volume 1
number 1
relay
supplement
FeATuRIng
sponsored by
5 Things About Relays
Fundamentals of Relays
Application Ideas: Relays
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EE3124
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WelCOme
TO The RelAy SupplemenT
In this supplement the critical elements of relays, as well as their many variations and
countless applications, will be discussed. We’ll focus on their fundamentals and talk
about both operational basics and product variants. Included in this coverage, we will
explain five things you’ll want to know about relays, including addressing common
relay applications and what types of relays exist. Lastly, we will spotlight simple
solutions relays offer in current and voltage switching applications.
On the product side, we feature a wide array of industry essentials. Crydom products
covered here include panel mounts, PCB mounts, DIN rail mounts and plug-in
mounts, as well as applications and ratings for these products. Likewise, we also
discuss power relays and solid state relays, as well as time delay and sensor relays,
all manufactured by Schneider Electric. Panasonic provides vital information on their
automotive, signal and power products and TE Connectivity offers information about
its low power PCB relays, solar and general purpose relays, among others. Rounding
out our list of relay manufacturers is Omron Electronic Components, which features
a host of power PCB relays, high frequency relays, DC power relays and more,
including information on the benefits and applications of each of its products.
This supplement is indispensable for the front-line industry professional, both for
reference and purchasing opportunities, and for insight on the latest and greatest
relay product lines in the business.
COnTenTS
FeATuReS
14 5 Things you wanted to
know about relays, but
didn’t think to ask
16 Fundamentals of Relays
18 Application Ideas: Relays
SupplIeRS
04 Crydom
06 Omron Electronic
Components
08 Schneider Electric
10 Panasonic
12 TE Connectivity
hIghlIghTS
03 About Us
20 Glossary
Cover photo courtesy of TE Connectivity.
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AbOuT
mASTeR eleCTROnICS
Master Electronics is a leading electronic component
distributor, dedicated to stocking breadth of part
numbers offering customers over 350,000 part numbers
for immediate delivery.
uSeFul lInkS
Master Electronics has over 40 years of industry
experience and has 12 sales branches to support you.
Master is authorized for over 230 suppliers, including
ebm-papst, Honeywell, Omron, Panasonic, and TE
Connectivity.
Relay Master
Home Page
Master Electronics
Home Page
1-888-4-RELAYS
Master Electronics, a leading distributor of
electromechanical, interconnect and passive
components, announces the launch of RelayMaster.com
(www.relaymaster.com), a web site dedicated to helping
engineers and buyers find, research, and purchase
relays. Users can locate relays through a simplified
multi-category search without specifying a part number
or manufacturer.
The web site features almost 30,000 unique part numbers
that can be found by part number/keyword, relay
category, manufacturer, current rating, coil voltage,
contract arrangement, or mounting type. Once the
possible matches have been found users can compare
parts, view specifications, data sheets, and availability
from distributors where they can purchase the item.
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For more products and complete technical information, visit our web site: www.crydom.com
Mounting type
panel Mount
These Solid State Relays and
Contactors are designed to easily
mount on panels or heat sinks for
single, dual or 3 phase applications
din Rail Mount
“Ready-to-use” SSRs and Contactors
available in single, dual and 3 phase
output ratings with housing widths
varying from 6 mm to 45 mm
pcB Mount
Includes SSRs in the popular industry
standard SIP footprint, Mini SIP and
DIP configurations. Most Crydom SIP
type SSRs are also offered as DIN
Rail mountable Assemblies
plug-in Mount
Designed to install in industry
standard relay sockets, they
can also be soldered directly on
PCB assemblies. They offer the
speed and dependability of solid
state switching in a traditional
mechanical relay format
accessories & assemblies
Comprehensive offer including
sockets, protective covers, thermal
pads and a variety of heat sinks
and ready-to-use heat sink/SSR
assemblies
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Ratings
new pRoducts
• AC Output current
ratings up to 150 Amps
Single channel, 50
Amps Dual channel
& 50 Amps 3 Phase @
140, 280, 530 & 660 Volts
• DC Output current
ratings up to 100 Amps
& up to 1000 Volts
• AC Output current
ratings up to 65 Amps
Single channel, 6
Amps Dual channel
& 25 Amps 3 Phase
@ 140, 280, 530 & 660
Volts
• DC Output current
ratings up to 30 A @
200 Volts
• AC Output current
ratings up to 25 Amps
Single channel, 15 Amps
Dual channel & 15 Amps
3 Phase @ 140, 280, 530 &
660 Volts
• DC Output current ratings
up to 20 A @ 200 Volts
• AC Output current
ratings up to 5 Amps
Single channel @ 280
Volts
• DC Output current
ratings up to 5 A @ 100
Volts
• 13 Heat sinks with
thermal impedance
ratings from 5.0 to
0.5 ºC/W and perfect
match for Crydom
SSRs
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what is a solid state Relay/contactoR?
A Solid State Relay or Contactor (SSR or SSC)
is an electronic component that switches
Power (AC or DC current) to a load circuit
and provides electrical isolation between an
application’s control circuit and load circuit. It is
a competitive technology to Electromechanical
Relays (EMRs) and other switching technologies
such as Mercury Displacement Relays (MDRs).
why use solid state switching technology?
•Long life
•Quiet operation
•Minimum electrical noise
•Low power consumption
•Shock & vibration
resistant
•Ideal for harsh environments
•High compatibility with control systems
•Fast switching
•Position insensitive
solid state Relays & contactoRs applications
Although there are literally thousands of individual uses for Solid State Relays and Contactors, most can be
categorized into the following applications:
heating control
This encompasses the largest segment of solid state relay users. Applications include,
but are not limited to: Professional food equipment, plastic molding/extrusion machinery,
HVAC&R and soldering equipment.
Benefits: Long life, no maintenance, safe product, easy to interface, as well as enabling
temperature accuracy. Suitable for heaters, fans, blowers and valve control.
lighting control
These applications are usually broken down into three categories: Theatrical, warehouse
and commercial. Many of the products used in this segment are custom designed.
Benefits: Dimming, silent operation, fast switching, long life, no maintenance, safe
product, easy to interface, reduced parts count.
Motion control
Includes elevators, lifts, hoists, exercise equipment, conveyor systems, solar trackers,
fans, solenoids and valve control.
Benefits: Endurance, shock & vibration resistance, Soft Start, reversing, no arcing, fast
switching, long life, no maintenance, easy to interface, reduced parts count.
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Omron is one of the most recognized world leaders in relay design and manufacturing. Omron delivers innovative
control components that allow customers to reduce product size, add more function capability, and assemble product
more cost effectively. As a world class manufacturer, the company provides local technical support and global logistics
coordination to expertly facilitate design in one country, fabrication in another, and on-time delivery wherever product
is needed. Omron’s comprehensive quality commitment means improved manufacturing yields, reduced field failures,
on-time product launches, and a reputation for product reliability.
Omron Electronic Components is pleased to partner with Master Electronics to bring our products to market with
a broad and wide inventory offering, unmatched relay technical resource available on relaymaster.com, and
knowledgeable sales and support staff.
pRoduct categoRies
FeatuRes
Key pRoducts
low signal Relays Extremely small with very high
• Perfectly suited for
Telecommunications
• Ideal for Thermostat
Controls
• Designed for Medical
Instrumentation and
Equipment
• The best options
for any Security
Equipment
• Precise and accurate
operation for Test and
Measurement
• Fully RoHS compliant
Families
• G5V1
• G6J-Y
• G5V2
• G6K2
• G6A2
• G6L
• G6E
• G6S
Mos Fet Relays
• Solid-State performance
and accuracy
• Smallest SSOP footprint
in the industry
(SSOP Models)
• Low ON-resistance
(as low as 100mOhms)
• Low OFF-state
capacitance
(as low as 1pF)
• Low 3-5mA trigger
current
• High-speed switching
• 100,000 hours of
continuous operation
• Dielectric Strength of
10,000VAC for 1 second
on some models
g3VM series
• A/D
• GR
• B/E
• H
• BR/ER
• J
• C/F
• LR
• G
reliability and low power consumption, Omron Signal
Relays are the worldwide standard for precision
components. Available in PCB and SMT configurations,
latching and non-latching models, high surge withstand,
and many, many more options.
Perfectly suited for Automated Test Equipment, Medical
Equipment, Instrumentation, Security Equipment,
Automated Meter Reading, Automotive Diagnostic
Equipment, and Communications. Available in 1 and
2-Pole configurations in PCB, SMT, DIP, SOP, and SSOP
packaging. Other options include current limiting,
high dielectric voltage withstand, normally open
and normally closed contact forms, high voltage and
current load handling, and high-speed switching.
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pRoduct categoRies
FeatuRes
Key pRoducts
power pcB Relays
• Wide range of rated
loads in compact
housings ensures
versatility
• Class A, Class B and
Class F coil classes
are available Power
Consumption starting as
low as 120mW
• Several models offer
superior Impulse
Withstand Voltage up to
10kV between coil and
contacts
• The G4A, G5CA, and
G8PT Series offer PCB +
Quick Connect Terminals
RoHS Compliant
Families
• G2R
• G2RG
• G2RL
• G4A
• G4W
• G5CA
• G6RL
• G5LA
• G5LE
• G5NB
• Patented microstrip line
and coaxial designs
eliminate cross-talk and
improve isolation
• Y and E terminal
arrangements including
reverse terminal options
• 1 and 2-Pole models
• Latching and nonlatching options
• SMT and PCB thru-hole
versions
• SPDT RF MEMS Switch
has LGA12 footprint
Families
• 2SMES
• G6K-RF
• G6Z
• The switching section
and driving section
are gas-injected and
hermetically sealed,
allowing these compact
relays to interrupt
high-capacity loads.
Families
• G9EA
• G9EB
• G9EC
Omron makes a wide range of power PCB relays, all of
which are 100% factory tested to ensure 100% total
satisfaction. Omron’s power relays set the standards for
industry, providing switching capacity of up to 30A in
a variety of contact configurations. Long life spans are
assured, even with the most arduous of loads.
high Frequency Relays
Omron’s High Frequency Relays help give your
products the needed edge to win new business in
today’s demanding marketplace. By delivering the
industry’s widest range of design options, Omron’s HF
Relays boast superior frequency characteristics and
much broader bandwidth.
dc power Relays
DC Power Relays that
Interrupt High-capacity DC
Loads and High-voltage DC
Circuits in a Compact, Lownoise Design.
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• G5NB-E
• G5RL
• G5Q
• G7L
• G5SB
• G6B
• G6C
• G6DASI
• G6RN
• G8PT
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Relay pRoduct line
applications
Key pRoducts
photoMos®
• Automatic Testing
• Instrumentation
• Battery Monitoring
• Data Acquisition
aQy2 (Vssop package)
• Vertical Design: 4.6mm2
• High Density Mounting
Optically Isolated MOSFET
aQy2 (son package)
• Smallest horizontal
design package
aQs (sop16 package)
• Built-in resistor for
space saving
• 4 Form A multi-channel
output
signal
Low Level Switching <2A
power
Polarized & Non-Polarized (<2A)
automotive
High DC Voltage up to 300A Current Levels
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• Telecom
• Data Communication
• Thermostat
• Security & Fire Systems
• LAN/WAN/SAN
• Test & Measurement
• Consumer Electronics
• Audio/Video Equipment
• Cooling Fan Load
• Motor Load
• Heater Load
• 20-30A Steady State
tX-th
• 2.4V coil voltage
availability
• 2 Coil latching for energy
savings
tQ-sMd
• Flat and compact design
• 2A maximum switching
current
agQ
• 2A maximum switching
current for 50,000 cycles
• High frequency
characteristics up to 1GHz
• Solar Inverters
• Lighting Control
• Power Supplies
• Medical Devices
• HVAC
• Industrial Equipment
• UPS
• PLC Controls
• Smart Meters
adw1 (dw)
• 8A 250VAC
• Energy savings (latching)
• Breakdown voltage
(between contact & coil):
5,000 V
he-pV
• Compliance with
European PV standard
VDE0126
• 35A & 48A versions
available
• Reduced coil holding
voltage up to 40% of
nominal coil voltage
• Electric Vehicles
• Construction
Equipment
• Outdoor Power
Equipment
• Main & Pre-Charge
Relays
• Battery Management
• Stop Idle Systems
• EV Charging Stations
• Power Locks &
Windows
cB
• 70A maximum current
rating
• Sealed and fluxresistant versions
acw (cw)
• High current cutoff
and carrying current
performance
aeV (eV)
• 400V DC/300A
switching cutoff
• Stable contact
resistance regardless of
ambient conditions
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Panasonic Electric Works
http://pewa.panasonic.com
The World’s First Vertical PhotoMOS in a Space Saving SSOP Package
OPTICALLY ISOLATED MOSFET
• 4.6mm2 Mounting Area
• Low On Resistance
• Low Output Capacitance
•
•
•
•
AQY221R2T
AQY221N3TY
AQY221N3TW
AQY221N2TW
Reduction in Board Space
Requirements
Footprint
VSSOP
65%
P
50%
SO
P
SS
O
N
SO
VS
SO
P
SON
SSOP
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pROduCT InFO
pRoduct categoRies & FeatuRes
general purpose Relays
• Can handle current loads from
10 mA to 20 A
• Socket, panel & DIN mount options
• Multiple features & contact
configurations available
• Optional protection, mounting &
identification accessories
power Relays
• Rated up to 50 A
• Socket compatible models available
• Semi-sealed versions available
• Blowout magnet options for high DC
voltage switching
• Feature-rich covers, mounting options
& accessories to suit a multitude of
applications
solid state Relays
• 100% solid state design
• Modern appearance & advanced
technology
• High cycling rates
• High breaking capacities (up to 125 A)
• A wide range of input voltages
• Industry first design (861 & 861H series)
• Various styles to fit multiple applications
time delay & sensor Relays
• Multi-function up to ten
• Wide voltage range from 12 to 240 V
• Flexible timing range 100 ms to 10 days
• DIN or Panel mounting styles
• Conform to international standards
including UL, CSA, RoHS & CE
pcB & Reed Relays
• Space-saving package design
• Single & double pole switching
• Ratings range from 0.25 to 20 A
• Fully sealed for wash-down processes
• Wave solderable
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applications
Key pRoducts
• Automation Control
Panels
• Packaging Machinery
• Processing Machinery
• Lighting Controls
• Power Supplies
• Industrial Appliances
• Motor/Pump Controls
• Oil & Gas
• 781, 782, 783 & 784
Series Plug-In Relays
• 750 & 755 Series Octal
Plug-In Relays
• 788 Series Plug-In Relays
• 785 Series Latching Relays
• 782H Series Hermetically
Sealed Relays
• 750H Series Octal
Hermetically Sealed Relays
• Automation Panels
• Processing Equipment
• Packaging Machinery
• Lighting Controls
• Power Supplies
• Motor/Pump Controls
• 199 Open Frame Power
Relays
• 725 Series Power Relays
• 389F Series Power Relays
• 300 Series Power Relays
• 92 Series Power Relays
• 9A Series Power Relays
• Industrial Heater Controls
• Process & Material
Handling
• Lighting Controls
• Medical Equipment
• Automatic Door Controls
• Oil & Gas
• 861 Series Relays
• 861H Series UL Class I
Div 2 Relays
• SSRDIN Series Relays
• 6000 Series Relays
• 70S2 Series Relays
• Automation Panels
• Packaging Machinery
• Lighting Control
• Material Handling
• Motor/Pump Control
• 820 Time Delay Relays
• 831 Voltage
Sensing Relays
• 841 Current
Sensing Relays
• TDR782, TDRPRO &
TDRSOX/SRX Time
Delay Relays
• Automotive
• Lighting Controls
• Electronics &
Communication
• Security
• Automated Test
Equipment (ATE)
• Medical Equipment
• 117SIP, 107DIP, 171DIP
& 172DIP Series Reed
Relays
• 49, 276, & 976 Series PCB
Relays
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inteRactiVe ecatalogs
•Built-in search tool for each individual catalog
•Catalog part numbers are linked to respective online
product pages
•Add notes to catalog pages
•Print or export single or multiple pages in PDF format
•Email links to ecatalog pages and share via Social
Media
inteRactiVe weB tools
•Available for timers, latching relays, pcb relays and
sockets
•Learn about timing functions, how to wire latching
relays, the differences between pcb and reed relays
and how to choose a socket and accessories
2d & 3d dRawing liBRaRy
•2D line drawings and 3D modeling files available
•Download to desktop or send via email
•Import files into an open CAD/Modeling pane
•Available in most major 2D and 3D drafting platforms
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pRoduct categoRies & FeatuRed pRoducts: panel/plug in Relays
KRpa/Mt
• Industry standard octal/undecal
type termination for quick
installation
• DC and AC Coils
• Mechanical indicator, indicator
lamp and push-to-test options
Kup/KuMp/Kuip
• Wide selection of termination and mounting styles
• Broad range of contact forms
• PC terminals available
• Push to test button and indicator lamps
K10
• Mounting options include socket, PCB, Top flange
• DC and AC coils
• LED versions available
pt/Kh
• Sensitive coil
• Low height 29/33mm
• Cadmium-free contacts
• Mechanical indicator
• Manual test tab, optionally lockable
• optional LED, protection diode
RM8/c/d
• Power relay with push-on and solder terminals
• Various mounting options
• Class B coil insulation
R10
• Broad range of coil options provide sensitivity ranging from
25 to 750mW
• Various contacts switch from dry circuit to 7.5A
• Many mounting and termination options
pRoduct categoRies & FeatuRed pRoducts: low poweR pcB Relays
oMi/oMih/oMit
• Meet 5kV dielectric voltage
• 10kV surge voltage between coil
and contacts
• Version with 1 form A, 1 NO
contact TV-5 rating (OMIT)
Rt
• Sensitive DC and AC coil
• Bistable version
• 5kV/10mm coil-contact
• Reinforced insulation
• Ambient temperature 85°C
Ryii
• 5kV/8mm coil-contact
• Reinforced insulation
• Low height 12.3mm
• Pinnings 3.2 and 5mm
MsR/t75
• High inrush currents with AgSnO
contacts
• 4kV/8mm coil-contact
• Reinforced insulation
snR
• Only 5mm wide
• Cadmium-free contacts
• Sensitive coil 170mW
• 4kV coil-contact
• 6/8mm creepage/clearance
• Protection class II
Re/Rel
• Sensitive coil 200mW
• 4kV coil-contact (REL)
• PCB area 200mm2
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pRoduct categoRies & FeatuRed pRoducts: low poweR pcB Relays and FoRce guided Relays
RZ
• Sensitive coil 400mW
• 5kV/10mm coil-contact
• Reinforced insulation
• Ambient temperature 85 or 105°C
• Height 15.7mm
• In acc. to IEC 60335-1
pcn
• Only 5mm wide slim type, permitting high density spacing
• Sensitive coil 120mW
• Cadmium free contacts
sR2M
• 2 pole relay with force guided
contacts according to EN 50205
• Reinforced insulation between
poles
sR4 d/M
• 4 pole relay with force guided contacts according to
EN 50205
• Compact design, space efficient
oJ/oJe/t77
• Miniature size
• Meet 4kV dielectric between coil and contacts (OJ/OJT)
• Sensitive coil 200mW type available
• Meet UL TV-5 ratings (OJT)
sR6
• 4/6 pole relay with force guided contacts according to
EN 50205
• Reinforced insulation between all contacts
pRoduct categoRies & FeatuRed pRoducts: pcB high poweR, MeteRing and solaR Relays
t9a/t9e/t90
• High breaking
capacity
• PCB and QC
connections and
chassis mount version
• UL-class F as
standard
• Ambient temperature
85°C
• Open version
available
eF
• Low profile max. 20.0mm
• QC terminals for load
• Meet 4kV dielectric voltage between coil
and contact
t92
• Switching capacity
7500VA
• DC or AC coil
• 4kV/8mm coil-contact
• PCB or QC connections or
chassis mount
pcF
• QC terminal for load (PCF only)
• Height 26.5mm
• Meet 4kV dielectric voltage between coil and contact
• Ambient temperature 85°C
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t9s
• Specially designed to meet the requirements
for the solar inverter industry
• Contact gap >1.5mm
• 350mW hold power
• Product in accordance to IEC 60335-1
• EN 61095: AC7 at 85°C
pcFn solar
• Specially designed to meet the requirements for the
solar inverter industry
• Contact gap >1.5mm
• 200mW hold power
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5
14
FeATuRe
things
—or not quite everything—you wanted to know
about relays, but didn’t think to ask BY CAROLYN MATHAS
F
ound in a multitude of devices, relays are
simple electromechanical switches featuring
an electromagnet, one or more electrical
contacts, a movable iron armature that is
attracted by the magnetic field, and a simple spring to
hold the armature in place. This article will provide a
brief intro to relays, touching on some of the basic five
things you might want to know.
#1: what aRe Relays, and what do they do?
Relays enable the control of a high-current electrical
load with a low-current electrical signal. Especially
useful for devices that consume a lot of power and
that would normally require the use of a substantial
length and/or distance of wire, a relay uses a single
relatively small and low-power wire to control on/off
electrical flow. Relays control the maximum current
and voltage that can run through the armature and
the contacts; the number of armatures used; the
number of contacts used; and whether the contact is
normally open or normally closed.
#2: what types oF Relays eXist?
There are several different types, sizes, and styles of
relays in use. Although not an exhaustive list, some of
the more common relays include:
• Solid-state relays (SSRs) used to switch electrical
current. They do not have any moving components,
which increases long-term reliability and longevity.
• A solid-state contactor relay is a heavy-duty
SSR version that has a heat sink and is used for
applications where there’s frequent on/off switching.
• A latching relay has two relaxed states. When
current is turned off, the relay remains in its last
state. The coil consumes power only for an instant,
and relay contacts retain this setting across a
power outage.
• Reed relays are reed switches in a solenoid with
contacts inside an inert gas-filled glass tube to
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protect them against atmospheric corrosion. Reed
relays switch faster than larger relays, and only
require a little power from the control circuit, yet
have low switching current and voltage rating.
• A mercury-wetted relay is a form of reed relay
whose contacts are wetted with mercury to reduce
contact resistance and associated voltage drop.
This relay is used for low-current signals where
surface contamination makes for poor contact and
in high-speed applications, mercury eliminates
contact bounce. Given mercury’s toxic nature and
expense, this relay is rarely used.
• Machine tool relays are used for industrial control
of tools and transfer machines, and feature a
large number of contacts that are easily converted
from normally open to normally closed status.
Programmable logic controllers (PLCs) have
replaced most machine tool relays.
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#3: what do the Ratings Mean,
and how aRe Relays conFiguRed?
A relay is rated for its ability to handle power and the
rating is given in terms of amperes, such as 20 A, 30 A,
etc., and amp levels must be as large as the maximum
rating of the target device. For coil and voltage types
of relays, there is a rating for relay coil expectations
that need to be matched to that of the trigger wire.
Important is the number of things the relay can control
simultaneously. With one input, if the output goes to
one purpose, it is a simple on/off switch, or a single
throw. With two outputs, there are two throws. Pole
and throw information is designated as follows:
• Single pole, single throw (SPST)
• Double pole, double throw (DPDT)
• Three pole, double throw (3PDT)
• Single pole, three throw (SP3T)
Contacts are either normally open (NO) or normally
closed (NC), describing what exists when no power is
applied.
#4: what aRe coMMon Relay
applications?
All relays basically operate in the same way, but can be
selected to best work in specific applications. Anything
hooked up to the factory wiring of a car has a relay.
The ability to use power tools without recharging is
also an important application. SSRs are used in such
environments as humidity, salt water, and vibration
where the failure rate is higher for electromechanical
relays. Recently, the ladder logic of programmable
logic controllers is replacing the relay logic to carry out
complex functions in industrial settings.
Other applications include digital signal
amplification, aerospace, defense and marine uses,
finding and isolating faults on transmission and
distribution lines, vehicle battery isolation, and
switching to standby power.
#5: eVolution oF the technology—
what’s happening?
During the past couple of decades, semiconductor
technology has threatened to provide advanced
smart power solutions for solid-state relays that
would reliably deliver high current. Why then are
electromechanical relays still the choice relay for
many apps? Three reasons exist—cost, efficiency,
and ease of use. Electromechanical relays still have
an edge over their solid-state competition. Parity is
coming closer, especially considering advances in
packaging. •
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Figure 1.
contacts are
either normally
open (no) or
normally closed
(nc), describing
what exists
when no power
is applied.
Figure 2. inside the 4-lead/pin spst relay. on the
left, the coil is not powered. on the right, the
coil is powered.
All RelAyS bASICAlly
OpeRATe In The SAme
WAy, buT CAn be
SeleCTed TO beST
WORk In SpeCIFIC
ApplICATIOnS.
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FundaMentals
oF Relays
T
BRIAN DIPERT
PRINCIPAL
SIERRA MEDIA
hink of the word “relay” and the image that
might come to mind is of an old-fashioned
telegraph system consisting of a series of
magnetic switches that replicate-thereforerecreate and propagate a message from source to
destination nodes (thereby indicating one common
use of a relay, to amplify the current or voltage in a
source switching signal). Or you might have a slightly
more modern vision, of an initial-generation computer
created out of relays in the before-transistor era, or of an
early-iteration and relay-constructed telephone system.
While you might think that relays have fallen by
the wayside in the contemporary semiconductor era,
reality indicates otherwise. Relays remain valuable
commodities in situations where complete electrical
isolation is needed between the signal doing the
switching and the signal being switched. They can
tackle high-current and –voltage transients that would
fry even the most robust silicon transistor. And even
when a modern solid-state relay is being employed
instead of its electromagnetic forebear, a conventional
transistor structure may not be the optimum means of
implementing it.
opeRational Basics
Figure 1 shows an elementary electromagnetic relay.
This particular relay contains two fixed-position
contacts, one by default implementing a closed-circuit
connection between it and the armature (containing
two movable-position contacts), with the other fixedposition contact normally creating an open-circuit
no-connection. A spring holds the armature in its
default position; gravity can alternatively implement
the default-position function in some designs. Passing
current through the coil creates a magnetic field in
the iron core within it, pulling the armature closer
and thereby both “breaking” the normally closed
connection to one of the contacts and “making” the
normally open connection to the other.
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Figure 1: the functions of traditional mechanical
relays are easy to understand, but they’re fraught
with innumerable potential failure mechanisms.
Rapid relay-switching speed is desirable both
to minimize injected switched-signal noise and to
suppress armature-to-contact arcing. In a dc-based
relay configuration, either a diode or a resistor/
capacitor series pair also commonly connects to
the coil in order to dissipate voltage spikes caused
by collapse of the magnetic field. Ac-based relays
sometimes include small copper rings crimped to the
end of the coil, generating miniscule out-of-phase
current that increases the minimum armature pull.
pRoduct VaRiants
As you may have already ascertained from the above
description, the constant application of source current
and/or voltage is necessary in order to maintain a
conventional relay’s switch function. A latching relay
conversely is bistable, i.e., with two “relaxed” states.
When the applied current and/or voltage is removed,
the latched relay remains in its last state (including
through power outages). Three common means of
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implementing the latching function are:
• Asolenoidoperatingaratchet-and-cam
mechanism (the first pulse turns the relay “on,” the
next turns it back “‘off”)
• Twoopposingcoilsplusaspringorpermanent
magnet to hold the armature and contacts in
position (a pulse to one coil turns the relay “on,” to
the other coil turns it back “off”), or
• Aremanentcore(requiringopposite-polarity
pulses to turn the relay “on” and “off”).
Instead of leveraging an intermediary armature,
a relay might instead be constructed of magneticmaterial contacts that directly move toward or
away from each other in the presence of an applied
magnetic field. Also, in order to suppress the corrosive
effects of conventional atmosphere chemistry, a relay
design might surround tube-contain contacts with
a vacuum or an inert gas (reed relay) or a liquid
(mercury-wetted relay).
seMiconductoR successoRs
As the earlier paragraphs suggest, traditional
mechanical relays are prone to innumerable
operating life-limiting effects. Repeated arcing
(sparking) between contacts causes pitting and other
structural and impedance degradations that will
eventually cause permanent function compromise.
Both the armature and iron core can over time inherit
“permanent” magnetism that adversely affects the
relay switching function. Contact bounce can occur as
a result of ambient vibration. The armature can also
become bent with time, or even break. And similarly,
should the spring stretch or (worst-case) snap into
multiple pieces, you can imagine the resultant
deleterious effect on the relay function.
Enter the SSR (solid-state relay), which promises
to resolve the mechanical shortcomings of traditional
electromechanical devices but which has tradeoffs
of its own to consider. Short-duration or low-intensity
current and voltage transients within the switching
signal (that a conventional relay would disregard)
might for example, unintentionally trigger it. Also,
complete electrical isolation between the switching
and switched signals isn’t feasible with conventional
transistor-based SSRs, although it can be achieved if
necessary via an intermediary optoisolator, albeit at
added cost and design complexity.
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Keep in mind, too, the voltage drop across the
source-to-drain junction of a transistor, which will
both impedance-alter the characteristics of the signal
being switched and act as a current limiter on the
peak capabilities of the SSR. Admittedly, high-end
devices capable of switching several thousand amp
signals are available, albeit at corresponding high
prices. To wit, the term “solid state contactor relay”
refers to such a robust unit, usually containing an
integrated heat sink.
Where a latching i.e. bistable function is desirable
in a SSR, a thyristor (also commonly known as a
silicon-controlled rectifier) commonly finds use
instead of a transistor (Figure 2).
Constructed of alternating regions of p- and
n-doped semiconductor material, you can
simplistically think of a thyristor as a pair of a pair
of coupled bipolar transistors. Beginning with the
anode in the above diagram, the three p-n junctions
are serially named J1, J2, and J3. With the anode at a
positive voltage potential with respect to the cathode
and no applied gate voltage, junctions J1 and J3 are
forward-biased with J2 reverse-biased, leading to no
anode-to-cathode conduction.
A further increase in the anode-to-cathode voltage
potential, however, results in avalanche breakdown of
junction J2, transitioning the thyristor to its conductive
“on” state. This “off” to “on” transition will occur at
a much lower anode-to-cathode voltage if a “pulse”
gate voltage (which acts as the switching signal)
is applied. And past this point, conduction will
continue until either the anode-to-cathode voltage
potential or current flow is removed (or sufficiently
lowered); further gate switching signal application is
unnecessary. •
Figure 2: a thyristor (versus a conventional transistor)
enables a solid-state relay to mimic the bistable
function of its mechanical latching-relay forebear.
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application ideas:
Relays
Relays offer simple solution in current
and voltage switching applications
BY STEPHEN EVANCZUK, CONTRIBUTING EDITOR
I
n this digital age, the need to control large currents
and voltages safely remains critical. Relays fill this key
role in many applications thanks to their simplicity,
reliability, and life span.
In the classical electromechanical relay (EMR), current
flow through a coil induces a magnetic field that impels
an armature to open or close contacts. EMRs feature high
breakdown voltage, no leakage current, and true galvanic
isolation across open contacts, but have a relatively short
life due to mechanical wear. Although EMRs remain the
component of choice for applications operating at high
voltage or current levels, their utility in many low-voltage
analog and digital designs is limited due to their relatively
slow response time, reliability issues, physical size, and
sensitivity to EMI, shock, and vibration.
By eliminating the physical contacts of EMRs, solidstate relays (SSR) offer significantly higher reliability
and lifespan than EMRs. Photo-coupled SSRs are based
on conventional semiconductor switching elements
such as thyristors, triacs, transistors, and diodes—using
optoelectronic components to isolate input and output
(Figure 1). The result is a device that is compatible with
digital logic and capable of much higher switching speeds
and greater resistance to harsh operating environments
than EMRs. On the other hand, SSRs exhibit small but
significant leakage in the off state as well as higher output
resistance than EMRs.
While engineers can find EMRs capable of switching
voltages in the 10K+ V range, SSRs are typically limited
to switching at most a few hundred volts, but can achieve
higher-voltage operation when wired in series. In these
series (or stacked) SSRs, the total load voltage is the sum
of the load voltage rating of each individual SSR. In this
configuration, however, differences in turn-on time for
individual SSRs can lead to potential problems. Even a
few microseconds difference in turn-on time can translate
into significant differences in on-resistance (Figure 2)
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Figure 1: a typical photo-coupled
solid-state relay uses leds to
isolate input from switched output.
(source: Vishay semiconductors.)
If an SSR in a stacked configuration should turn on
significantly later than the others, it could be subject
to the full load voltage and be driven into avalanche
breakdown—potentially destroying the switch itself.
To prevent avalanche breakdown during switching,
engineers can place an RC snubber circuit or metal oxide
varistor device across each SSR output to ensure voltage
sharing during switching. SSRs also often integrate
current-limiting circuitry designed to protect the relay from
transients during switching. In addition, engineers can
find matched SSRs fabricate on the same die to minimize
differences in performance characteristics.
SSRs are current-controlled devices, requiring a
specified forward current, If, for turn on. For operation
from a voltage source VCC, an SSR with forward drop
voltage Vf would then require a current limiting resistor
Rf £ (VCC - Vf)/If, using appropriate Vf and If values taken
from curves of temperature-dependent performance
provided in SSR datasheets.
SSRs require only very low drive currents to turn on—a
characteristic that can result in inadvertent switching from
leakage current in digital drivers. In this case, a shunt
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Figure 4: a typical application of ssRs involves
switching power to a resistive load such as a room
heater based on sensor input or logic functions.
(source: omron.)
Figure 2: in a stacked ssR configuration, small
differences in turn-on time can lead to significant
differences in on resistance, risking avalanche
breakdown in unmatched ssRs.
(source: Vishay semiconductors.)
resistor placed in parallel with the SSR will ensure the
gate leakage current will not turn on the LED and cause
the SSR to switch (Figure 3). Assuming a logic gate has
leakage current IOH driving into an SSR with a turn-on
voltage Vf, a suitable shunt resistor RX < VF/IOH would
suffice to prevent inadvertent switching. For an SSR with
an LED on drive current If, a suitable LED current setting
resistor then becomes Rf = (VCC - VOL - Vf)/If.
The flexibility and ease-of-use of SSRs allows relatively
simple solutions in many applications. In the classic
application, an SSR simply opens or closes a circuit
supplying power to a load, such as a room heater (Figure
4). In practice, heaters built from some materials exhibit
temperature-dependent load resistance, so care must be
taken to prevent overcurrent in the SSR with the use of
current limiting protection across the SSR.
SSRs are often an ideal, low-cost, safe solution for
many motor-driven applications. For a reversible motor,
for example, a pair of SSRs can be used to drive the motor
in forward and reverse directions (Figure 5a). To eliminate
the possibility of switching both SSRs on simultaneously a result that could overheat the motor - the SSRs could be
driven with control signals that insert a delay in switching
direction (Figure 5b). In practice, an SSR-based motor
driver design would require SSRs with sufficient ability to
withstand the large inrush currents encountered during
motor startup — as well as suitable overvoltage protection
to handle the back EMF generated by the motor when the
SSR is turned off. •
a.
b.
Figure 3. a shunt resistor can prevent logic
driver leakage current from inadvertent
switching an ssR.
(source: avago technologies.)
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Figure 5: a simple reversible motor solution (a)
uses a pair of ssRs to drive the motor in forward
and reverse directions, using a delay to ensure
safe operation (b).
(source: sharp Microelectronics.)
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glOSSARy OF RelAy TeRmS
armature—A moving magnetic segment of an
electromagnetic relay, or a piece of soft iron connecting
the poles of a magnet.
Bosch relay—The most commonly used relay in car
audio and security is the Bosch type relay, also known
as a 5-pin, 5-prong, or 5-terminal relay.
coil—Also known as electromagnetic coil is formed
when a conductor is wound around a core to create an
inductor or electromagnet. When current is set up in the
coil, it makes a relay switch positions.
contacts—The portion of current-carrying elements
where electrical circuits are opened or closed. The
current carrying portion of a relay that engages or
disengages to open or close an electrical circuit.
contactor—A relay that handles the high power
necessary to directly control an electric motor or other loads.
dpdt—Double pole, double throw, which is equivalent
to two SPDT switches controlled by one mechanism.
dpst—Double pole, single throw, equivalent to two
SPST switches controlled by a single mechanism.
electromagnet—A magnet consisting of a coil of
insulated wire wrapped around a soft iron core that is
magnetized only when current is flowing through the
wire.
electromechanical relay—An electromechanical
relay uses a magnetic coil and mechanical contacts.
When current flow through a coil it moves an armature,
causing mechanical contacts to touch and close an
electrical circuit.
latching relays—Latching relays feature one or
two coils and have no default position, staying in
their last position when current stops flowing. Used in
applications where power consumption and dissipation
need to be limited since, once actuated, they need no
current flow to maintain their position.
Mercury-wetted relay—A reed relay where contacts
are wetted with mercury and used to switch low-voltage
signals as mercury reduces contact resistance and
associated voltage drop for low-current signals where
surface contamination may make for poor contact, or in
high-speed applications where the mercury eliminates
contact bounce. Given mercury’s toxicity and expense,
they are seldom used.
non-latching relays—In this case, the electromagnet
pulls on a switch that is spring-loaded to one side,
which is called the “normal” or “reset” side. It has an
initial position of normally closed (NC) maintained by
the force of a spring or permanent magnet while no
current flows. Non-latching electromechanical relays
are used in control applications when a switch must
return to a known state when power is lost.
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normally closed (nc)—Describes the position of
contacts in a spring-loaded switch. NC contacts are
closed when the spring is in a resting position and open
only when a force causes the contacts to move from
their resting position.
normally open (no) contacts—These contacts are
normally open when a spring is in a relaxed position
and close when forced to its other position. Many
switches come with one set of contacts NO and another
set of contacts NC.
polarized relay—Polarized relay places an armature
between the poles of a permanent magnet to increase
sensitivity. Polarized relays were formerly used in old
telephone exchanges to detect faint pulses and correct
distortion.
protective relays—Digital instruments that protect
electrical circuits from overload or faults in electric
power systems.
Reed relay—A reed switch enclosed in a solenoid
that has a set of contacts inside an evacuated or inert
gas-filled glass tube to protect the contacts against
atmospheric corrosion. Reed relays switch faster than
larger relays, require only little power from the control
circuit, but have low switching current and voltage
ratings.
solid state relay (ssR)—An electronic device with
no mechanical moving parts that switches electrical
current using transistors. The relay controls a larger
electrical current by using a small control signal.
spdt—Single Pole, Double Throw, a simple type of
changeover switch.
spring—A current-carrying spring to which a
contact is fastened or which can also serve as a contact.
spst—Single pole, single throw, a simple on-off
switch whereby two terminals are either connected
together or disconnected from each other. A common
example is a light switch.
throws—The number of throws equal the number
of separate contacts that power can be sent to, or the
number of places a switch can be “thrown” to.
Valet switch—A standard SPST switch wired
between a coil power source and a specific pin. In this
type, the relay is turned off as a security measure. You
can turn the switch on, but others do not know where it
is. An example is car security.
Voltage spike—A rapid and sharp voltage increase
resulting from a relay coil reversing its voltage to try
to keep the current flowing. Devices must be protected
from voltage spike which can kill the driver transistor. •
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