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I came across a circuit that switches line-connected heating coils using relays. The relays are SPDT, and are connected as so:

schematic

simulate this circuit – Schematic created using CircuitLab

Why connect the relay like so? My intuition would be to just leave the NC contact open. Is there some subtlety that I'm missing?

The only thing I could come up with is that connected these two leaves more room for thicker copper on the LINE trace. Behold the relay's pinout:

Relay pinout

This way one could route a beefy trace through the NC and common (left and center, respectively) instead of slimming down at the common pins to avoid the NC ones.

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  • \$\begingroup\$ I would say no reason. \$\endgroup\$
    – Eugene Sh.
    Aug 3, 2018 at 17:18

2 Answers 2

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It could simply be layout, especially if it's this kind of relay:

enter image description here

If the relays are stacked close together a fatter trace may be possible if the PCB layout person runs the COM close to the N.C., in which case it should be connected if it is too close to safely withstand the expected voltages.

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    \$\begingroup\$ This was my thought, too, yet I cannot verify due to not having access to the layout or physical board. \$\endgroup\$ Aug 4, 2018 at 15:47
  • \$\begingroup\$ Yes, that's possible with your relay type too. It's common to adjust the schematic to make the layout better. \$\endgroup\$ Aug 4, 2018 at 16:03
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One reason would be this: floating nodes are bad, and can create problems (especially in a vacuum, the metal can reach any potential).

This rule also applies to inputs or amplifiers that are in a dual package (one amplifier or IC that is unused can cause problems by oscillating for example.)

But in this case it's probably irrelevant if it's tied to the line or not.

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