Right... There is no "simple answer" because there are different ways to use a Zener.
If you go over the Zener voltage the diode conducts. If there isn't something to limit the current you'll fry something (the Zener or something else).
[u]Here[/u] are a couple of over-voltage protection circuits. One uses a Zener and one uses regular diodes but they BOTH have current-limiting resistors.
Sometimes when a resistor won't work because you need the current (such as in a power supply) the resistor is replaced with a fuse (to protect the Zener). In that application, it's assumed that an over-voltage condition is rare or will "never happen" because if it does you'll have to replace the fuse.
Generally speaking, zener diodes are for reference voltages and TVS diodes (zeners on steroids) are for protection.
But neither might be ok for your electromagnet.
An electromagnet (solenoid) just needs a diode across the coil to protect the transistor.
Google something like "solenoid circuit diagram", and go to images.
Leo..
Wawa:
An electromagnet (solenoid) just needs a diode across the coil to protect the transistor.
Ah, so you did the forensic research and attempted - vainly - to imagine what a solenoid, a bridge rectifier, a Zener diode and an 18B20 have to do with each other in a "fire-fighting robot"!
Your crystal ball doesn't show that?
Time to put a new one on your Christmas wishlist.
Crystal ball are becoming more and more the default tool to help new posters.
They seem to think we are mind readers.
Leo..