Switches

Switch Contacts - pole, throw etc.
Standard Switches - SPST, SPDT, DPST, DPDT.
Special Switches - multiway, key, tilt, reed etc.

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.)

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.