Using zener diode to protect test circuit?

How would you connect it? If I understand correctly then DZ1 is not being used as a shunt regulator. As soon as the voltage at the connection of R2 and R3 goes above Vref, the TL431 turns on all the way and Q1 snaps on. The TL431 is either all the way on or all the way off, never in between.

That means that the overvoltage trigger point is insensitive to the Vbe of Q1. It also means that M2 will be either on all the way or off all the way, instead of turning off slowly as Vin goes above the limit.

Jonathan

Track back through the posts to where I added a comment about sizing resistors when using the TL431... you can dramatically reduce the wasted current with some value changes.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Yeah. I remember our discussion on low voltage zeners from last year now. I agree with you, but I'll forget again.

Jonathan explains better too. I suspect the Engineer -- Scientist gap dominates the EE -- Chemist gap on this one.

I started with a zener because I didn't know what the TL431 even was this morning, and didn't have a model, and I wanted to poke around in other parts of the circuit. Not good engineering, but fine for a rainy morning's pasttime.

I placed

online. This zip file has an LTSPICE model for the TL431 in it. Perhaps more interestingly are the two pdf files, one for the circuit similar to Jim's, and one with the pesky zener. The discrepancies in part numbers are because I used what was at hand in LTSPICE.

What I noticed is the increase in power dissipated by the control circuit at about 5.29 - 5.34V for the zener, against lower power dissipation overall. I don't know if the model that I grabbed off the net gives realistic power dissipation in the TL431 or not.

Now I'm off to play with Win's hex inverter circuit. I actually may have parts for that, left over from the Goder booster.