NC contacts relay boards vs NO contact relay boards - Page 1
Forum section: electronics / projects

author: Cicada
author_rank: Regular Contributor
timestamp: April 27, 2022, 09:55:07 am
content: HiI have 100+ of these relays. G6B-2014P-US DC5https://www.digikey.co.za/en/products/detail/G6B-2014P-US%2520DC5/Z3640-ND/1815886Would like to use them for something.Usually, the hobbyist /RPi / Arduino multi-channel relay boards have SPDT (change over contacts) on them. This is good because one would mostly want NO contacts.But using the G6B-2014P-US DC5 relays on a board will leave you with 2 x NC contacts per relay.Are there any use cases for such configurations that anybody came across? Hopefully, some general use cases that I am not aware of. You can mention the obscure ones as well.Cicada

author: SeanB
author_rank: Super Contributor
timestamp: April 27, 2022, 10:30:21 am
content: You can double up the contacts, so getting more reliability, or 7A capability, or wire the 2 in series to make it more reliable against welding shut, though for that better to use the other spare contact as feedback, to have positive contact close indication. Just need to have more than 10mA of load on the signal side to get reliable switching, as they are rated for a minimum load of that.Yes NC is not the most common, though for a few applications, where the load is powered most of the time it will be better, and can also be put in power supply lines to a microcontroller, so that a watchdog reset also turns the relay on, and thus depowers the board, and when the voltage decays below hold on voltage for the relay the power is applied again, resetting the board against latch up or malfunction, and doing a hard reset on all logic, as against a soft reset alone. Otherwise not many applications, as you always are using power when off, which can be a problem.

author: Cicada
author_rank: Regular Contributor
timestamp: April 27, 2022, 12:27:50 pm
content: Hi Sean,You describe a handy application of a NC relay. Thanks.Below are examples that I have experienced myself.Manual emergency stop switches are usually normally closed i.e. when the button is not activated. A normally closed circuit is used to implement a "safe circuit" since it also inherently checks that the cable is not broken or disconnected along the path to the safety control electronics.Also. PIR sensors in alarm systems have relays with NC contacts for the same reason mentioned above. To check that the circuit is not accidentally or deliberately broken/disconnected.

author: Cicada
author_rank: Regular Contributor
timestamp: April 27, 2022, 12:29:38 pm
content: I found this.https://amperite.com/blog/normally-closed-relays/Here are just a few systems that require normally closed relays.Air conditioners:Normally closed relays are used to keep AC unit blowers running even after the air compressor shuts off, allowing the system to continue generating residual cool air without running the compressor.Gas valve control systems:In these systems, normally closed relays act as safety features, stopping power to the gas valve in the event of a malfunction.Telephone circuits:With normally closed relays, telephone calls can be terminated if no one answers within the specified amount of time.Dead Man’s Seat:Certain machines are set to shut down if the driver is no longer in the seat. However, an immediate shutdown would be inconvenient if the driver merely bounces or shifts and accidentally activates that safety feature. The normally closed relay provides a time delay that takes into consideration occasional driver bounces, but it maintains that safety precaution by allowing for a shutdown if the driver is out of the seat for too long.Explosive atmospheres:G-series time-delay relays are hermetically sealed, so they can be used in explosive environments without risking fires or explosions caused by sparks.Not sure about the last one.

author: Circlotron
author_rank: Super Contributor
timestamp: April 27, 2022, 12:57:48 pm
content: Some relays with N/C contacts are sensitive to vibration because the contact pressure is limited, so can give unexpected results in an automotive situation.

author: Cicada
author_rank: Regular Contributor
timestamp: April 27, 2022, 01:39:05 pm
content: Quote from: Circlotron on April 27, 2022, 12:57:48 pmInterestingAutomotive relays.https://greggdistributors.ca/electrical/Relays/hel965400071Disconnect lights from battery when cranking the starter.

author: elekorsi
author_rank: Regular Contributor
timestamp: April 27, 2022, 07:14:53 pm
content: Quote from: Cicada on April 27, 2022, 12:27:50 pmFor manual emergency switches, yes NC contacts are used. But if we speak about relays, NO contacts should be used, with the logic that in normal (safe) state relay is allways on. Why? Because in case of the device failure (power failure, relay coil failure, etc.) with the NC you will never get the safety signal disconnected, NC contact will just sit there connected.Same goes for all devices where you need to signal that everything is OK and not that something is wrong(for example alarm systems).Even safety related end-switches should be NO, with the mechanics made in such way, that the roller or lever of the switch is triggered in the safe state. In such case if there is a mechanical failure (broken lever of the switch), system detects the non-safe condition of the machine...