First off:
Just because the data sheet says it is rated for 220v AC, DO NOT CONNECT MAINS AC TO YOUR NVR!!
This is dangerous for you and for the NVR because there is very little isolation between the relay and motherboard. You might fry your NVR!
What is the contacts?
This little green contact block on the NVR;
It typically has a screw connector plugged in like this:
Here's the back of an NVR I own, this photo doesn't show the screw terminals plugged in:
The relays are marked 'NO#' & 'C#'.
Top row is top two terminals. Bottom row is bottom two terminals.
NO - Normally open, C - Common. These NVRs don't typically give you an NC (normally closed) contact.
To wire to a device, connect in series to the relay.
When the relay is triggered, it closes the circuit and your device powers up.
Why is AC rating different to DC rating?
AC - Alternating Current
DC - Direct Current
A diagram for voltage to time:
You see DC is constantly at a voltage, whereas AC is a sinewave and goes to 0v at points in time.
Why is the AC rating for a relay (a switch) higher than DC?
- AC is easier to disconnect when under load.
- A switch is two metal contacts, these physically move apart to switch off.
- Both AC and DC then have electrons jump the air gap as it moves apart (arcing).
- Depending on voltage, this can arc could be very big.
- AC will arc, but as the sinewave crosses 0v the arc extinguishes itself.
- DC never reaches 0v, so it can continue arcing.
- Arcing damages the relay contacts and creates a lot of heat. This is how relays melt and catch fire.
