Electronic – How to decide between a TVS or Zener diode for overvoltage protection

The device is made to handle signals which normally operate between GND and V++. Any signal line transient that goes below GND is clamped by the lower diode to a voltage that is equal to the forward voltage drop of the lower diode. So you can see that clamping will be to less than about -1V in that case. (Clamping level really depends upon how much current surge there is caused by the negative voltage spike as the forward voltage varys depending on the diode forward current).

The device can be used with or without the pin 5 connected. If pin 5 is open any positive voltage spike on the signal pins is clamped to a level that is the sum of the forward voltage drop of the upper diode plus the reverse zener voltage of the TVS diode. Once again actual clamp level depends upon the amount of energy in the voltage spike and how much current it causes to flow through the clamp diode path.

If pin 5 is attached to a low impedance V++ voltage rail then the positive voltage spikes are clamped to a level about one forward voltage drop of the upper diode over the supply voltage level. In many cases though the leading edge of the positive voltage spike will have a very fast rise time and any series inductance in the wiring from pin 5 back to the V++ supply will prevent the supply from immediately being able to sop up the energy in the spike. This will cause the voltage on the pin 5 to rise up and then eventually be clamped by the TVS device.

On these devices the TVS device is not normally meant to be used for clamping an over voltage condition on the V++ supply rail. The device is just not rated for that behavior and the V++ supply output is rated for much higher energy output so as to be able to supply power to the rest of the circuit. If over voltage of the V++ rail is something that needs to be protected against then it is advisable to use a proper circuit for this purpose. Usually that would be called a supply crowbar and usually consists of a high power SCR and a trigger circuit. The SCR, once triggered is designed to short the supply output in either its current limited load or to overload the internal fusing circuit and cause that to blow out.

As for your #2, there is nothing "hard" about choosing the value of the resistor if you understand what is going on.

You take the desired peak voltage across the resistor and divide it by the operating current of the coil. That's it- good old George Simon Ohm's Law. The current immediately after the switch turns 'off' is the same (or a bit less) than it was before, so the voltage the switching device will see is the power supply voltage plus a diode drop plus the operating current times the resistor.

A bipolar TVS could be used in place of the zener + diode if the voltage you wish to clamp at is substantially more than double the power supply voltage. TVS devices are designed to clamp voltage under high current conditions so they're made to withstand massive surges in power (often in the 1kW range). A normal coil doesn't have that kind of surge, so it's a bit of a waste. If you put (say) a 6V bipolar TVS across a 12V coil intending to clamp to 18V, you will have a bad day, since the TVS will conduct in parallel with the coil and probably destroy the driver.

You could use a unipolar 18V TVS across the transistor though.