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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?

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  • \$\begingroup\$ Do you have an example of this or a schematic you can share? \$\endgroup\$ – AlmostDone Mar 29 '18 at 19:51
  • \$\begingroup\$ what is an active low and active high relay ?? .... are you talking about the logic level required to energize the relay? \$\endgroup\$ – jsotola Mar 29 '18 at 19:58
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    \$\begingroup\$ it's about the relay state while off and or booting up the MCU \$\endgroup\$ – dandavis Mar 29 '18 at 20:46
  • \$\begingroup\$ the only difference is what the relay does when the power is off. if you don't care about that there is no difference (except power consumption) \$\endgroup\$ – Jasen Mar 29 '18 at 20:59
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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.

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A relay by itself is neither "Active High" or "Active Low". The Normally-Open contacts will close when power is applied to the coil.

I assume you are referring to the relay boards sold for use with Arduinos and other microcontrollers. Those boards have some additional ciruitry - Opto-coupler, transistor, etc. - that may provide an "Active Low" or "Active High" input. With an "Active Low" interface, the relay should operate when the input is low. With an "Active High" interface, the relay should operate when the input is High.

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In logic both are equivalent but the power offstate needs to be what is needed.

There is also the inverted logic case, low side coil drive to NO contacts.

The advantage here is a simple SPST-NO or 1P1T is cheaper than SPDT and is compatible with open collector drivers or bipolar or CMOS alike.

This latter option which you did not include is the most common. Although off-the-shelf arrays of opto isolated relays permit you to drive either anode or cathode side to choose normal or inverted logic.

THe choice is NC vs NO is always determined by your application needs and nothing else. i.e. "What is Normal" is relative to your operation.

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