Photorelays: A More Power- and Size-Efficient Alternative to Conventional Relays?

September 14, 2020 by Jake Hertz

Photorelays are gaining traction. In many cases, these devices outpace their mechanical counterparts in terms of size and power efficiency.

Relays are a fundamental electronic component that designers use for applications where they need high current switching. Many different types of relays exist, but recently, a new variation—the photorelay—has gained popularity. 


Some Background on Relays: Electromechanical vs. Semiconductor

Relays are electric switches that are used to provide high current and high power outputs in an otherwise low-power circuit. Generally, there are two types of relays available: electromechanical relays and semiconductor relays. 

A normally-open electromechanical relay relies on a current flowing through an input circuit, activating an electromagnet. This generates a magnetic field that causes an armature to move, closing the switch in a second, larger circuit and allowing current to flow through.

When the source of power is removed, a spring draws the contact away from the second circuit, stopping the flow of electricity and turning off the end device.


Electromechanical relay

Electromechanical relay. Image used courtesy of Circuit Globe


A semiconductor, or contactless relay, is a device that can be used in place of a mechanical relay to switch electricity to a load. Solid-state relays are purely electronic and do not rely on any physical moving parts or mechanical mechanisms. Of this classification of relays, one that is quickly gaining popularity is the photorelay.


What are Photorelays?

Photorelays are a subgroup of contactless relays, which, as their name implies, utilizes light as an impulse signal.


Photorelay working principle

Photorelay working principle. Image used courtesy of Toshiba


A photorelay works by emitting light when current passes through an LED. The emitted light crosses the isolation boundary to fall on the light sensor of a PDA chip that, in turn, powers and drives the gate of a common source-connected MOSFET pair. This turns the MOSFET on, allowing AC/DC current flow through the power terminals of the MOSFET. 


Benefits of a Photorelay

Compared to a traditional mechanical relay, the photorelay offers many benefits

Since photorelays operate by light, there are no mechanical parts. This means less wear and component degradation over time, giving designs higher reliability and longer lifespans.

Furthermore, mechanical relays require a large current to move the armature and drive the output. Photorelays, on the other hand, are driven by an LED on the input side, meaning the necessary input current for the device will be as low as 3 mA to 5 mA. This is a huge advantage for power savings.


Switch bounce visualized

Switch bounce visualized. Image used courtesy of Epec Engineered Technologies


On top of this, mechanical relays can suffer from contact bounce (i.e the unwanted closing and opening of the relay contact). Photorelays provide better reliability by removing contact bounce and offering faster switching speeds. 


Toshiba’s New Photorelays

Adding to its already-impressive line of photorelays, Toshiba recently announced the release of three new photorelays

These new products—the TLP3407SRA, TLP3475SRHA, and LP3412SRHA—are said to be the industry's smallest voltage-driven photorelays with an extended operating temperature rating of 125°C. 

Having a maximum operating temperature rating of 125°C, compared to the traditional 110°C, allows these photorelays to be mounted in high-temperature areas, opening them up to a new world of applications.


The new photorelays come in Toshiba's S-VSON4T package

The new photorelays come in Toshiba's S-VSON4T package. Image used courtesy of Toshiba

Further, Toshiba claims that its S-VSON4T package has the industry's smallest mounting area of 2.9 mm². The company hopes this package size will capitalize on PCB real estate and increase the number of photorelays in an existing layout.

With photorelays gaining popularity, this news from Toshiba shows healthy growth in the field. Continual development of better photorelays will hopefully give rise to more power- and space-efficient designs in the future.