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White Paper
Advanced Photorelays Prove ‘Less Is More’
in Component Design
By Roger Shih, Business Development Engineer, Optoelectronics Discrete Business Unit
Toshiba America Electronic Components, Inc.
Highlights
• Paper discusses how
use of photorelays
instead of mechanical
relays can help design
engineers develop
systems with lower
power consumption,
less heat and
longer life.
• Provides a comparison
of mechanical relays
versus photorelays,
with illustrated
examples of each,
and discusses the
basic functions of
photorelays.
• Various photorelay
applications are noted
and a list of questions
to ask when specifying
photorelays.
Introduction
Design engineers, in their ongoing quest
to develop systems with lower power
consumption, less heat and longer life are
increasingly turning to photorelays over
mechanical and other types of relays.
With the proliferation of photorelays in an
increasing number of devices, especially
test and measurement equipment, even
small improvements can have a ignificant
impact on the performance of the end
product.
Many electrical applications require the
use of relays to control the application
of a high voltage (or load) by using an
electrical signal. This high voltage must
be isolated from the control electronics
section where the electrical circuits
might be more vulnerable to the voltage.
Isolation between circuits also protects
each one from being disturbed by the
mutually generated noise. Today relays
are found in the vast majority of
applications including motor control,
heating control, signal coupling and
interfacing, just to name a few.
The invention of relays came from the
quest for:
• the ability to turn a device on or off
• translation from a lower voltage signal
level to a higher voltage level, (or vice
versa)
• isolation of the control circuit section from
the much different drive section where
electrical potentials must not be the same
as the grounding used in the control circuit
The general idea is to utilize a weaker
electrical signal to control a much larger
electrical current in the device being
controlled. As electronic components go,
the mechanical relay is one of the oldest.
Solid-state relays, or photorelays, are a
relatively recent development, although
they have been in widespread use for
many years. The major characteristics
and benefits of each are summarized below,
and a comparison of their basic operation is
shown in Fig. 1.
Figure 1.
The two most widely used types of relays used to separate circuits with different voltage levels in electronic devices are
mechanical and photorelays. A mechanical relay is a switch controlled by an electro-magnet, while a photorelay uses an
optical signal from an LED to transmit a signal to a photo-detector/MOSFET.
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Mechanical Relays
An electro-mechanical relay consists of an electromagnet where the weaker control current flows through
its copper winding, and a set of contacts which are
mechanically manipulated by the energizing and
de-energizing of the electro-magnet to move them
to the on or off configurations.
photodiode array which generates a gate voltage signal
causing the output MOSFET to turn on (or off). See Fig.
2, Photorelay Function, which shows a diagram for a
normally-closed type of photorelay. In this case, when
the LED triggers the MOSFET, the switch opens (turning
“off ” the photorelay).
Figure 2. Photorelay Basic Function
This type of technology has its advantages and
disadvantages:
• High ruggedness and durability within its expected
lifetime
• When open, the set of contacts offers very high “off”
resistance and very low capacitance
• When closed, the set of contact offers reasonably
low on-state resistance
• Performance consistency is low due to the mechanical
wear and aging of the contacts, as well as the
contamination level of the surrounding environment
• Slow response
Photorelays
With the advent of light-emitting diodes, phototransistors and MOSFETs, a more advanced type of
relay evolved: photorelays or solid-state relays. In a
photorelay the control signal drives a light-emitting
diode. The emitted light stimulates a photo-transistor
or MOSFET, which in turn switches itself on or off. With
this configuration, most of the performance parameters
of the relay are now determined by the performance of
the light-emitting diode and the MOSFET device.
Photorelays hold several significant benefits over
conventional mechanical relays:
• High speed switching (typically one-fifth the speed
of mechanical relays, or 80 percent faster)
• Lower drive power consumption (typically two orders
of magnitudes smaller than mechanical relay)
• Longer life, which comes from not having any
mechanical components that might be worn out
with use. Reliability of photorelays surpasses
that of mechanical relays by design.
• No chattering noise since switching is controlled
optically
• Smaller devices require less board space
A typical photorelays consist of four chips. As an
example, in a Toshiba photo relay the input side
consists of a gallium arsenide (GaAs) infrared light
emitting diode (LED). The output side includes a light
receiving photodiode array and two output MOS field
effect transistors (MOSFETs). When the LED emits
light, the photons energize the optically-coupled
In the normally closed type of photorelay, the signal from the
LED turns off the photorelay. Photorelays are available in
normally-open or normally-closed configurations.
Photorelay Applications
Because of their advanced performance characteristics,
photorelays are ideal for a wide range of products.
These include, but are not limited to:
Analog modems
Measuring instruments
Digital line cards
Programmable controllers
Gate drives for thyristor Public phone line cards
Industrial robots
Relay output I/O boards
Logic testers
STBs
Memory testers
Various actuator drivers
Below are several examples of how design engineers
build photorelays into their product designs:
• In an induction-heating cooker, photorelays control
the electrical signals between the control circuit and
several subsystems within the product, such as the
fan motor, heater and coil.
• In an inverter air conditioner, photorelays control
electrical flow between the indoor and outdoor unit
controllers, the fan motors, control air-flow motor and
compressor motor.
• In a refrigerator, photorelays manage electrical flow
between the control circuit and multiple devices,
including the fan motor, defroster/heater, damper
motor, interior lamp and compressor motor.
• In a washing machine, photorelays control electrical
flow to and from the drain valve motor, water supply
valve, softener supply valve, and multiple other
devices within the unit.
• In a more complex system, such as an automatic
meter reading system, photorelays control electrical
flow between the telephone lines and the modem
and controller that process information from the
water, gas and electricity meters.
Photorelays play an especially important role in
automated test and measurement equipment, where
isolation requirements, along with low on-resistance,
are design parameters. Furthermore, because of their
size advantage (i.e., SSOP packages at 4.2mm by
1.9mm), they are ideal for such board-space intensive
applications. Recent developments in photorelays
have focused on miniaturization, both of the
photodetector and by integration of the MOSFET and
photodetector into a single chip. Another trend is low
CxR values for test equipment, with values for new
devices approaching 1pFW.
Questions to Ask When Choosing Photorelays
Photorelays are not all alike! It is important to ask
specific questions when selecting photorelays for
specific applications. Several key questions can
be found below:
• What technology did the manufacturer use to produce
the photorelay? For example, white mold technology
typically results in a higher level of optical coupling
efficiency than other technologies.
• How extensive is the manufacturer’s line of
photorelays? A broad line means the engineer
can likely spec all the photorelays he/she needs
for different applications from one manufacturer.
• What is the manufacturer’s experience in designing
and manufacturing photorelays? An experienced
manufacturer can often design photorelays to be more
cost efficient and reliable than competitive products.
• Have the photorelays received approval from relevant
national and international safety standards?
Standards bodies most involved with providing
approvals are UL, TUV, BSI and SEMKO.
Toshiba Photorelay Lineup for General Applications
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TAEC Regional
Sales Offices
NORTHWEST
San Jose, CA
TEL: (408) 526-2400
FAX: (408) 526-2410
Photorelays are an excellent example of
where the “less is more” adage rings true.
These innovative devices are an omnipresent
Portland, OR
TEL: (503) 784-8879
FAX: (503) 466-9729
Toshiba Photorelay Lineup for ATE (Tester) Applications
component of many everyday products.
Maximizing the performance of these
devices results in a superior end product.
SOUTHWEST
Irvine, CA
TEL: (949) 623-2900
FAX: (949) 474-1330
Richardson, TX
TEL: (972) 480-0470
FAX: (972) 235-4114
CENTRAL
Buffalo Grove, IL
TEL: (847) 484-2400
FAX: (847) 541-7287
NORTHEAST
Marlboro, MA
TEL: (508) 481-0034
FAX: (508) 481-8828
Parsippany, NJ
TEL: (973) 541-4715
FAX: (973) 541-4716
SOUTHEAST
Duluth, GA
TEL: (770) 931-3363
FAX: (770) 931-7602
The information contained herein is subject to change without notice.
• The Toshiba products listed in this document are intended for usage in general electronics applications (computer, personal
equipment, office equipment, measuring equipment, industrial robotics, domestic appliances, etc.). These Toshiba products are
neither intended nor warranted for usage in equipment that requires extraordinarily high quality and/or reliability or a malfunction or
failure of which may cause loss of human life or bodily injury (“Unintended Usage”). Unintended Usages include atomic energy
control instruments, airplane or spaceship instruments, transportation instruments, traffic signal instruments, combustion control
instruments, medical instruments, all types of safety devices, etc. Unintended usage of Toshiba products listed in this document
shall be made at the customer’s own risk.
• The products described in this document may include products subject to foreign exchange and foreign trade laws.
• The products contained herein may also be controlled under the U.S. Export Administration Regulations and/or subject to the
approval of the U.S. Department of Commerce or U.S. Department of State prior to export. Any export or re-export, directly or
indirectly in contravention of any of the applicable export laws and regulations, is hereby prohibited.
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