Electronic Parts

Discrete Components

Poles, Throws, Contact States, and Ratings

The words used to describe switch contacts are compact once you know what they mean. They tell you how many separate circuits the switch controls and how many connections each circuit can select.

Pole

A pole is an electrically separate moving contact or circuit controlled by the same switch mechanism. A single-pole switch controls one circuit. A double-pole switch controls two separate circuits together.

Throw

A throw is one of the contact paths a pole can select. A single-throw switch normally makes or breaks one path. A double-throw switch selects between two paths.

• SPST — single pole, single throw. A simple on/off switch.
• SPDT — single pole, double throw. One common connection selects either of two outputs.
• DPST — double pole, single throw. Two separate circuits are switched on or off together.
• DPDT — double pole, double throw. Two SPDT sections operated together.

The same naming pattern continues beyond two poles. A 3PDT has three SPDT sections operated together, a 4PDT has four, and so on. A two-position switch with twelve ganged SPDT sections can therefore be called a 12PDT switch. Manufacturers may instead describe the same idea as twelve SPDT sections, twelve poles with two throws, or a multi-deck selector.

Common switch markings

Contact arrangements in schematic form

A dashed mechanical link is often used to show that two contact sections are operated by the same actuator. Exact symbol styling varies among schematic standards, but the electrical relationships are the important part to recognize.

Center-off and multi-position switches

A double-throw switch does not always have only two mechanical positions. An SPDT or DPDT switch may have a third center position in which the common contact is connected to neither throw. This is commonly marked ON-OFF-ON. Rotary selectors can extend the same idea to three, four, six, twelve, or more selectable contacts per pole.

Mechanically ganged contacts

When two or more contact sections are drawn with a dashed mechanical link, the dashed line is not an electrical connection. It means the contacts move together because they share the same lever, button, shaft, or other mechanism. This distinction becomes important with relays and large rotary switches, where many electrically isolated circuits may be controlled by one actuator.

Normally open and normally closed

The word normal describes the switch when it is not being actuated. A normally open (NO) contact does not conduct in its normal state and closes when operated. A normally closed (NC) contact conducts in its normal state and opens when operated. Changeover contacts often provide a common terminal plus both an NO and an NC terminal.

Momentary and maintained

A momentary switch returns to its normal position when the operator releases it. A maintained or latching switch stays in the selected position. Some switches combine the two ideas, such as ON-OFF-(ON), where the position in parentheses is momentary.

Break-before-make and make-before-break

In a multi-position selector, break-before-make means the old connection opens before the new one closes. This avoids momentarily joining the two selected circuits. Make-before-break, sometimes called a shorting contact, briefly connects both paths during the transition. Which is desirable depends on the circuit.

Contact bounce

Real contacts do not always change state once and stop. When they strike each other they can bounce mechanically for a short time, producing several rapid electrical transitions. Lamps and motors usually do not care, but digital circuits may count those transitions as several presses. Hardware or software debouncing is therefore often used with mechanical switches connected to digital inputs.

Identifying an unknown switch with a meter

When markings are missing, a continuity tester or ohmmeter can help identify the contacts. With power removed from the circuit, check which terminals are connected in each switch position. An SPDT section will usually reveal one terminal that alternately connects to either of two others. Repeating the test for every position makes it possible to draw a small contact table before wiring the switch.

Never use an ohmmeter or continuity tester on a circuit that is energized.

Ratings matter

A switch must be suitable for the voltage, current, and kind of load it is expected to control. A contact rating for a resistive load does not automatically mean the same switch can handle the same current through a motor, transformer, solenoid, or large capacitor. Inductive loads can create a high voltage when opened, and lamps or capacitors can draw high inrush current when first switched on.

AC and DC ratings may also differ. Direct current does not naturally pass through a zero-current point every half cycle as AC does, so an arc can be harder for a small switch to extinguish on DC. Never assume that an AC rating is also a valid DC rating unless the manufacturer specifies it.

Other useful specifications

• Contact resistance
• Insulation resistance
• Dielectric or withstand voltage
• Mechanical life and electrical life
• Operating force and travel
• Temperature and environmental sealing
• Terminal style and mounting method

Return to the Switches overview.