Download Electrical Sensing Devices

Electrical Sensing Devices
Original Power Point Created by
Casey Osksa
Modified by Georgia Agricultural
Education Curriculum Office
June 2002
What are the basic functions
of electrical control?
 safety
 convenience
 comfort
Non-Automatic Controls
 plug
at end of cord
 snap switch
 magnetic switch
Parts of Automatic Devices
 Sensing
 Switch
Element
Types of Switches
 Single
(120v)
Power
Throw: break one line of wire
Types of Switches
 Double
(240v)
Pole: break two lines of wire
Types of Switches
 Three
Pole: 3-Phase:
Types of Switches
 Normally
 door
bell
 Normally
 stop
Open (NO)
Closed (NC)
button
How are electric control
switches classified?
 Number
of poles: moveable contacts
 Number of throws: stationary contacts
 NO or NC
How does a Mercury Switch
operate?
 switch
is opened or closed by liquid
mercury in a closed tube
 as tube is tilted, mercury makes contact
with wires, activates switch
 as tube is tilted opposite, mercurey
moves away from wires, breaks circuit
 livestock water tanks
What is an overload
protection device in a switch?
 safety
protection
 snap switch is a thermal overload
device
 often have a small light on switch
which indicates when a motor is
running
What is a relay device?
 control
a circuit carrying a large current
by means of a low-current circuit
 photoelectric
cell
 Advantages:
 smaller
control wires
 less expensive
 large distances between switch and load
 low current in switch, lasts longer
 can sequence certain loads
Parts of a relay
 Solenoid
or electromagnet
 coil
and a movable armature
 set of contact points
 Two
electronic circuits
 low
voltage control circuits
 load circuit
What is an Electro-Magnetic
Switch?
 similar
to a relay, without the low
voltage line
 creates a fast “make” and “break”
circuit
 easy to apply remote control
 easy to add overload protection to
switch
 car or tractor starting switch
Sensing Devices
 Thermostats:
air temperature
 Humidistats: changes in moisture &
humidity
 Time control: clock
 Photocell: light intensity
 Each reacts to a specific condition
 High or low voltage circuits
Thermostats
 Bimetallic
 different
metals expand at different rates
 Liquid-gas
 gas
filled bellows expand/contract
 Hydraulic
 same
as liquid-gas, uses liquid
 Heating
or Cooling