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Regarding the wiring of a relay, it seems to me it is customary to wire the hot side (L1, +) of power to the COM terminal, and not NO/NC terminals (which would be the outputs). Is there a reason for this? Is there any reference? Or does it just support logical sequential flow when reading a schematic wiring diagram?

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  • \$\begingroup\$ Could you make it a bit clearer? With schematics to illustrate your points. \$\endgroup\$
    – Eugene Sh.
    Nov 29 '17 at 18:31
  • \$\begingroup\$ In a Form C contact (one with a COM / NO & NC) there are two states, energized and not energized, in these state there are two possibilities, COM is connected to NC and COM is connected to NO. In no state is NO connected to NC. \$\endgroup\$
    – Tyler
    Nov 29 '17 at 18:44
  • \$\begingroup\$ if you are routing power to two places then use the COM terminal. ... if you are using the relay as SPST switch, then it makes no difference ... NOTE: accessibility to measuring probes could be a factor to consider \$\endgroup\$
    – jsotola
    Nov 29 '17 at 19:18
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There are two schools of thought on this...

Option 1: Line to common..

schematic

simulate this circuit – Schematic created using CircuitLab

This is the classic way to connect it, and gives you the option to pick off the normally closed connection later if you need to.

Option 2: Line to normally open..

schematic

simulate this circuit

This method stops you from later connecting in the normally closed line, however it also means you do not have unconnected live terminals in the relay which may provide a safety benefit.


For most relays, electrically, and mechanically, it does not make much any difference. If your relay has special features, it may list a preferred connection method in the specification data sheet.

Failing any manufacturer instructions, which method you chose is really arbitrary.


ADD: If you are using multiple relays within the same system, or even multiple products, it is however prudent to follow the same wiring on them all. Set a company standard as it were.

Do not have some connected live on common and others live on normally open.

If your do you will just confuse, and potentially harm, whomever is working on it, and probably even yourself.

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Personally, I usually prefer to wire the line to the COM terminal because it's typically closely coupled to the coil, especially with the cheaper types of relay- there is often a metal part that runs right beside the coil. Fancier types sometimes have pusher construction that better isolates all of the contact metal- and often sport high coil-contact breakdowns of many kV.

The N.O. terminal goes to the load, where there is a often lot of noise and high voltage transients at switching, especially with an inductive load. The photo below is of an automotive type relay, but the internal construction is similar to inexpensive common types such as JS1 from Matsushita (but turned on its side).

enter image description here

All the metal with red arrows is connected to the com terminal, only the metal with the green arrow is connected to the N.O. terminal.

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