Switches

Also see:
Relays |

Circuit symbol for a

simple on-off switch

Selecting a Switch

There are three important features to consider when selecting a switch:

• Contacts (e.g. single pole, double throw)


• Ratings (maximum voltage and current)


• Method of Operation (toggle, slide, key etc.)
  

---

 See different types of switch

Switch Contacts

Several terms are used to describe switch contacts:

• Pole - number of switch contact sets.


• Throw - number of conducting positions, single or double.


• Way - number of conducting positions, three or more.


• Momentary - switch returns to its normal position when released.
  


• Open - off position, contacts not conducting.


• Closed - on position, contacts conducting, there may be several
  on positions.
  

For example: the simplest on-off switch has one set of contacts (single pole)
and one switching position which conducts (single throw). The switch mechanism
has two positions: open (off) and closed (on), but it is called 'single throw'
because only one position conducts.

Switch Contact Ratings

Switch contacts are rated with a maximum voltage and current, and there may be
different ratings for AC and DC. The AC values are higher because the current
falls to zero many times each second and an arc is less likely to form across
the switch contacts.
For low voltage electronics projects the voltage rating will not matter, but
you may need to check the current rating. The maximum current is less for
inductive loads (coils and motors) because they cause more sparking at the
contacts when switched off.

---

Standard Switches

ON-OFF

Single Pole, Single Throw = SPST

A simple on-off switch. This type can be used to switch the power supply to
a circuit. When used with mains electricity this type of switch must be
in the live wire, but it is better to use a DPST switch to isolate both live and
neutral.

  

SPST toggle switch

---

(ON)-OFF

Push-to-make = SPST Momentary

A push-to-make switch returns to its normally open (off)
position when you release the button, this is shown by the brackets around ON.
This is the standard doorbell switch.

Push-to-make switch

ON-(OFF)

Push-to-break = SPST Momentary
A push-to-break switch returns to its
normally closed (on) position when you release the button.

Push-to-break switch

---

SPDT Centre Off

ON-ON

Single Pole, Double Throw = SPDT
This switch can be on in both
positions, switching on a separate device in each case. It is often called a
changeover switch. For example, a SPDT switch can be used to switch on a red
lamp in one position and a green lamp in the other position.
A SPDT toggle switch may be used as a simple on-off switch by
connecting to COM and one of the A or B terminals shown in the diagram. A and B
are interchangeable so switches are usually not labelled.
ON-OFF-ON



A special version of the standard SPDT switch. It has a third switching position
in the centre which is off. Momentary (ON)-OFF-(ON) versions are also available
where the switch returns to the central off position when released.

SPDT toggle switch

SPDT slide switch

(PCB mounting)

---

SPDT rocker switch


Dual ON-OFF

Double Pole, Single Throw = DPST
A pair of on-off switches which
operate together (shown by the dotted line in the circuit symbol).
A DPST switch is often used to switch mains electricity because it can
isolate both the live and neutral connections.




 DPST rocker switch

---

Dual ON-ON

Double Pole, Double Throw = DPDT
A pair of on-on switches which operate
together (shown by the dotted line in the circuit symbol).
A DPDT switch can be wired up as a reversing switch for a motor as
shown in the diagram.
ON-OFF-ON

DPDT Centre Off

A special version of the standard SPDT switch. It has a third switching position
in the centre which is off. This can be very useful for motor control because
you have forward, off and reverse positions. Momentary (ON)-OFF-(ON) versions
are also available where the switch returns to the central off position when
released.
DPDT slide switch




Wiring for Reversing Switch

---

Special Switches

Push-Push Switch (e.g. SPST = ON-OFF)
This looks like a
momentary action push switch but it is a standard on-off switch: push once to
switch on, push again to switch off. This is called a latching action.

---

Microswitch (usually SPDT = ON-ON)
Microswitches are
designed to switch fully open or closed in response to small movements. They are
available with levers and rollers attached

---

Keyswitch
A key operated switch. The example shown is SPST.

---

Tilt Switch (SPST)
Tilt switches contain a conductive liquid
and when tilted this bridges the contacts inside, closing the switch. They can
be used as a sensor to detect the position of an object. Some tilt switches
contain mercury which is poisonous.

---

Reed Switch (usually SPST)
The contacts of a reed switch are
closed by bringing a small magnet near the switch. They are used in security
circuits, for example to check that doors are closed. Standard reed switches are
SPST (simple on-off) but SPDT (changeover) versions are also available.
Warning: reed switches have a glass body which is
easily broken! For advice on handling please see the
Electronics in Meccano

---

DIP Switch (DIP = Dual In-line Parallel)
This is a set of
miniature SPST on-off switches, the example shown has 8 switches. The package is
the same size as a standard DIL (Dual In-Line) integrated circuit.
This type of switch is used to set up circuits, e.g. setting the code of a
remote control

---

Multi-pole Switch
The picture shows a 6-pole double throw
switch, also known as a 6-pole changeover switch. It can be set to have
momentary or latching action. Latching action means it behaves as a push-push
switch, push once for the first position, push again for the second position
etc.

---

Multi-way rotary switch

Multi-way Switch
Multi-way switches have 3 or more
conducting positions. They may have several poles (contact sets). A popular type
has a rotary action and it is available with a range of contact arrangements
from 1-pole 12-way to 4-pole 3 way. The number of ways (switch positions)
may be reduced by adjusting a stop under the fixing nut. For example if you need
a 2-pole 5-way switch you can buy the 2-pole 6-way version and adjust the stop.
Contrast this multi-way switch (many switch positions) with the
multi-pole switch (many contact sets) described above.




1-pole 4-way switch symbol

---

 Thanks for using this blog.................