Well, me not doing that initially, lead to the excellent explanations by @matthewb with much better illustrations than I could do, so…
Seriously though, I’ve been watching quite a few Chernobyl documentaries lately and it only reinforced my belief that engineers need to get used to understanding and planning for the worst case scenario no matter how unlikely that might be.
While the worst case scenario that could happen here is a loss of an expensive V5 brain in the middle of the Worlds qualifying eliminations, my assumption is that some of the students reading this will be designing systems in the future that may have serious catastrophic failure consequences and it is important for them to learn how to properly think about the worst case scenarios. It is called “culture of safety” and only works when all: design engineers, operators, managers, and regulators are on the same page.
We already identified the first mistake when Taran (and I guess majority of people who watched the tutorial) must have subconsciously assumed that black wire going out of MC29 is synonymous with electrical ground and didn’t see anything wrong with joining them together.
@matthewb gave us a very good illustration how this could lead to the electrical short and, while some could argue that they will program it correctly, I am convinced that you must never underestimate how easy it is for someone to make a programming mistake or a typo in the middle of the competition.
Hardware solutions like protective diodes or connecting LED’s common negative to the V5 electrical ground is always a better option.
I am sorry if it sounded all gloom and doom, but the key point here is that there are way too many unknowns to conclude that accidental shorting of the ADI 5v power rail is not going to be a problem in all cases.
VEX didn’t publish 5v power regulator schematics and part numbers for us to lookup and read the specs, as well as do an online search for any reported issues.
There are two ways power regulators implement protection: thermal shutdown and current limiting. Thermal shutdown is designed to protect from the long term slow heating by currents well below those that would trigger current limiting protection in case of the short.
In top of the line parts those two protections are tightly integrated, but in the cheaper designs they are not necessary integrated or tested together (because they want to sell parts advertised with max specs for the given size of the power drivers, which are the most expensive part).
Such parts will shutdown correctly when you test current limiting by shorting the cold part and then correctly thermal shutdown after the steady load slowly heats the part.
However, in the real life, you may have the long period of steady load that will “preheat” the part to the temperature just below that of triggering thermal protection and then hit it with the burst of high current just below the current limiting threshold. By the time the heat wave arrives to the temperature sensing element - the power driver could release the magic smoke and turn into a blob of the molten metal.
In the best case scenario, you will end up with a permanently damaged part that doesn’t conduct any current and, in the worst case, you may end up with high input voltage shorted to the regulator output or even to the ground. Well designed parts will have an additional integrated piece of thin wire that acts as the last resort fuse to break the circuit, but you never know what corners have been cut in the cheaper part, since they don’t report those specs.
The bottom line is: without looking up specs of the parts inside the V5 brain and running variety of tests on them, you cannot rule out the possibility of 5v power rail regulator being permanently damaged or even (although unlikely) melting down and passing the short to the next circuit level.