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relays - SPST, SPDT, DPST, DPDT?


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From what I understand, a relay has a control circuit and a power circuit.  If you energize the control circuit, the power circuit is closed.  I have seen relays advertised as single-pole single-throw (SPST), single-pole double-throw (SPDT), double-pole single-throw (DPST), and double-pole double-throw (DPDT).  I am not familiar with these terms.  I was wondering if anyone could give me an explanation, or point me towards something I can read to learn more.


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Well the power circuit is just a switch, the control circuit is just what moves the lever.

Now think of a simple On-Off switch with two connectors. It's a simple throw (one On position) simple pole switch (SPST). Two such switches operated by the same lever would be DPST.

If it was On-Off-On it would be SPDT and DPDT respectively.

HTH, Nikolas

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Hi Paul

SPST - Single Pole Single Throw - Otherwise known as an on-off switch.
SPDT -Single Pole Double Throw - Single pole changeover switch, ie, with no power to relay, centre connects to one side. When power applied, centre connects to other side.
DPST - Just two independent switches like SPST.
DPDT - Just two independent switches like SPDT.

Hope the illustration helps



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Thanks Ed,

That was a big help.  Just to make sure I understand correctly, whey you say...

DPST - Just two independent switches like SPST.

...By "independent', you mean that you can switch two separate circuits ON or OFF.  They are still controlled by just one controlling coil, right?  So, the two circuits are either both ON or both OFF, right? 

Please excuse my ignorance.  I am sort of new to this stuff.

Thanks again,

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