SPST = Single Pole, Single Throw SPDT = Single Pole, Double Throw
DPST = Double Pole, Single Throw DPDT = Double Pole, Double Throw
Several terms are used to describe switch contacts:
Pole - number of switch contact sets.
Throw - number of conducting positions (only used for single and double)
Way - number of conducting positions.
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.
A simple on-off switch
A simple on-off switch has one set of contacts, single pole,
and one switching position which conducts, single throw.
This is type of switch is called SPST (single pole, single throw)
and its action is described as ON-OFF.
The switch mechanism has two positions: closed (on) and open (off), but it is called 'single throw'
because only one position conducts.
A simple push switch
A simple push-switch, such as one for a doorbell, has one set of contacts and the on position
is only momentary, as soon as you release the switch it goes back to off.
This action is called push-to-make (push to close contacts).
The momentary action is shown by using brackets like this: (ON)-OFF.
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.
This is a special version of the standard DPDT switch shown above.
It has a third switching position in the centre which is off.
This can be useful for motor control because you have forward, off and reverse positions.
Microswitches are designed to switch fully open or fully closed in response to small movements and small forces.
They are available with levers and rollers attached.
Microswitches are often used as sensors in machinery to detect the position of parts including doors, for example
they may be used to stop a machine if a door or panel is opened which exposes moving parts.
Normal switches are likely to suffer from damaging arcing (sparking) at their contacts when
they are not fully open or closed, microswitches are designed to avoid this problem.
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.
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 website.
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 switches have 3 or more conducting positions and they may have several poles (contact sets).
The symbol shows a 1-pole 4-way switch.
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 a multi-way switch (many switch positions) with a multi-pole switch (many contact sets) described above.
have kindly allowed me to use their images on this website and I am very grateful for their support.
They stock a wide range of switches and other components for electronics and I am happy to
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