On/On vs On/Off/On Toggles

[DLW]

Hello,
Is there a standard nomenclature to know if an SPST/SPDT is On/On or On/Off/On. I can usually tell by looking at the schematic, but this information isn't always available. I wish this information was just specified on the build doc or BOM...

[EBK]

You've nearly spelled out the nomenclature in your question.

ON-ON
ON-OFF-ON, sometimes called center-off

If the switch is momentary type, parentheses are used around the corresponding ONs and OFFs.

There are also ON-ON-ON DPDT switches, which are very useful and can even be wired as a SP3T.

[DLW]

I should have worded my question better... If it only says SPDT in a build doc, how am I supposed to know if I should use an On/On or On/Off/On switch?

Also, is it a safe assumption that SPST will always use an On/On switch?

[EBK]

If it only says SPDT, it should be on-on.  On-off-on is more of a specialty switch, so it should certainly be spelled out in the docs if it is used.

A SPST is always an on-off switch.  It only has two contacts, so it can't have two on positions.  Although a SPDT is usually used, with only two terminals connected, where a SPST is called for.  In this case, an on-on SPDT makes a good SPST switch.

[madbean]

I should spell it out more clearly (although I think I have in more recent docs). SPDT is always On/On unless otherwise specified.

[DLW]

Got it. Thanks, guys.