Low cost mains power supply

For a small amount of power (and efficiency in the single digit percentage range), a rectifying bridge, a zener for regulation, a current limiting resistor and a capacitor or two is all you need. A switch and fuse are probably a good idea too.

Isolation is key for safety on something like that.

I've got one here it has a capacitive dropper and a tiny 4mAh NiCd cell (I assume NiCd because the charge rate printed on it is C/10) the AC switch is a 48V relay and there's a TO92 device which probably switches it. image in ABSE

I one time needed a super efficient 5Vdc [allowed to be directly connected to the mains] power supply but had NO room for the size of caps needed for the normal drop stage. So, I designed up a small circuit that connectd the mains to the rectifier diode starting at zero crossover and DISCONNECTING when the mains input went above around 7V and started to take the 5 Vdc output voltage up to 5.1, or so. thus I had efficiency AND regulation.

Look at some of the smart meter app notes from Atmel, Microchip, etc. They have examples of small offline supplies.

The mains voltage rises to about 7V input (5V output) in about 100us and to about 7.1V input (5.1V output)in 111us. So you have about 11us to charge your storage capacitor. Then, the storage cap must hold up the 5v for about 16ms until the next rise (this assumes 170V peak, 60Hz, half-wave).

I must be missing something very important and I would appreciate your help in understanding what it might be.

Many thanks, John

John S schrieb:

I consider it useful to mention mains voltage (and frequency) when presenting such calculations.

I also think that "7.1V" is somewhat tight.

The Harris HV-2405E used this principle, of course with wider thresholds.

Oliver

They were mentioned (170V peak, 60Hz, half-wave) in the part that you snipped.

And of course, it all depends on the current consumption. With very low currents used, the cap would only discharge slowly.

correct, sorry!

Oliver