mains inline powertap with no neutral.

[PetrosA]

A lot of the occupancy sensors sold here in the US explicitly said to use the earth wire if there was no neutral available and with states like California mandating the use of these sensors, some buildings have pretty large loads on their earth wires. The 2011 code addresses this by mandating that all switch boxes must have a neutral run to them. This will also change how we wire three way switches and what kind of cables we need, but it will stop the loading of the earthing system.

[SeanB]

It is not a good idea to use a protective conductor as a supply return inside the house. It is not an expectation that disconnecting it at any point will lead to a shock hazard. Outside the supply authority treats all wiring as having a shock hazard unless proven otherwise, so the risk there is mitigated by having trained crews and rigorous use of procedures and test gear for work done.

[Zero999]

Try the attached schematic.

The MCU needs to be programmed to switch Tr1 for no longer than the amount of time required to discharge C1 and C2. Tr1 could easily be replaced with an SCR or IBGT.

[atwoz]

Try the attached schematic.

The MCU needs to be programmed to switch Tr1 for no longer than the amount of time required to discharge C1 and C2. Tr1 could easily be replaced with an SCR or IBGT.

Hero, have you tried this circuit? What happens when you use a CFL? Does it blink? That's what happened to me when I tried a method of suppling power through it.

[johnnyfp]

So I put  hero's circuit in a spice simulator. And there seems to be a  18v to -240v at the zener diode. Is that normal?

[HackedFridgeMagnet]

There shouldn't be 240v across the diode, if that is what you mean.
if the spice ground is at neutral then you will get a large voltage but not exactly 240v. maybe something closer to sqrt2  x 240v.

post your sim.  (.asc)

Personally I think the way to do it is with an extra wire. ( neutral ) Ten years down the track people will see that black wire and know what it is for.

[Uncle Vernon]

Personally I think the way to do it is with an extra wire. ( neutral ) Ten years down the track people will see that black wire and know what it is for.

More like 20 to 30 years. But your point is quite correct. Wiring practice has changed to reflect more complex needs and now it is more common to see Neutral and Earth at switch locations. Older practice was to run a twin of active and switched to switch locations and it was as a work around to this that the neutral less schemes were developed.

It's never going to be easy however, systems like C-Bus are becoming more common and these will throw an entirely different curve ball to many control solutions. 

[johnnyfp]

I moved the and to the low voltage part of the circuit from the neutral and now am getting sensible readings.

So trying to understand the circuit and why a zero threshold detect is needed. I can see that when the  fet is on the light bulb lights but the  control part of the circuit does not get any  charge through it. So does this mean that with this circuit you have  to switch of the fet to allow the main cap to charge to allow power to the control circuit?

What if the fet was moved to be in series of the bridge rectifier and a resistor put in parallel with the fet for when  the fet is of to allow continued power to the control circuit?

[Zero999]

No I haven't tried the circuit.

Yes, the MOSFET needs to be off to allow some of the power to be taken from the lamp to power the control circuit. The zero crossing is required so the MOSFET only turns on/off when the mains voltage is near zero. Obviously, the MOSFET needs to be off for long enough to allow C1 and C2 to charge.

There's no way to do this without a neutral, other than to take some power from the load to power the control circuit or use another power source such as solar power or a battery.