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Case 6: Using a solid-state relay to reduce the size and prolong the service life of power supply units for Programmable Logic Controller (PLC)

Programmable logic controller input output module

Reducing the size of a control relay unit, semiconductor, long service life, board size reduction, countermeasure for the effects of magnetic fields

Our customer was having difficulty in reducing the size of a control relay unit.
Its design team also had to reserve board space for devices necessary to protect against radiation noise and magnetic fields. Was there any solution?
Solving a problem by replacing a mechanical relay with a semiconductor device, namely, the TLP3906 photovoltaic-output photocoupler

Product Overview


A photocoupler consists of an LED on the input side and a photovoltaic cell on the output side. Replacing a mechanical relay with a solid-state relay helped prolong the service life, reduce the size and achieve silent operation of the relay unit.


  • Semiconductor devices eliminate the need to take a relay’s maximum cycles of make and break into consideration during the design process.
  • Photocouplers, which have no mechanical contact, generate no acoustic noise.
  • This photocoupler can offer sufficient isolation voltage where industrial control equipment requires 1500V to 2500V. And it can drive variety ofvarious MOSFETs.

Problem-Solving Examples

Solution for Company D, an industrial equipment manufacturer (Employee population: 600)

Company D started to design a new power supply unit for programmable logic controller (PLC) applications. Its design targets were a reduction in chassis size and a further improvement in efficiency. The project team of Company D improved and reduced the size of each section of the unit. The project was going smoothly until the design team faced the final challenge.
The team was using a mechanical relay, which is physically large and requires sufficient space to protect against radiation noise and the effects of magnetic fields. The team proceeded by trial and error and consulted a distributor’s field application engineers (FAEs). However, despite its best efforts, it was having difficulty in finding a solution.
Reprimanded by top management and unable to find a way to break the deadlock, the members of the team were growing increasingly anxious.


Solving the problem using the TLP3906 photovoltaic-output photocoupler

The solution lay in the use of a photorelay. A sales representative of Toshiba introduced the project team to its field application engineer. He proposed replacing a mechanical relay with a solid-state relay, or more specifically, with a pair of the TLP3906 photovoltaic-output photocoupler and a MOSFET.
Mechanical relays control the make/break of an electrical signal using a metal contact whereas solid-state relays control a signal conduction passage using a MOSFET. Mechanical relays generate grinding or clicking noise and have shorter life expectancy than solid-state relays because of metal-to-metal wear and tear. While mechanical relays require maintenance every few years, solid-state relays, which have no mechanical parts, eliminate the need for regular maintenance. The team was concerned about whether solid-state relays can switch as high a current as mechanical relays. However, with the recent progress of microfabrication technology, the latest MOSFETs can handle a large current. Furthermore, MOSFETs are physically smaller. The TLP3906 photovoltaic-output photocoupler can generate electricity on its own using light from the integrated light-emitting diode, eliminating the need for an external gate power supply.
The project team assembled the TLP3906 photovoltaic-output photocoupler and a MOSFET on a printed circuit board. Consequently, the relay board exhibited satisfactory performance, meeting the design targets. The switching speed increased significantly while the chassis size became smaller than initially planned.


List of Problem-Solving Examples


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