Class Notes - Year 4 - 2020-2021
Year 4

Lesson 403

Solid State Relays and Phase-Loss Lab

EMC5 25-1 - Solid-State Relays

Solid state relay

SSR is a switching device that has no contacts and switches entirely by electronic means
Used silicon controlled rectifier (SCR), triac, or transistor output instead of mechanical contacts
Output is optically coupled to an LED inside the relay
Relay is on when LED is energized, off when de-energized

SSR Circuits

Consists of:

Input circuit
Control circuit
Output (load) circuit

Input Circuit

Control component is connected here
Same function as coil on EMR
Activated by applying voltage higher than pickup voltage
De-activated by applying voltage lower than dropout voltage
Some are fixed voltage (12 VDC)
Most have voltage range (3 VDC - 32 VDC

Control Circuits

Determines when output is energized or de-energized
Interface between input and output circuits
When control circuit receives input voltage, circuit is switched

Output (Load Switching) Circuit

Is the load switched by the SSR
Performs same function as mechanical contacts of EMR
Normally only one output contact
Faster switching via thyristors
Common output switching devices
SCRs - high current DC loads
Triacs - Low current AC loads
Transistors - Low current DC loads
Antiparallel thyristors - High current AC loads
These dissipate heat better than triac
Thyristors in diode bridges - Low current AC loads

SSR Circuit Capabilities

2 Wire Control
Can use momentary push button
3 wire memory control
Identical to standard
SCR used to add memory
Does not allow current to flow until current is applied to gate
Equivalent NC contacts
Contact is NO
Connect control voltage via resistor, contact closes
Push button, current takes path of least resistance
Contact opens
Transistor control
SSRs can be controlled electronic control signals from logic gates & transistors
Series and parallel control
Creates multicontacts that are controlled by one input device
Same control voltage to all
Remember control voltage will be additive

SSR Switching Methods

4 Types
Zero switching (ZS)
Instant on (IO)
Peak switching (PS)
Analog switching (AS)
Zero switching
Turns on when control voltage is applied and voltage at load crosses zero
Turns off when control voltage is removed and current in load crosses zero
Designed to control resistive loads
Temperature of heating elements, incandescent lamps
Instant on switching
Turns on immediately when control voltage is present
Turns off when control voltage is removed and current in load crosses zero
Designed to control inductive loads
Contactors, magnetic valves, small motors
Peak switching
Turns on load when control voltage is present and voltage is at its peak
Turns off when control voltage is removed and current crosses zero
Transformers, large motors, DC loads
Analog switching
Infinite number of possible output voltages within rated range
Built in synchronizing circuit that controls output voltage as a function of the input
Allows for ramp up function
Turns off when control voltage is removed and current in load crosses zero
Typical input is 0 VDC-5VDC
Example: temp sensor

Temperature Considerations

Failure of most SSRs doubles for every 10 degrees Celsius above 40
Cooling accomplished by installing SSR in heat sink
Chosen based on maximum load

EMC5 25-2 - Electromechanical and Solid-State Relay Comparison

Advantages and Limitations

Equivalent Terminology Chart
EMR advantages
EMR disadvantages
SSR advantages
SSR disadvantages
Input signals
EMR takes more voltage and current pull in coil than to hold it
This is minimized with SSR
Response time
SSR is much faster
Voltage and current ratings
SSRs have arcless switching
Useful in explosive environments
EMRs have replaceable contacts if too much current is applied
With SSRs the whole unit needs replaced
Insulation and leakage
SSRs have possibility for small amounts to leak

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