PSG COLLEGE OF TECHNOLOGY
DEPARTMENT OF INSTRUMENTATION AND CONTROL SYSTEMS ENGINEERING
19U604 - POWER ELECTRONICS AND DRIVES
SOLID STATE RELAYS
Presented By
Akalya M (22U201)
Madhumita K (22U226)
Nemi Siva Saravani M J (22U228)
Date : 17/02/2025 Premalatha P (22U231)
Sivarekha K (22U243)
Sowndariya Sree D (22U244)
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Overview
01. Introduction to Solid-State Relays
02. Types of Solid-State Relays
03. Working Principle of SSRs
04. Snubber Circuits and Protection techniques In SSRs
05. SSRs in Motor Drives and Control
06. Selection Criteria of SSRs in Industrial Applications
07. Challenges and Future Trends in SSR Technology
08. Applications of SSRs in Power Electronics
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Introduction to Solid-State Relays
● Solid-State Relays (SSRs) are
electronic switching devices that use
semiconductors (SCRs, TRIACs,
transistors) instead of mechanical
contacts to control electrical loads.
● Unlike Electromechanical Relays
(EMRs), SSRs have no moving parts,
making them faster, more durable,
and maintenance-free.
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Key Features
● Fast Switching: Instantly responds to control signals, ideal for
high-speed automation.
● High Durability: No moving parts → Longer lifespan, resistant to
wear and tear.
● No Mechanical Wear: No contact degradation, ensuring consistent
performance over time.
● High Input-Output Isolation: Electrically isolates control and load
circuits, enhancing safety and reliability.
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Types of Solid-State Relays
Reed-Relay-Coupled SSRs:
● Uses a reed relay to trigger
a thyristor.
● Provides excellent isolation
but operates at a slower
switching speed.
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Types of Solid-State Relays
Transformer-Coupled SSRs:
● Uses a transformer to isolate
and trigger the thyristor.
● High sensitivity, commonly used
for AC loads.
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Types of Solid-State Relays
Photo-Coupled (Optically
Coupled) SSRs:
● Uses an LED and a
photosensitive semiconductor
(phototransistor, thyristor) for
switching.
● Best electrical isolation,
widely used in automation and
industrial applications.
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Types of Solid-State Relays
Direct-Control AC SSRs:
● Uses external contacts to
trigger a TRIAC.
● No input-output isolation,
making it less safe for some
applications.
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Types of Solid-State Relays
Direct-Control DC SSRs:
● Uses a simple transistor switch
for DC loads.
● Lacks isolation but is
cost-effective and easy to use.
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Types of Solid-State Relays
SCR-Based SSRs for DC:
● Uses an SCR with a
commutating circuit for
turn-off.
● Suitable for DC applications
requiring controlled
switching.
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Reed - Relay coupled SSR
● A reed relay is used to provide
isolation between the control and
load circuits.
● When a low-power control signal
energizes the reed relay, the reed
switch closes, triggering the thyristor
(typically SCR or TRIAC) to turn on
and switch the load.
● The reed switch ensures excellent
isolation as there is no direct
electrical connection between the
input and output.
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Transformer coupled SSR
● The control signal is applied to the
primary winding of a small
transformer.
● The transformer generates a
secondary voltage, which triggers the
thyristor (SCR or TRIAC) to control
the load current.
● This type of SSR provides isolation
between the control and load circuits,
with the degree of isolation
determined by the transformer
design.
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Photo-Coupled SSR
● The control signal activates an LED,
which emits light.
● The emitted light is detected by a
photosensitive semiconductor
(phototransistor, photo-thyristor, or
photo-triac) to trigger the load
circuit.
● The key advantage is that the input
and output are electrically isolated
by the light transfer, making it ideal
for protecting sensitive control
circuits from high-power or noisy
loads.
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Direct-control AC SSR
● No Isolation: The control signal
directly triggers a TRIAC, which
switches the AC load.
● There is no isolation between the
control (input) and load (output)
circuits. External contacts (e.g.,
mechanical switches) are used to
apply the control signal to the
TRIAC.
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Direct-control DC SSR
● The control signal directly triggers
a TRIAC, which switches the AC
load.
● There is no isolation between the
control (input) and load (output)
circuits. External contacts (e.g.,
mechanical switches) are used to
apply the control signal to the
TRIAC.
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SCR based SSR for DC
● Uses an SCR to control the DC
load, and a commutating circuit
is used to turn off the SCR after
switching.
● The SCR remains on after
triggering, allowing current to
flow. The commutating circuit
(e.g., a capacitor or another
SCR) is used to momentarily
reduce the current to zero,
turning off the SCR.
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Snubber Circuit & Protection in SSR
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● Designed to protect the device by suppressing sudden voltage spikes or
transients that can occur during switching.
● Capacitor (C): Absorbs the sudden voltage surge when the SSR switches
.Resistor (R): Determined to limit the current flow through the capacitor
during discharge.
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Protection in SSR
1. Overvoltage protection:
● Diodes: Can be used to protect against inductive flyback energy .
● Zener diodes: Can protect against positive voltage transients when the SSR is
off.
● Metal oxide varistors (MOVs): Can protect against voltage transients by
diverting surge current.
2. Short circuit protection:
● Fuses: Fast-acting fuses can protect against load short circuits.
● Circuit breakers: Can be used to protect SSRs with an I²t value greater than
5000 A²s.
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3. Overcurrent protection:
● Fuses: Fast-acting fuses can protect against load short circuits.
● Circuit breakers: Can be used to protect SSRs with an I²t value greater than
5000 A²s.
➔ Important considerations when using snubber circuits:
● Sizing
● Internal snubbers
● Application specific
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Solid State Relay In Power Drives
High Switching Speed: Vibration and Shock
No physical Resistance:
movement(mechanical The lack of mechanical
contact) components
Quiet Operation: Long Lifespan:
Does not produce audible do not undergo
clicking sounds mechanical stress
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Solid State Relay In Power Drives
Low Power Consumption: Reduced risk of contact
reducing overall energy failure:No risk of
consumption ‘bouncing’ contacts
Size: Arc-Free Switching:
manufactured in far smaller switch without
and more compact generating any arcs
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Selection Criteria
Load type
Electrical ratings
Voltage and current ratings Resistive load-Zero-Crossing SSR
Switching type Inductive load-Random Turn-On
Surge current capability Capacitive load-Fast Turn-Off
Switching characteristics
Mechanical & safety considerations
Turn-on type
Turn- off time Mounting type
dV/dt & dI/dt Ratings
Environmental considerations
Heat dissipation
Operating Temperature
Protection features
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CHALLENGES
1. Heat Dissipation
2. Leakage Current
3. Voltage Drop
4. Surge and Transient Sensitivity
5. Limited Overload Capacity
6. Cost
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FUTURE TRENDS IN SSR
1. Improved Efficiency and Thermal Management
2. Miniaturization and Higher Integration.
3. Enhanced Switching Speed and Accuracy.
4. IoT and Smart Control Integration
5. Enhanced Noise Immunity
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APPLICATIONS
1.Smart Grids and Power Distribution:
SSRs are integrated into smart grids for
efficient load switching, power routing, and
remote monitoring.
2. Telecommunications:
They are used in network switching, signal
routing, and protection circuits in
communication systems.
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4. Industrial Automation:
SSRs are widely used in automation systems for controlling motors, solenoids, heaters,
and other industrial equipment due to their fast switching and durability.
5. Heating and Temperature Control:
They are commonly used in thermostats, ovens, furnaces, and other heating systems
for precise temperature control without mechanical wear.
6. Lighting Control:
SSRs are used in commercial and residential lighting systems, including dimming
applications and smart lighting solutions.
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CONCLUSION
Solid State Relays (SSRs) provide reliable, fast, and silent switching, making
them ideal for modern automation and control systems. Despite some
challenges, ongoing advancements are enhancing their efficiency and
expanding their applications, ensuring their continued importance in future
electronic systems.
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REFERENCES
1.www.electronics-tutorials.com
2.bchindia.com
3. br.omega.com
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