crowbar circuit OrCAD simulation

[Skyland]

I have a couple of issues with the simulation;

1. : the zener diode has Vz at 4.7V, it is not behaving as expected since it is reducing the supply voltage by 4.7 instead of setting it to that voltage.
2. : the SCR is not triggering and short circuiting the PSU.
3. : Is there a way to add a spice model to a thermal fuse in capture.
4. : Any recommendations on the values of the resistors and capacitor used?

 

[crutschow]

1. That's what zeners do. Your expectations are incorrect. Zeners have a voltage drop across them equal to their rated voltage when conducting current.

2. The SCR should be triggered. Don't know why it's not.

3. Don't think so.

4. Not at this point.

 

[dick_freebird]

That SCR has a "typical" holding current of 5mA so you are
marginal for getting the SCR to latch on (. Try reducing your
1K series resistor to (say) 100 ohms. It's not like you're going
to "crowbar" the supply with all that series resistance anyhow,
why is it there in such a high value?

I suspect that the SCR model may be a bit funny, the 5.001
clamped voltage is just a little too cute and too close to
the 1K * 5mA holding current result. Maybe the model
deserves a closer look, re realism.