Great suggestion Don. I'm wondering if a TL431 could do D109's job. Also wondering if some robustness would be obtained by paralleling a pair of 3.6V 500mW zeners in D109's place without sacrificing any of the desired properties of a single component.
Someone previously said "most of these (zeners) fail shorted" implying that some fail open...could that be the case in Han's example that he shared was demonstrably reliable? The radio continues to work without D109 so without it crowbarring the 3.3 rail one would not notice that it failed open (like a fuse) until the next microcontroller blasting spike occurs.
'73 Robert AD6XJ
