Switches.pdf

Switches

Switches

Commonly used to control the “on/off function” of a component and/or circuit Also used to “direct the current” in an electrical circuit May also be used as “momentary contact” switches The term “pole(s)” refers to the number of input circuits of the switch The term “throw(s)” refers to the number of output circuits of the switch Switches may be “normally open” (NO), or “normally closed” (NC) depending upon the application

Introduction:

Classifications of Switches

SPST Switches

• Single pole input

• Single pole output

• A set of contacts inside the switch opens or closes the circuit

• The contacts carry the current load of the circuit when closed

Switches

SPDT Switches One input circuit (pole) Two output circuits (throws) Only one output is energized at a time Contacts carry the current load of circuit

Switches

Ganged Switches (MPMT) Contain multiple “wipers” that operate in unison Contacts may carry current load of circuit Contacts may supply current to an “electromagnetic switch”

Push-button

relays

Control relays are widely used in control circuits to switch multiple control circuits and to control light loads such as starting coils, indicator lights, and audible alarms. Relay can contain normally open, normally closed, or both types of contacts.

relays

A control relay’s pole number is the number of isolated circuits that can pass through the relay. This is the total number of circuits that can be controlled by the relay. Control relays often have multiple poles, but they need not all be used.

Throw

A control relay’s throw number is the number of closed-contact positions per pole. The following abbreviations are frequently

used to indicate contact configurations: SPST (single pole,

single throw), SPDT (single pole, double throw), DPST (double

pole, single throw), and DPDT (double pole, double throw).

Break

A control relay contact break number is the number of separate contacts that open or close a circuit.

Electromagnetic Relay Operation

An electromagnetic relay is a magnetic switch. It uses electromagnetism to switch contacts.

A relay will usually have only one coil but may have any number of different contacts.

Electromagnetic Relay Operation

With no current flow through the coil (coil de-energized),the armature is held away from the core by spring tension

When the coil is energized, the electromagnetic field moves the armature causing the contact points of the relay to open or close.

Electromagnetic Relay Operation

CR1

Coil

Normally open (NO) contact

Normally closed (NC) contact

Contacts are open when no current flows through the coil but close as soon as the coil is energized.

Contacts are closed when no current flows through the coil but open as soon as the coil is energized.

Relay Circuit Operation

CR1

R

G

ON

OFF

S L2 L1

With switch S open: coil CR1 is de-energized contacts CR1-1 are open light R is off contacts CR1-2 are closed light G is on

Relay Symbol

With switch S closed: coil CR1 is energized contacts CR1-1 are closed light R is on contacts CR1-2 are open light G is off

solid-state relay

A solid-state relay accomplishes the same result as the electromechanical relay but with nonmoving electronic devices such as silicon controlled rectifiers. SSR applications include office machines, medical equipment, display lighting, elevator control, vending machines, test and measurement instrumentation, and so on.

AC and DC SSR

AC Relays are another solid state relay that are built

primarily from a Triac circuit. A Triac is a component

made of two "Thyristors" which are voltage controlled

unidirectional switches

DC Relays utilize transistors and MOSFETS to switch

larger circuits.

advantages

Solid-State Relays Electromechanical Relays

No contacts to wear out. Normally have multithrow, multipole contact arrangements.

No contact arcing to generate electromagnetic interference.

Contacts can switch AC or DC.

Very long life when properly applied.

Low initial cost.

Very fast switching capability.

Very low contact voltage drop, thus no heat sink is required.

Resistant to shock and vibration because they have no moving parts

Some EMRs allow replacement of contacts.

disadvantages Solid-State Relays Electromechanical Relays

Normally only one contact available per relay.

Contacts wear out.

Heat Sink required due to voltage drop across switch.

Generates Rapid switching application or high current loads will shorten contact life.

Can switch only AC or DC.

electromagnetic noise and interference on power lines.

Environmental considerations (heat, dust, dirt, water).

Poor performance when switching high inrush currents.