Toshiba Applications of photorelays to FA equipment Application Notes

Applications of photorelays to FA equipment

Application Notes

Applications of photorelays to FA equipment

Overview

Compared with mechanical relays, photorelays are superior in terms of long life, low current

drive, and high-speed response of contacts, and are increasingly being installed in FA equipment.

This document introduces examples of photorelay applications in FA equipment and describes

precautions when using photorelays.

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Applications of photorelays to FA equipment

Application Notes

Table of cntents

Introduction ............................................................................................. 3

2. Output Terminals (I/O), Alarm output applications ......................................... 4

2.1 Types of output terminals ........................................................................ 5

2.2 Application example of photorelay to transistor output terminal .................... 5

2.3 Example of photorelay application to contact output (mechanical relay) ........ 8

3. Analog measurement applications ............................................................. 11

3.1 Switching between Current and Voltage Measurements (Shunt Resistor) ..... 12

3.2 Gain control ......................................................................................... 12

3.3 Switching various sensor signals and eliminating mutual effects (noise) ....... 12

4. Conclusion ................................................................................................ 15

Revision History .............................................................................................. 16

RESTRICTIONS ON PRODUCT USE ..................................................................... 17

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Application Notes

1. Introduction

FA equipment used in various production sites and processes is evolving to meet a variety

of needs, including improved facility availability and production efficiency, higher precision,

improved safety, and energy conservation.

Specifically, in order to rationalize preventative maintenance from monitoring and

analysis of operating conditions, and to respond to predictive maintenance, we are

improving functions such as strengthened coordination between instrumentation and

control and equipment, and maintenance-free.

This application note introduces the case of output terminals for equipment I/O and the

case of analog signal measurement functions as examples of photorelay applications that

contribute to the improvement of these functions.

A brief introduction to the photorelay is given in front of this manual.

Photorelay is a type of solid-state relay that uses MOSFET for its contacts (switches). It is

also known as a photo-MOS relay or a MOSFET relay.

A photorelay consists of an LED that converts an electrical signal for controlling a contact

into an optical signal, a photodiode array (hereinafter referred to as PDA) that converts an

optical signal into an electrical signal, and a MOSFET that performs a contact function.

The operation of the photorelay is as follows (Fig. 1):

(1) To drive the contacts, apply current to the LEDs. The LED emits light and an optical

signal is generated.

(2) The optical signal is converted into an electric signal by the PDA on the contact side,

and an electric signal is input to the control terminals (gates) of MOSFETs.

(3) When an electric signal is input to the gate, MOSFETs turns ON and the contact is

connected.

Since the contact part is a semiconductor element, there is no mechanical wear, and even

if it is opened and closed repeatedly, there is no deterioration of the contact.

In order to obtain the electromagnetic force to move the contact, the mechanical relay

requires that a current of about tens of mA flow through the control coil.

In contrast, photorelays can operate MOSFETs with LED currents as small as a few

milliamperes. Therefore, it has features of long life, low drive input and high-speed

response compared with conventional mechanical relays.

Semiconductor relays use triacs with similar features, but photorelay with MOSFET has

the advantage of being able to control not only alternating current signals but also direct

current signals with lower leakage currents.

The basics of photorelays and precautions for their use are described in our application

note "How to replace mechanical relays with photorelays". Please also refer to this.

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(Photorelay Application Point Image)

Application Notes

Fig. 1 Principle of photorelay operation

2. Output Terminals (I/O), Alarm output applications

In addition to the contact output units of programmable logic controllers (PLCs), general-purpose

inverters, servo amplifiers, various sensors, measuring and weighing equipment are increasingly

equipped with output terminals that control external devices and loads (Fig. 2(a), (b)).

Fig. 2(a) PLC Function Block Diagram

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Application Notes

Fig. 2(b) Servo Amplifier Function Block Diagram

(Photo Relay Application Point Image)

2.1 Type s of output terminals

Typical output pin types are shown below.

1) Transistor/MOSFET outputs (sink type, source type)... Control of DC-load, responsiveness of

1 ms or so

2) CMOS high-speed output... Equipment control signal (serial communication, analog (PWM)

signal, etc.), response of 1 μs or so

3) Tr i ac output: Control of AC (alternating current) load, response of about 10 ms

4) Contact output (mechanical relay)... Control of DC/AC load, response of about 5 to 10 ms

5) Contact output (photo relay)... Control of DC/AC load, response of about 0.5 to 5 ms

This section describes the advantages of replace 1) transistor/ MOSFET output, 4) contact

output (mechanical relay) with 5) contact output (photorelay).

2.2 Application example of photorelay to transistor output terminal

Photorelays have mainly been used to solve mechanical relay problems, but in recent years,

photorelays have also been applied to eliminate the disadvantages of transistor and MOSFET

outputs.

The transistor and MOSFET outputs are used to output signals to weak electrical equipment

ranging from 12 to 48 VDC.

There are two ways to connect the power supply and load: a sink type in which current flows

from the load to the output terminal when the output transistor is turned on, and a source type

in which current flows from the output terminal to the load (Fig. 2.2.1(a), (b)).

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and Common Board Sink Type

Fig. 2.2.1(b) Transistor output source type

Application Notes

In sink type, if the wiring connected to the equipment output terminal is grounded, load current

flows regardless of the status of the output terminal. In contrast, the source type is preferred in

European countries because it is highly safe because it is open to ground fault accidents.

In this way, the sink type and source type can be used according to the user's design philosophy.

However, since the wiring is different from each other, the assembly of the source type and sink

type must be changed according to the destination, and the contact must be changed manually

by attaching a switch on the board (Fig. 2.2.2(a) and (b)).

Fig. 2.2.1(a) Transistor output sink type

Fig. 2.2.2(a) Example of Transistor Output

Fig. 2.2.2(b) Transistor output/common board

source type example

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