Input Devices and Sensors
Types of Switches
The following sections outlines common switching devices currently used by the industry.
Hand switches are used as digital input devices and in hardwired electrical control
circuits associated with digital outputs. Hand switches come in numerous sizes,
shapes, and configurations. Common switch types include rotary, selector type switches,
toggle switches, and pushbuttons. Selector and toggle switches are almost always
maintained contact type. Pushbuttons may be momentary or maintained contact type.
Selector switches can have key operators to prevent tampering.
Figure 2.2- Pushbuttons and Selector Switches (courtesy IDEC)
Limit switches convert mechanical motion or proximity into a switching action. Limit
switches are most commonly used in DDC control systems for HVAC to provide position
status feedback to the controller for valve and damper positions. A wide variety
of configurations are available. Common types include industrial limit switches,
mercury, and proximity switches.
Figure 2.3-Industrial Limit Switches
Figure 2.4-Mercury Limit Switches
Figure 2.5-Proximity Switches
Temperature switches (also called thermostats, aquastats or freezestats depending
on application) are commonly used in DDC control systems to provide a digital input
when a process medium temperature rises or falls to a set temperature. Switches
with a number of different operating principles are manufactured. Some of the common
types include bimetallic, fluid thermal expansion, freezestat and electronic.
Bimetallic temperature switches use a bonded "bimetal" strip consisting of two dissimilar
metals with different thermal coefficients of expansion. When the temperature changes,
the metals expand or contract at different rates causing the strip to bend. Various
configurations such as coiled elements are used to increase the thermal movement
to cause two contacts to come together or separate. Some configurations use the
bimetallic principle to change the orientation of a bulb containing liquid mercury
so that the mercury flows into contact with two electrodes, completing the circuit.
Fluid thermal expansion temperature switches use the principle of thermal expansion
of a fluid to cause displacement of a bellows, diaphragm, bourdon tube, or piston
to open or close a set of contacts. Fluid system based temperature switches can
be connected to a remote fluid containing bulb by a capillary tube, allowing the
switch element to be remote from the sensing bulb.
Figure 2.6- Remote Bulb Thermostat
The freezestat is commonly used to prevent water or steam coils in air handling
units from freezing. Freezestats use a fluid that is a saturated vapor at the switch
set point temperature. This fluid is confined within a long capillary tube. The
tube is installed in a serpentine fashion over the area of the air stream to being
monitored. If any point along the tube falls below the saturation temperature, the
vapor begins to condense causing a rapid change in pressure in the system and actuating
the switch mechanism.
Electronic temperature switches use the same sensing technologies used for analog
temperature sensing to electronically operate a set of output contacts. Refer to
the Temperature Measurement portion of the Analog Input Device Section for more
details of sensing technology.
Humidity switches, or humidistats, are used in DDC control systems to provide a
digital input when a process or space humidity level rises or falls to a set level.
Common applications are high limit safety interlocks for humidifiers, space or process
humidity alarms, and simple on-off humidity control.
Mechanical humidistats use a hygroscopic material such as animal hair, nylon or
other plastic material that changes dimension with changes in relative humidity.
The dimensional change is amplified via a mechanical link to causing a switch to
Mechanical humidistats are rapidly being replaced by electronic humidistats that
use thin film capacitance or bulk polymer resistance analog humidity sensing technologies
combined with electronic switching circuitry to produce a switching action at an
adjustable set point. These sensing technologies are described in the Humidity Measurement
portion of the Analog Input Device Section.
Flow switches are used to provide a digital input to DDC controls systems when a
fluid flow rate has risen above or fallen below the set value. Common applications
include safety air and water flow interlocks for electric heaters and humidifiers,
chiller safety interlocks, and burner safety interlocks. Numerous technologies are
available, but the most common types used in DDC systems for HVAC control are mechanical
and differential pressure types.
Mechanical flow switches operate on the principle that the kinetic energy of a flowing
fluid creates a force on an object suspended in the flow stream. The magnitude of
the force varies with (the square of) the velocity of the fluid. Various configurations
are used to transfer this force into operation of a switch. Common configurations
include paddles or sails, pistons or discs.
Differential pressure type flow switches (Figure 2.8) operate on the principle that
a difference in pressure is always associated with fluid flow, or the principle
that the total pressure of a flowing fluid is always greater than the static pressure.
These differences in pressure can be accurately predicted for a given situation
and related to the fluid flow rate. For more information see the Flow Measurement
portion of the Analog Input Section.
Level switches are used in DDC control systems (for HVAC) to provide a digital input
when the fluid level in a tank, vessel or sump has reached a predetermined height.
Common applications include cooling tower sump level control and monitoring, steam
condensate tank level, storm water and sewage sump level monitoring and control
and thermal storage tank level monitoring. Numerous mechanical and analog technologies
are currently available. Some analog technologies include capacitance, ultrasonic,
and magnetostrictive-based devices in combination with solid-state electronics to
provide a switching action based on level. More commonly used technologies include
devices that employ the use of a float (integral, rod and float, submersible), conductivity
probe, or differential pressure mechanism.
Integral float type level switches typically combine an metal or plastic float attached
to the arm of a submersible rotary switch mechanism, or a float that encloses a
magnet which slides on a hollow rod enclosing one or more reed switches.
Submersible float switches utilize an encapsulated integral float type switch or
mercury switch suspended on a fluid tight cord in the vessel being monitored. When
the level is below the cord attachment, the float hangs down and the switch is in
its normally open or closed position. When the fluid level rises, the float rises
above the cord attachment point, changing the float orientation. When the float
has position has inverted sufficiently, the internal switch changes position.
Conductivity probe-type level switches work for conductive liquids only and use
the liquid itself to conduct low level electrical signals between two or more electrodes
to operate higher level electronic switching devices such as transistors or triacs.
Pressure switches are used in DDC systems to provide status indication for fans,
filters and pumps, and to provide flow and level status indication by virtue of
the predicable relationships between pressure and these values. Pressure switches
may be mechanical or electronic.
Mechanical pressure switches use a piston, bellows, bourdon tube or diaphragm and
a magnetic or mechanical linkage to convert the forces resulting from the measured
pressure into repeatable motions used to operate one or more switches (Figure 2.3).
Low pressure switches commonly used to measure air pressures in the range of 0.05
inches water column to 1 psig typically use a flexible diaphragm. Piston, bourdon
tube and bellow type switches are available
Vibration switches are used to provide a signal when vibration levels in rotating
machinery such as fans, reach unsafe levels. Vibration switches are commonly applied
on large cooling tower and air handling unit fans.
Moisture detecting switches are commonly used to detect moisture under raised floors,
in piping and tank containment areas and in the drain pans of air handling units
to alert system operators before damage or flooding occurs. Most moisture detecting
switches are instruments of the float type or conductivity type. Float types are
adapted to actuate at very low levels. Conductivity types may consist of point sensitive
probes located very close to the bottom of a low point or sump where water will
collect, or they may be ribbons or strips with wires separated by a non-conductive
material, such that when any portion of the ribbon is exposed to liquid moisture,
the electrical circuit is completed and the switch mechanism activates.
Current sensing relays are used in DDC systems to monitor the status of electrical
devices. The devices typically have one or more adjustable current set points. Common
applications include fan and pump on/off status feedback. Current switches can detect
broken fan belts if properly adjusted. Current relays can also be used for phase
Getting Started |
Chapter 1: Switches Intro |
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