Let's say you have a standard capacitive dropper power supply like the following (from Wikipedia):
I understand that these work well for driving LEDs since they act like a constant current source. I was wondering if the design could be (safely) simplified further by removing the zener and decoupling capacitor - the LEDs don't care what voltage they "see" as they're current-limited anyway, and human eyes cannot detect 120Hz flicker.
I mentioned this to a friend of mine and he pointed out that the mains is subject to voltage transients from things like lightning and inductive loads switching off, and that C2 would shunt the majority of these transients away, protecting the LEDs - that seems to make sense, I suppose C2 + the input impedance of the circuit would behave like an RC filter during a transient?
If that's the case, what purpose does the zener serve if voltage regulation isn't important, and why do so many commercial designs seem to include one? Simulating a similar circuit with a 50us/1kV spike (a la IEC 61000-4-5), the LEDs only see a small increase in voltage and current - C2 does limit the transient significantly...but if C2 were gone, is a sub-millisecond overcurrent even that bad anyway?