristics play an increasingly important role.

Low capacitance results in improved switching times and better isolation characteristics when switching high-frequency load signals. You need low on-resistance for reducing power dissipation when switching high currents. This also improves switching speeds improving the precision of measurement. Temperature range of the relay is an important factor when considering on-resistance values, as rising temperatures drive up the on-resistance.

To enhance precision by minimizing noise, physical isolation between the input and the output of a relay plays an important part. Expect isolation voltages as high as 5 KV AC for optically isolated relays as these offer a truly physical separation between their input and the output. Solid-state relays also offer high linearity leading to accurate measurements.

Industrial applications also benefit from using optically isolated relays, although the requirements here are different. For instance, an industrial plant using several relays, the low power consumption of optically isolated relays offers substantial savings. Where an electromagnetic relay requires 50-100 mA to actuate, a typical optically isolated relay requires only 5 mA.

Latching-type models of solid state relays have built-in protective circuits that safeguard power supplies, motors, and other industrial devices susceptible to disturbances from the output side. Such disturbances come from voltage peaks or overcurrent conditions arising from short circuits or improper use. Their reliability and small form factor saves space, while speeding up development.