how to deal with voltage spikes from a mechanical relay that is triggered by AC mains

Thread Starter

studbike

Joined Feb 22, 2018
12
Goodmorning, I am a DIYer, and have worked with relays many times before for heaters, motors, etc, but not for computers/sensitive electronics.

I am creating a battery backup for my home security system (see picture below). I got nervous after reading about voltage spikes from the coil de-energizing. I have tried searching for quite a while, and I don't understand what to do when the relay is triggered by AC mains... or if this is even an issue worth worrying about.

Before someone mentions it: the security system has its own batteries internally, but they only last about a day. Therefore, the switching time of a mechanical relay will be a non-issue.

MVIMG_20180223_104717.jpg
 

ericgibbs

Joined Jan 29, 2010
15,705
hi stud,
Welcome to AAC.
A simple AC relay coil snubber circuit would help in reducing 'electrical noise'.
A snubber is a capacitor usually 100nF thru 220nF and a series resistor 47R thru 220R, connected across the relay coil.
The capacitor must be mains rated.

An alternative would a MOV [ metal oxide varistor].
E
 

Thread Starter

studbike

Joined Feb 22, 2018
12
thank you for the prompt replies!

i would lean towards the varistor in the interest of using 1 component instead of two. can you recommend a varistor spec range?

Out of curiosity, I have another question:

is there any reason not to use a single pole relay with the hot wires running through the NC and NO terminals, and just attach the ground wires together? Here's another drawing for clarity:

MVIMG_20180228_122802.jpg
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
R/C Snubbers come in one component when purchased off the shelf.
By Ground I assume you mean power common, if so, yes you could just switch the +ve side.
Max.
upload_2018-2-28_11-40-29.png
 

MaxHeadRoom

Joined Jul 18, 2013
25,469
The reason I prefer R/C snubbers is they are always active, the MOV does not come in to effect until a certain level is reached.
Also, if drastically exceeded, they self destruct, often without showing any symptoms if internal to an enclosure.
Max.
 

ebp

Joined Feb 8, 2018
2,332
In that situation there is pretty much zero concern. There will be other devices on the line that will happily eat the tiny amount of energy when the AC goes off. Any transformer connected to the AC line (e.g. for doorbell, thermostat, etc. will dump a far higher amount of energy into the line under the same circumstances.
 
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