Overvoltage protection with a twist

[Ian.M]

Sounds/looks good, except you use the drop from 5V to 3.3V (nominal Vclamp) to calculate the pullup resistor for approx 2mA quiescent current shared between the TL431 and its voltage set divider. 2.5K starves the TL431.  Try  1K, as the divider only takes 1/4 mA , that gives about 1 1/2 mA for the TL431.

N.B. the cap wired A-K across the TL431/TLV431 ie *IMPORTANT* for the clamping transient response as the high frequency gain of both shunt regulators sucks. 

[GarthyD]

Sounds/looks good, except you use the drop from 5V to 3.3V (nominal Vclamp) to calculate the pullup resistor for approx 2mA quiescent current shared between the TL431 and its voltage set divider. 2.5K starves the TL431.  Try  1K, as the divider only takes 1/4 mA , that gives about 1 1/2 mA for the TL431.

N.B. the cap wired A-K across the TL431/TLV431 ie *IMPORTANT* for the clamping transient response as the high frequency gain of both shunt regulators sucks. 

Thanks. I've adjusted the R3 value. I might use slightly less resistance in the end as I suspect that my 5V input might actually end up being a fair bit less, and power efficiency isn't much of an issue on test equipment. Still, this is something I can experiment with to determine the precise value.

Many thanks for all of your input and guidance thus far. If I had continued down my original path of shunting to my 3.3V power rail, there is a good chance that, due to the fairly low current in use on that rail, that I would have eventually damaged both the test equipment and the boards I was testing.

[GarthyD]

Just a quick update: I ended up using the overvoltage protection based on Ian.M's recommendations when testing the prototype board I have recently assembled.

Here is the test setup:



The left two breadboards contain the test setup. The other breadboards contain the breadboard prototype, which drives the test setup. Buried at the top left of the second-to-left board is the overvoltage protection:



The breadboard prototype is being used to test a partly-assembled PCB prototype:



The board has been fully assembled since:



I talk about that a bit more here:

https://www.eevblog.com/forum/projects/general-purpose-sam4s-mcu-and-max-v-cpld-development-board/

Thanks all, especially Ian.M for setting me on the right path.