Electrical Sensing Devices

What are the basic functions of electrical control? u safety u convenience u comfort

Non-Automatic Controls u plug at end of cord u snap switch u magnetic switch

Parts of Automatic Devices u Sensing Element u Switch

Types of Switches u Single Throw: break one line of wire (120v)

Types of Switches u Double Pole: break two lines of wire (240v)

Types of Switches u Three Pole: 3-Phase:

Types of Switches u Normally Open (NO) v door bell u Normally Closed (NC) v stop button

How are electric control switches classified? u Number of poles: moveable contacts u Number of throws: stationary contacts u NO or NC

How does a Mercury Switch operate? u switch is opened or closed by liquid mercury in a closed tube u as tube is tilted, mercury makes contact with wires, activates switch u as tube is tilted opposite, mercurey moves away from wires, breaks circuit u livestock water tanks

What is an overload protection device in a switch? u safety protection u snap switch is a thermal overload device u often have a small light on switch which indicates when a motor is running

What is a relay device? u control a circuit carrying a large current by means of a low-current circuit v photoelectric cell u Advantages: v smaller control wires v less expensive v large distances between switch and load v low current in switch, lasts longer v can sequence certain loads

Parts of a relay u Solenoid or electromagnet v coil and a movable armature v set of contact points u Two electronic circuits v low voltage control circuits v load circuit

What is an Electro-Magnetic Switch? u similar to a relay, without the low voltage line u creates a fast make and break circuit u easy to apply remote control u easy to add overload protection to switch u car or tractor starting switch

Sensing Devices u Thermostats: air temperature u Humidistats: changes in moisture & humidity u Time control: clock u Photocell: light intensity u Each reacts to a specific condition u High or low voltage circuits

Thermostats u Bimetallic v different metals expand at different rates u Liquid-gas v gas filled bellows expand/contract u Hydraulic v same as liquid-gas, uses liquid u Heating or Cooling