Question/Help

Tinder Surprise

I have to centrally power qty:7 magnetic door locks, but not all the time (seer below)

Each lock will draw 12v 1amp

I have been recommend a central power supply at 12v 10amps

Here is where it gets interesting...

I will need to wire each of the magnetic door locks via a very expensive piece of kit.

The kit has a relay rated at 12v 2amps that will energise the lock (via the - cable) when it hears a radio transmission from a tag.
So essentially speaking not all 7 mag locks will energised at the same time, but they could be at some time if each has a tag near.

If my central PSU is doing 12v 10amps am I risking blowing the relay on the VERY EXPENSIVE PIECE OF KIT who's relay is only rated at 12v 2amp?

Questions :
1. Does the extra amps across the relay matter?

2. If it does matter is there something I could put in front of the relay to protect it against the extra amps?


All advice welcome

Thanks

2011

I would imagine that the very expensive piece of kit has a relay per door.
In the event that you allow a higher current to be switched by a relay than it is rated for there is a high chance that if will fail prematurely.

A simple solution would be to allow the relay from the expensive kit to drive another relay which is suitably rated that would switch the high current load(s).

Remember most relays are designed to switch AC or DC, not both.

Tinder Surprise

2011 wrote: »

I would imagine that the very expensive piece of kit has a relay per door.
In the event that you allow a higher current to be switched by a relay than it is rated for there is a high chance that if will fail prematurely.

A simple solution would be to allow the relay from the expensive kit to drive another relay which is suitably rated that would switch the high current load(s).

Remember most relays are designed to switch AC or DC, not both.

Thanks for the reply.

There is a very expensive piece of kit at each door.
So each of the 7 doors has the 1 kit and a 1 mag lock.
The problem is all power is coming from a central point/PSU, so 7 cable drops (one to each)

The 12v power for the lock will be from a central PSU/cable lopped through the relay of the kit.

When the kit is activated it closes the relay (negative cable loop via N/O and COM) giving the lock its 12volts when that relay closes.

My concern is that the relay on the kit is rated at 12v 2amp, but the central PSU will be pushing 12v 10amp down the cable.
When the relay closes then that relay (potentially) could have 12v 10amp across it.

Do amps really matter when considering damaging relays etc?

brightspark

Just because the power supply can supply 10A, it doesn't mean it will.

The current drawn through the relay will be whatever the load current is on that part of the circuit.

It might help if you posted a circuit diagram so we can be sure of what you are doing.

A fuse could help protect the relay (if for example the cable or the door magnet shorted)

2011

Tinder Surprise wrote: »

There is a very expensive piece of kit at each door.
So each of the 7 doors has the 1 kit and a 1 mag lock.

So get the relay from each kit to drive a suitably rated relay. This relay could even switch 100A if required.

The problem is all power is coming from a central point/PSU, so 7 cable drops (one to each)

So you need to ensure that the cables are appropriately sized or the volt drop may be excessive.

The 12v power for the lock will be from a central PSU/cable lopped through the relay of the kit.

It sounds like each lock only draws 1 amp.

When the kit is activated it closes the relay (negative cable loop via N/O and COM) giving the lock its 12volts when that relay closes.

..........and you can use this to drive an external relay. That way the kit realy will see very little load.

My concern is that the relay on the kit is rated at 12v 2amp, but the central PSU will be pushing 12v 10amp down the cable.

No. The PSU will be supplying a total of 10A but only 1 amp through each individual relay.

When the relay closes then that relay (potentially) could have 12v 10amp across it.

Why? Only the current of one lock will flow through the relay which is 1 amp.

Do amps really matter when considering damaging relays etc?

Yes. Overloading a relay can destroy it.

2011

Just to add, I would select a larger power supply unit if I were you. When run near capacity they run hotter and this can impact the reliability and lifespan of the unit.

Tinder Surprise

Thanks guys.. Really great advice.

I've sourced a Central 12v 10amp PSU that has fused 1amp outputs (x8) so that will guarantee the relay downstream will only ever see the 12v 1amp

So that's shloud sort that.

Next concern ; one of the kits is ~50metres from the central PSU so voltage drop concerns rears its ugly head.

Gonna use a 1pair BELDIN equiv cable :

http://www.edlireland.ie/iopen24/228760-p-88.html?cPath=0_1462_1407


.. Any thoughts welcome

2011

Tinder Surprise wrote: »

I've sourced a Central 12v 10amp PSU that has fused 1amp outputs (x8) so that will guarantee the relay downstream will only ever see the 12v 1amp

As already suggested it would be better to select a larger power supply unit. Why not go for a larger PSU?

A strange cable choice in my opinion as this is normally used for some sort of signal rather than for supplying DC power.
Anyway here is the data sheet for 8760 Belden cable. As can be seen the conductor resistance is 21.3265 Ω/km. That means that your 50m cable run (there and back) will have a total resistance of about around 2.13 Ω which is quite high due to the small conductor size.

As the volt drop on the 50m cable run will be the current (1 amp) times the resistance this will mean that the voltage at the door will be around 9.9 volts which may not be enough.