SPST vs SPDT and Whether You Need a Changeover

SPST vs SPDT is a decision about one terminal: SPST connects or disconnects two terminals, while SPDT adds a third so that in either stable position of a conventional two-position part the common sits on one throw rather than leaving the circuit open. The buying decision is whether you need a second destination, a diagnosable fault state, or somewhere for a future revision to go. Swiclick builds both configurations widely, with the KAN series tact switches predominantly two-terminal and many G series micro switches offered as changeover parts alongside explicit SPST-NO variants. Price does not follow terminal count reliably, because on some bodies the three-terminal version is the higher-volume part. Both sit under the same standard and certification route, so the decision starts with circuit behaviour. No tact switch product name reviewed in July 2026 explicitly stated a changeover configuration.

Detail on each configuration sits on the SPST switch and SPDT switch pages. This page is only about the choice.

SPST vs SPDT Comparison Table

Decision rule: choose SPDT when you need a second destination, a diagnosable state, or a spare throw for a later revision. Otherwise SPST is sufficient.

Fault Detection Is the Real Divider

The most useful reason to buy the third terminal has nothing to do with routing signals.

A two-terminal switch produces an open circuit both when it is unactuated and when its wire has failed. Your controller cannot tell those apart. In a consumer product that may not matter. In a machine where an undetected broken wire means a guard reads as closed, it matters a great deal.

A changeover contact creates diagnosable states. Wire both throws to separate monitored inputs and exactly one should be active at any time, so both low or both high indicates a fault rather than a position. It does not by itself diagnose every open or short circuit, and a working diagnostic usually also needs pull-ups or pull-downs and controller logic that recognises the impossible states. The NO NC and COM labelling that goes with this is worth reading before you wire it.

None of this makes a switch suitable for a safety function. Personnel safety needs a device with certified positive-opening operation and a circuit architecture matched to the assessed risk level. Fault detection through a changeover contact is a reliability measure.

Where Each One Dominates

Product format pushes the choice more than most engineers expect.

Tact switches favour SPST. A metal dome pressing onto a fixed contact is naturally a two-terminal arrangement. No tact switch product name reviewed in July 2026 explicitly stated a changeover configuration, though product names do not always state the contact form, so confirm with Swiclick rather than treating that review as complete. If a finger presses a button through a keypad, the tact switch is usually the answer and SPST comes with it.

Micro switches often favour SPDT. Many snap-action bodies transfer a movable contact between two fixed contacts, so the changeover comes with the mechanism. Explicit SPST-NO micro switches exist in the same range, so check the part rather than assuming the format. Where a changeover body is the volume part, buying single-throw can mean paying for it anyway and leaving one terminal unused.

Slide switches offer both. Three-terminal bodies are the volume parts, and a two-position on and off body also exists.

The practical consequence is that the mechanism you need for the actuation usually decides the configuration before you get to choose.

Using an SPDT Part as SPST

Wiring a changeover part as SPST is standard practice, although it retains the three-terminal footprint and any associated cost.

Use the common and whichever throw gives the resting state you want. Connect to NC for a normally closed result and NO for a normally open one. Leave the third terminal unconnected and keep its pad on the board.

Two habits make this safer. Note on the schematic which throw is unused so a later reviewer does not delete the pad. And check the datasheet for whether the unused throw is left floating internally, because a small number of designs do not leave it electrically separate.

What It Costs

Terminal count is a poor proxy for price on this product family.

On micro switch bodies the changeover version is frequently the volume part, so the two-terminal variant can quote higher. On tact switches the two-terminal part is the usual form, and no reviewed product name stated a changeover. On slide switches both are common.

Ask for both prices at RFQ rather than assuming. The difference is usually small enough that fault detection or a spare throw is worth more than the saving.

Selected part numbers in both configurations may be covered by UL component files, with approval granted per model and rating rather than per series. Confirm the exact model against the current certificate.

FAQ