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