The Solid State Relay
While the electromechanical relay (EMR) are
inexpensive, easy to use and allow the switching of a load circuit controlled
by a low power, electrically isolated input signal, one of the main
disadvantages of an electromechanical relay is that it is a “mechanical
device”, that is it has moving parts so their switching speed (response time)
due to physically movement of the metal contacts using a magnetic field is
slow.
Over a period of time these moving parts will wear out and fail,
or that the contact resistance through the constant arcing and erosion may make
the relay unusable and shortens its life. Also, they are electrically noisy
with the contacts suffering from contact bounce which may affect any electronic
circuits to which they are connected.
To overcome these disadvantages of the electrical relay, another
type of relay called a Solid State Relay or (SSR) for short
was developed which is a solid state contactless, pure electronic relay.
The solid state relay being a purely electronic device has no
moving parts within its design as the mechanical contacts have been replaced by
power transistors, thyristors or triac’s. The electrical separation between the
input control signal and the output load voltage is accomplished with the aid
of an opto-coupler type Light Sensor.
The Solid State Relay provides a high degree of
reliability, long life and reduced electromagnetic interference (EMI), (no
arcing contacts or magnetic fields), together with a much faster almost instant
response time, as compared to the conventional electromechanical relay.
Also the input control power requirements of the solid state
relay are generally low enough to make them compatible with most IC logic
families without the need for additional buffers, drivers or amplifiers.
However, being a semiconductor device they must be mounted onto suitable heat sinks
to prevent the output switching semiconductor device from over heating.
Solid State Relay
The AC type Solid State Relay turns “ON” at the zero crossing
point of the AC sinusoidal waveform, prevents high inrush currents when
switching inductive or capacitive loads while the inherent turn “OFF” feature
of Thyristors and Triacs provides an improvement over the arcing contacts of
the electromechanical relays.
Like the electromechanical relays, a Resistor-Capacitor (RC)
snubber network is generally required across the output terminals of the SSR to
protect the semiconductor output switching device from noise and voltage
transient spikes when used to switch highly inductive or capacitive loads. In
most modern SSR’s this RC snubber network is built as standard into the relay
itself reducing the need for additional external components.
Non-zero crossing detection switching (instant “ON”) type SSR’s
are also available for phase controlled applications such as the dimming or
fading of lights at concerts, shows, disco lighting etc, or for motor speed
control type applications.
As the output switching device of a solid state relay is a
semiconductor device (Transistor for DC switching applications, or a
Triac/Thyristor combination for AC switching), the voltage drop across the
output terminals of an SSR when “ON” is much higher than that of the
electromechanical relay, typically 1.5 – 2.0 volts. If switching large currents
for long periods of time an additional heat sink will be required.
Input/output Interface Modules
Input/Output Interface Modules, (I/O Modules) are
another type of solid state relay designed specifically to interface computers,
micro-controller or PIC’s to “real world” loads and switches. There are four
basic types of I/O modules available, AC or DC Input voltage to TTL or CMOS
logic level output, and TTL or CMOS logic input to an AC or DC Output voltage
with each module containing all the necessary circuitry to provide a complete
interface and isolation within one small device. They are available as
individual solid state modules or integrated into 4, 8 or 16 channel devices.
Modular Input/Output Interface System
The main disadvantages of solid state relays (SSR’s) compared to
that of an equivalent wattage electromechanical relay is their higher costs,
the fact that only single pole single throw (SPST) types are available, “OFF”-state
leakage currents flow through the switching device, and a high “ON”-state
voltage drop and power dissipation resulting in additional heat sinking
requirements. Also they can not switch very small load currents or high
frequency signals such as audio or video signals although special Solid
State Switches are
available for this type of application.
In this tutorial about Electrical Relays, we have
looked at both the electromechanical relay and the solid state relay which can
be used as an output device (actuator) to control a physical process. In the
next tutorial we will continue our look at output devices called Actuators and
especially one that converts a small electrical signal into a corresponding
physical movement using electromagnetism. The output device is called a Solenoid.
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