How does a time delay contact remain open
They're not just a relay.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Pulse relay.

This relay will turn on with Vin but switch off automatically when the time delay is complete.


On power up (Vin goes +) Vc1 will be 0 V. The output of the Schmitt trigger will be high, Q1 will turn on and the relay will be energised.
After a time delay (approximately R x C seconds) C1 will have charged up enough to trigger the Schmitt trigger and its output switch low. Q1 will turn off and the relay drop out.
D1 protects the transistor from inductive kick when switching off.
The circuit is simplified and doesn't have a quick discharge for C1 when the power is switched off.
The timing diagram on the right shows the relationship between input and output.
Making R2 variable allows user adjustment of the time.


There are many configurations of these timer relays with delay-on, delay-off, one-shot, pulse-train, etc.




  The symbol inside the timer--two parallel lines--sometimes means
  capacitor and sometimes means relay contact, I guess. What is it
  representing here?








simulate this circuit

Figure 2. A capacitor and draftsman's contact.

The symbols look similar but the lines on a capacitor are close together giving a simple representation of the two plates of a capacitor separated by a small gap. The draftsman's contact is just an easier way to draw a contact than the switch shown in the relay in Figure 1. (Ignore the dots in the symbol. The schematic editor puts them in when wires touch.)


  I'm not advanced enough to know what a triac is, but how could you
  tell there was one present here? Continued thanks to you for your
  instructive answers!!








simulate this circuit

Figure 3. Symbol for a triac.



Figure 4. The TRS51A13S2A dual timer module.

There are a couple of clues that it's not a relay (with coil and moving contacts).


The physical size makes it unlikely.
There's no need. Relays are expensive relative to solid-state devices.
The datasheet says, "Fully Solid State And Encapsulated". This one settles the issue.


A triac is a semiconductor device with two power terminals (top and bottom of the symbol) and a gate (sticking out the side of the symbol).
It acts like a normally off switch but if a voltage pulse is given to the gate the triac turns on and remains on until the current somehow falls to zero.
With AC supply the current is reversing 50 or 60 times a second, depending where you live. Since it's reversing it crosses through zero 100 or 120 times a second. All we have to do is trigger when we want current to flow and turn off the trigger when we want it to stop. The triac will turn off on the next zero-cross.
Triacs have no moving parts and are cheap.