mains inline powertap with no neutral.

[johnnyfp]

Hi,

Any one got a schematic or advise on how I can put together a switch that  can control a load line, either transformer or candesant light bulb  without a neutral  line?

I basically want to replace a wall light switch with a cpu controlled circuit., which tend to only have live going to the switch where neutral  is at the light fitting.

I've seen some touch sensitive switches capable of doing this,

Cheers

Jfp

[jeroen74]

I looked into this some time ago. One way is to basically very lightly 'dim' the load and use the resultant voltage over your widget during the interruption as supply for your own local supply.  You could devise some regulation circuit that only turns off the load when the mains voltage is quite close to the required voltage.

[c4757p]

I was just going to suggest that. When the load is off, you can get power just by being in series with the load. In fact, if your circuit's power consumption is small, you may be able to get some power while it is on as well by inserting a small step-up transformer in series with it, and running off the stepped-up voltage drop of the transformer. Obviously you need some good protection for that, because if your load suddenly takes a large current it will induce a similarly large voltage in your circuit.

[IanB]

You could also have a charging circuit and a small backup battery. The circuit charges the battery through the load when the switch is off and runs off battery power when the switch is on. Of course this would only be effective if the switch is off for a decent amount of time every day.

[ptricks]

Safest way is with a battery powered control. Anything else you use to try to take power from the switched device is going to be overly complicated. Whatever you use you will still need battery power during the off stage so why not just make it battery powered from the start.

[c4757p]

I am in agreement with ptricks. It's fun to think of ways to pull power from the load line, but safety is important with something like this, and this sounds like a good way to go.

If you wanted, you could then add a separate battery charging circuit that just sits in parallel with the load switch, which should be much easier (I'm sure there lots of reference designs for mains-powered battery chargers out there). It may be prohibitively big, though, in which case I'd just recommend traditional batteries.

[jeroen74]

Unless you are going to mass-market your widget, I'd say go for battery power too. It will save you a lot of hassle

I've witnessed a project basically end up in a (sales) disaster because marketing thought that customers wouldn't mind adding a neutral wire. Well, it turned out they *do* mind, even in new constructions So they had to design a new version that required no neutral wire in a widget without any significant space left for even more components.

[c4757p]

I've witnessed a project basically end up in a (sales) disaster because marketing thought that customers wouldn't mind adding a neutral wire.

I don't understand why marketing people are permitted to be so clueless about the field they work in. Anyone would decent electrical knowledge would immediately recognize "no neutral wire" as an unusual enough request that there's probably a good reason for it.

[DarkPrince]

A generic off-the-wall light dimmer (cheap ones) uses the potential between the switch to determine firing angle. So they use the same concept, except only charging a 100nF cap so a very light load. Nothing crazy about it.

[G7PSK]

Most lighting switches here in the UK will have 3 wire + earth going to them even if unused as it may be required later if multiway switching is required also unless the fitting is metal the earth wire is blind so you could be naughty and use that at a pinch.

[Monkeh]

Anyone would decent electrical knowledge would immediately recognize "no neutral wire" as an unusual enough request that there's probably a good reason for it.

Laziness and tradition is the reason.

Most lighting switches here in the UK will have 3 wire + earth going to them even if unused as it may be required later if multiway switching is required also unless the fitting is metal the earth wire is blind so you could be naughty and use that at a pinch.

*snort*

Most light switches in the UK have just two conductors. You're lucky to get a CPC in any house older than about 1975, let alone having three cores just provided in case!

And if you use a CPC as a live conductor.. I have no words.

[pachuma]

I designed about two years ago an interchangeable capacitive glass light switch that retrofitted on a regular wall lightswitch.

The switch was powered in series with the load while it was off and while the load was on, I used part of the waveform to power the switch, aka widget, and then the rest to the load (dimming slightly the load).

One thing to be aware of is that it only works spotlessly when switching a resistive load. Driving an inductive load this way is not a very good idea. Therefore, CFL are not recommended. Even when the switch is off, some CFLs glow with a few milliamps going through them.

The PSU of the widget is capacitive, with a cross by zero detection being done by a micro-controller.
I'll try to dig up a picture of the fitted gadget and its little explicative PDF.

To be honest the thing I am most proud that gadget is the way you only see a clear sheet of glass on the wall and the fixing mechanism of the glass to the switch/sensor.

[c4757p]

And if you use a CPC as a live conductor.. I have no words.

I'm sure it runs thoroughly afoul of every electrical code on the planet, but I doubt there is any real technical/safety reason why it couldn't sink a couple mA. Still, don't do it...

[Monkeh]

And if you use a CPC as a live conductor.. I have no words.

I'm sure it runs thoroughly afoul of every electrical code on the planet, but I doubt there is any real technical/safety reason why it couldn't sink a couple mA. Still, don't do it...

Safety reasons like the CPC being present in certain cable types to allow a protective device to operate in the event of cable damage? Can't be anything to do with that.

[T4P]

where i live in the electrical standards are similar to the ones in UK and ... i've got 2 earths going into my lights 

[SeanB]

In and out to the light. They just cut the cable to put in the switch, and leave the neutral uncut. The PE would be used to earth the metal parts if a switch surround if needed.

[Monkeh]

In and out to the light. They just cut the cable to put in the switch, and leave the neutral uncut.

Uh.. Yeah, nobody does it that way.

[NiHaoMike]

Maybe use a capacitive power supply with a Y rated cap to ground? And what about the idea of using a current transformer to derive a little more power when on?

[T4P]

In and out to the light. They just cut the cable to put in the switch, and leave the neutral uncut. The PE would be used to earth the metal parts if a switch surround if needed.

Yeah but nobody does it that way ... no sloppy neutral uncut switch here, mostly 2 poles unless you're talking about the 3 way switches controlling 3 light fixtures which sometimes are very ODDLY wired in the trunking. Like seriously the one's in my parent's room requires jumpers and some puzzle solving

[johnnyfp]

This is a  retro fit idea, so most places will have a 2 core wire going to the light switch with live and load. Sux because there is no neutral to put a decent psu circuit. Hence the query. I have an X10 light switch which I may tear down to find out how they do it.

I like the captive approach suggested and would indeed be interested in the circuit diagram.

[cybergibbons]

The earth conductor should never be used as a return path. It's so far outside of the regs for very good reason.

[somlioy]

I asked about something similar earlier: https://www.eevblog.com/forum/beginners/1-phase-motion-detector-how-does-it-work

[johnnyfp]

What I found weird was in NZ the neutral and earth are tied together in the fuse box.! Is that normal? I''ve seen this in a few houses in NZ so not dodgy wiring in my hours.

[Monkeh]

What I found weird was in NZ the neutral and earth are tied together in the fuse box.! Is that normal? I''ve seen this in a few houses in NZ so not dodgy wiring in my hours.

Yes, it's called TN-C-S. One combined conductor in the supply, separate earth and neutral in the installation.

Although sane countries leave doing this to the supplier.

[pachuma]

OK, so here is an image of one of the light switches I've got installed around the house that uses the capacitive PSU and load switching.