John, I only spent about 5 minutes trying 4 or 5 versions, and I accept that I might have made a mistake. What you see is the first arrangement that worked. I'll have a go tomorrow and see if I've maligned you.
Fair enough. regardless of who is right, I certainly don't feel 'maligned', and it may well be me who has, at least in some senses, been wrong, so I look forward to what your further exploration reveals!
If you weren't aware this is LTspice, is free, and a good way to check things like this fast.
Yes, I have LTspice, on the recommendation of someone here (probably you!) a good few moons ago. However, I have to confess that I never managed to climb very far up the (seemingly fairly steep) learning curve, and therefore have used it very little. Maybe you have stimulated me to have another go
I've had enough for today, so sorry for the wait.
No problem - I know the feeling ! In the meantime, doing some 'thinking aloud' (very rough ballparks, again 'ignoring' the rectification) aloud ....
It seems to me that probably the only way in which there could be a flaw in my thinking is if I am misunderstanding and hence underplaying the effect the rectification has, since if one just sticks to thinking about ('full wave') AC, none of it is much more than Ohm's Law ....
Your assumed 5 kΩ, load (at 15V) represents 3 mA, which sounds fairly credible for the timer module.
If there were no current going anywhere else (like through the zener, when one has rectification), that 3 mA travelling through your 15 μF 'dropping capacitor' (reactance/impedance about 212 Ω at 50Hz) would result in a voltage drop of a mere 0.6V ! On the other hand, with my suggested 0.14 μF dropping capacitor (reactance/impedance about 22 kΩ at 50Hz), the voltage drop would be about 66V.
Those drops are still too low. However, if, not unreasonably, one assumed that, an additional, say, 6 mA of the current going through the dropper was going somewhere other than through the load (e.g. like through a zener, when there is rectification), hence a total current through dropper of 9 mA, then your 15 μF capacitor would still only drop about 1.9 V, whilst my suggested 0.,14 μF one would drop about 198V - very much closer to 'what the doctor ordered'
My assumption was that none of the above is appreciably conceptually changed by introducing the rectifier. At first sight it might seem that the (average or RMS) currents would be roughly halved, but I think that may well be to some extent offset by the fact that the large 'smoothing capacitor' across the load maintains current through the load (which has to come from somewhere) which is pretty constant throughout both half cycles of the supply. However, as I've said, it is here (the impact of the rectification) that their might possibly be a flaw in my reasoning.
However, I have to say that my thinking above (about capacitor values) is consistent with my practical experiences. As I've often mentioned, in my house I make fairly extensive use of capacitors for 'switched dimming of (usually 5W - 12W) cheap LED 'lamps'. My experience is that adding anything like 15 μF has no noticeable effect on LED brightness. Depending on the degree of dimming I want (and the LED wattage), I generally use capacitors in the range 0.015 μF to 0.22 μF or thereabouts.
Kind Regards, John
