D. Protection Circuit (Relays)

There are 4 types of protection circuit

–Diode circuit

–RC circuit

–Zeners circuit

–MOV circuit

Different types of coil transient suppression

Coil transient suppression..cont.

-When circuit complete (transistor forward bias), current will flow into relay coil & not through diode.-relay coil energize, produce magnetic field from i=0 to i=max.-but if transistor is off, circuit is open, current accumulate at relay coil need to discharge.-Will go through diode, called freewheeling diode.-But if no protection device, it will go back through relay coil and can cause arching (cause

contact to wear/oxidized).-strength of the i=max will slowly decay its value until i=0

-When circuit complete (transistor forward bias), current will flow into relay coil & not through diode.-relay coil energize, produce magnetic field from i=0 to i=max.-but if transistor is off, circuit is open, current accumulate at relay coil need to discharge.-Will go through diode, called freewheeling diode.-But if no protection device, it will go back through relay coil and can cause arching (cause

contact to wear/oxidized).-strength of the i=max will slowly decay its value until i=0

Pressure switch

-Pressure is force exerted over a surface divided by its area ; P=F/A

-Pressure switch is control switch that detect amount of force & activates electrical contact when the set amount of force is reach.

-Application:To control the air pressure in the tank by turning the air compressor motor on whatever the tank pressure falls below the set value.

-Pressure is force exerted over a surface divided by its area ; P=F/A

-Pressure switch is control switch that detect amount of force & activates electrical contact when the set amount of force is reach.

-Application:To control the air pressure in the tank by turning the air compressor motor on whatever the tank pressure falls below the set value.

NC Pressure switch

NO Pressure switch

Pressure switch application

-When switch is closed, current flows through rung 1, motor will start compress the air until system pressure is reach.-When system pressure is reach, the NC contact will open & pump motor is turn off.-When pressure in the compressor falls below the set point, pressure switch in rung 1 will close and restart the motor again.-NO contact use to signal overpressure condition.-Alarm is sound when overpressure.

Example pressure switch.

-Deflecting mechanism that moves when a force (pressure) is applied

-Diaphragm switch use for pressure less than 200psi.

-Deflecting mechanism that moves when a force (pressure) is applied

-Diaphragm switch use for pressure less than 200psi.

Diaphragm

Example pressure switch..cont.

-Cylindrical device with several deep fold which expand or contract when pressure is applied.-The expanding bellow

moves against spring mechanism which operate elect. contact when source pressure

increase.-can use for pressure up to 500psi.

-Cylindrical device with several deep fold which expand or contract when pressure is applied.-The expanding bellow

moves against spring mechanism which operate elect. contact when source pressure

increase.-can use for pressure up to 500psi.

Bellows

Example pressure switch..cont.

-Move back & forth in a tight fitting chamber by pressure applied in a chamber.-Design for high pressure application of 10,000psi

-Move back & forth in a tight fitting chamber by pressure applied in a chamber.-Design for high pressure application of 10,000psi

Piston

Limit Switch

-require physical contact of the object

with the switch actuator.

-the mechanical motion physically open or close a set of contact within the limit switch enclosure.

-require physical contact of the object

with the switch actuator.

-the mechanical motion physically open or close a set of contact within the limit switch enclosure.

NO limit switch

NC limit switch

Reed Switch

-electrical switch operated by an applied magnetic field.- consists of pair of contact on ferrous metal reeds in a hermetically (insulated or protected from outside influences) sealed glass envelope.-contact maybe normally open, closing when magnetic field is present, or normally closed and opening when a magnetic field is applied -usage example-computer keyboards, petrol pump, automatic dentist drill activation.

BREAK TIME !

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