12V AC to 12V DC Relay?

http://australia.rs-online.com/web/c/semiconductors/discretes/bridge-rectifier/

Use a full wave bridge rectifier, not a relay.

Here is one I generally use; Radio Shack 276-1181.

This one works at 50 or 60 Hz.

You have not fully described your quagmire; I will make some assumptions here, let me know if I am off.

Your intercom system is powered by 12v ac, by way of a plug in transformer. When you are ready to open the door for a guest, you push a button and it sends a 12v ac signal to the door.

Your card reader and door strike are powered by 12v dc and since these are stand alone devices, you don't have a power supply for them.

You are actually mixing elements of a security/access control system with an intercom system, something others frequently do, albeit with the technical know how.

The door strike is probably powered closed and fails open, so it needs a constant 12v dc power supply. Your card reader also needs constant 12v dc to work. As the others have said, use a bridge rectifier to provide that power from your 12v ac supply.

Your card reader should have a built in relay to interrupt power to the strike (normally closed contact), and should have external activation terminals (signal from intercom).

WJMFIRE, you bring up a pretty good point: the rectified pulsed DC may not be suitable for the card reader. As you say, there are a number of things undefined.

This pulsed power is no trouble for a solenoid in a strike, but might present trouble for the electronics in the reader.

The true solution for this is to use a power supply with the correct filtered voltage output.

Totally agree.

Now I am confused. The OP stated that he had the DC Power Supply, this leaves me wondering if all he needs is a 12 volt AC relay with a set of dry contacts to route the DC to the card reader and door magnet.

I would think a fair sized electrolytic capacitor across the suggested bridge rectifier could solve the problem of the pulsed DC, as has been stated the number of undefined variables is problematic.

Since I know nothing about this system I guess I will ask the question. Am I wrong in thinking that there will be significantly more energy stored in a DC magnet, vs. AC? I'm just wondering if the discharging magnet could produce enough signal to cause popping in the audio system.

"I'm just wondering if the discharging magnet could produce enough signal to cause popping in the audio system."

That is a common issue with electric door hardware, mostly with larger solenoids. A MOV will usually solve that particular issue satisfactorily. Simple, easy, effective, and inexensive.

As you say, the question is neither framed nor worded well.