I've seen questions about selecting TVS diodes before (for example, How to choose the correct TVS diode and how to size it?; there are others, but they aren't about TVSs for protecting relay coils). However, the answers there are lacking easy-to-understand guidelines. The best information I've found is from https://www.eevblog.com/forum/projects/how-to-choose-tvs-diodes-to-dispel-flyback-surges-from-relay-coils/:
just ensure the peak current rating of the TVS exceeds the current drawn by the relay/contactor
However, even that leaves some open questions, which are not helped by this blog which asserts that higher is not better... although the example they give seems to assume that peak pulse power is directly related to maximum clamping voltage, which does not seem to be the case in practice?
For the sake of clarity, I'm going to use a specific example, however I am looking for answers that can be generally applied.
Say I have a G5LE-1A4 DC12. Coil voltage is 12V nominal, ~20V maximum, with a draw of 33 mA (0.4 W). My power supply (IRM-10-12) claims ±0.3 V (±2.5%) which, IIUC, means I shouldn't see more than 12.3 V on the relay coils when they're energized.
Question 1: What is an appropriate voltage for a TVS? The previously-cited forum thread advises choosing a breakdown voltage at least double the nominal coil voltage, but IIUC this guarantees that the coil will be exposed to an over-voltage condition (24 V > 20 V)? It seems that a diode with Vclamp-max < 20 V would be a better choice. Such diodes exist with common Vbr-min of 12.4 V or 13.3 V.
Question 2: What should I look for in peak power? (Alternatively, how do I whittle down the options once I've picked a breakdown/clamping voltage?)
Continuing with the concrete example, let's say I'm ignoring the 'double voltage' guideline and looking at something that isn't going to over-volt the coil, i.e. breakdown closer to nominal and clamping as low as possible. Let's say I'm also 'lazy' and want a bi-directional diode. P6KE13CA, SA12CA and 5KP12CA all look like viable candidates. One, however, has a much higher peak power (3 kW vs. 500 W and 600 W) compared to the others. Still, if I'm understanding the preceding advice correctly, even the smaller number exceeds what I need by orders of magnitude. (Is that bad? The previously cited blog seems to think so, but as noted, its reasoning seems suspect. Moreover, I'm not even finding TVS diodes with peak pulse power less than 400 W.)
Besides "obvious" criteria such as "does this fit on my PCB", or the even more prosaic "does anyone have this in stock", what criteria can be used to narrow the set of potential parts?
In summary, how do I go from a relay's coil properties to selecting a specific TVS diode?