With most analog circuits, it's a very wise idea to simulate the design before prototyping. SPICE is the de facto tool for this and comes in many different flavors and variants. Essentially a numerical differential equation solver for circuit primitives, SPICE can be used to give you a good approximation of real-world circuit performance. However, non-trivial circuits involving complex ICs can often be challenging to model.LTSpice IV is a popular simulator and most LT parts have free SPICE models available for download. I wanted to simulate the TRIAC trigger circuit and set out to find a model for the Zener diode. TI doesn't provide any models for the LM431, but they do offer several for the TL431, which should have similar circuit behavior.

After attempting to adapt the HSPICE and PSPICE models to LTSpice with lackluster results, I ended up downloading TINA-TI (TI's flavor of SPICE simulator) and using the encrypted TINA module of the TL431. My transient analysis file is located in the ZIP file at the bottom of the page in the sim directory.

##### TINA schematic

##### 1 ms transient analysis simulation (click for full size)

From the transient analysis, we can see that once the input voltage reaches just about 6 V, the Zener breaks down and the TRIAC gate voltage falls to 1.8 V this is below the gate threshold voltage for Q3 operation and so we can safely assume that the TRIAC will start conducting and latch until the fuse blows.