Digital Flashcards
Contact type electro-mechanical input field devices are ?
And sometimes the switching mechanism?
Switch or contact field devices.
Sometimes the switching mechanism is spring or snap activated which makes for a quick and positive switching action
Push buttons normally associated with?
Motor control stations
Switching mechanism
Often the switching machisim is spring or snap activated which makes for a quick and positive switching action
Over travel switches of limit switches
Single break snap action contacts for positive activation
Either NC or NO or combination
For different applications they have different activation mechanism
Explain the working principal of a contact or relay
When a coil is energized the magnetic field induces a field into the armature which is pulled across to the core coil forcing the contact to operate
What are contacts
Large relays often associated with switching large 3 phase loads like motor
What are solenoids? And now do they work?
They are electromagnetic devices which operate on a similar principle to contractors,
When a coil is energized it attracts an armature, this operates an actuator which opens on closes a valve for fluids and air etc.
Reed relays / switches
Constructed from hermetically sealed glass tubes with iron contact reeds
When the magnetic field is within range the iron reeds are magnetized and attracted together
Inductive proximity switches
The oscillator produces an alternating magnetic field. the magnetic field induces eddie current on the surface of the target. This Eddie current upset the AC inductance of the oscillator circuit caching it to reduce
When it reduced below a certain level the sensor triggers operating a switching circuit ( transistor type )
Detect ferrous metals like: iron, nickel, cobalt
Capacitive proximity switches
Two small plates forms a capacitor placed behind the sensor head. When an object approaches the sensor the dielectric constant of the capacitor is changed this changing the oscillation frequency which triggers the device operating a transistor switch
Detects non ferrous metals like: wood, glass, water, plastic
Hall effect switches
There is a thin sheet of semiconductor material, and a constant current is passing through it
If there is a magnet near it, a small voltage called hall voltage will appear a cross the sheet, this voltage triggers an electric switching circuit (transistorbased)
The hall voltage will disappear by removing the current or magnet
Photoelectric switches detect…?
Photoelectric sensor detect objects by means of a beam of light ( infrared in modern sensors)
Modulated type photoelectric switches
Tuned to respond to high frequency pulses of light, so it’s not affected by ambient light or light noise
Unmodulated photoelectric switches
Used when sensing distance is short, and when dust & dirt & ambient light is not a problem
They’re used for high speed counting applications cuz the beam of light is continuous and responds quickly
