Electrical – Relays with NC and NO connections: does high vs low matter


Is there any difference between:

  • An active low relay with the load wired to the NC (normally closed) connector, and
  • An active high relay with the load wired to the NO (normally open) connector?

In either case, when the control signal is low, the relay will be open and the circuit will be incomplete. When the control signal goes high, the relay will close the circuit.

I ask because I've come across a number of situations in which people build a logic inverter to get active-high behavior from their active-low relay (which seems to be the most common type, at least for the N-channel relay boards commonly available online).

If you have both NO and NC connections, is the logic inverter necessary?

Best Answer

The power-off state is usually required to be safe or safe-er. This is a system-level requirement.

In most cases that will require the normally open connection to be used.

There are cases (for example certain types of heat applications) where hot is safe-er than cold and you want to use the normally closed and depend on some other device as a cutoff.