How does a deadlatch physically work?

[silverfox17]

I understand that when a lock is properly installed, the deadlatch should rest against the striker plate (my terminology may be a bit off, so if I'm using a wrong word/term please correct me). The deadlatch physically prevents the lock from being shimmed open with something like a credit card.

What I am looking for is a schematic / information about how this is physically happening inside the lock. My guess is that the deadlatch has spring loaded "piston" on the back, and when the piston is pressed in far enough it locks down behind the latch preventing it from being pressed in.

That's just a guess, however - anyone know how they really work and preferably have a diagram?

[Robotnik]

There are numerous types of deadlatch, but I'm assuming you're talking about a residential-type, grade 2-ish latch. Patent image below illustrates an exploded view of such a latch (it's s huge image, so I've linked rather than embedded):

http://patentimages.storage.googleapis.com/US6536812B1/US06536812-20030325-D00019.png

When the deadlatch plunger is depressed with the latch extended, a small piece shifts position and blocks the latch from being pushed back.

[C locked]

They do basically operate as you described
As robotnik has covered the tiebolt type
Here is the adams rite commercial mortice deadlatch
http://www.google.com/patents/US5113676

[peterwn]

I came across an old British Union deadlatch nightlatch years ago. When latch retracted by key or turn knob and released it only extended 80% or so of usual distance. When the door was closed and it hit the strike it would extend the whole way and deadlock. There would be some mechanical toggle arrangement to achieve this. A problem with this is if someone pushes the latch bolt it would fully extend with the door open, the door will not properly close to the bewilderment of the user.