Clamping a negative voltage supply rail?

[humidbeing]

I'm designing a low power -5V supply circuit. How do I go about protecting voltage from going too negative? Like if the 7905 failed and decided to let -12V through to the output?

Normally, for a positive supply, I would use a polyfuse then a varistor across + and GND. Do varistors work on a negative supply? I googled but couldn't find the answer.

Here's an example of what I have so far. (Capacitance not shown).

[Zero999]

Yes, varistors work with either polarity, but a crowbar circuit would be better.

[humidbeing]

Seems impossible to find crowbar examples for negative supplies.

[Kim Christensen]

Just move the 0V, or ground reference point "to the top":
 

[pqass]

Or use a TL431 (vs zener) for better control (see attached screenshot and simulation).
To trigger, adjust the pots until 2.5V appears between TL431 ref and anode.  The simulator seems to have a problem (keeps tripping after backing-off the pots and clicking RUN/Stop then Reset buttons) so just reload the page for a fresh start.
Circuit taken from here.

You can adapt this to your specific need by placing the triac+fuse before your regulator and the rest (pot+resistor divider, TL431) after your regulator. Maybe even drop the pot for a fixed 1.1k; together with the 1k, the crowbar would trip at about +/-5.4V.

[Doctorandus_P]

There is no real difference between a "positive" and a "negative" voltage. It is only a matter of taking some arbitrary point and defining that as a zero or "GND" reference. Often a "logical" zero / gnd reference is chosen, but this is only a convention to make schematics easier to read.