Smart light switch powered without a neutral - how does it work?

Thread Starter

lesneypark

Joined Dec 31, 2015
47
I have come across a smart light switch made by LightwaveRF which contains some electronics to turn on/off the lights, dim the lights and allow the user to control the lights remotely using a smart device or remote control.

The switch appears to generate its power from the incoming unswitched live wire in and the switched live wire out, there is no neutral required at the light switch.

This is one of a handful of similar switches powering electronics in the same way - without a netural - and I'm intrigued as to how it works.

Does anyone have any ideas as to how this works?
 

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MikeML

Joined Oct 2, 2009
5,444
It uses the bulb as the Neutral feed, if you remove the bulb the circuit wont work.
So how does it get power after the light bulb is lit @ full brightness?

There is an impedance in series with the internal power switch (Triac) across which a small voltage is developed, likely implemented as a minimum firing angle of the Triac. This effectively "steals" a bit of voltage (~5V?) that would otherwise appear across the bulb. Saying it another way, instead of the bulb getting 120V at full brightness, it gets 115V...

The power supply inside the light switch has to contend with two conditions: bulb off, it gets ~120Vac. Bulb on, it gets a minimum of 5Vac; more if the bulb is only being dimmed.

Do an experiment. Wire said dimmer/sw to a 100W lamp. Measure the AC voltage across the dimmer at various brightness settings, especially at full-brightness. The minimum voltage/waveshape across the dimmer will tell all...

Even a $5 rotary-knob dimmer bought at the Home Depot works this way...

I'll say it again: If you have to ask, you shouldn't be screwing with this stuff...
 
Last edited:

GopherT

Joined Nov 23, 2012
8,012
Even if the switch powers from 100% to 95% of full power for 1 or 2% duty cycle to charge a capacitor! there will be enough power to keep a triac "alive". All within the spec of mains power.
 

WBahn

Joined Mar 31, 2012
26,156
Could you provide a specific part number for the module that you use in place of a wall switch?

I went to their site and the stuff that I am seeing all involved modules that plug into (or replace) standard wall outlets.
 

Thread Starter

lesneypark

Joined Dec 31, 2015
47
There is an impedance in series with the internal power switch (Triac) across which a small voltage is developed, likely implemented as a minimum firing angle of the Triac. This effectively "steals" a bit of voltage (~5V?) that would otherwise appear across the bulb. Saying it another way, instead of the bulb getting 120V at full brightness, it gets 115V...
I looked through the triac section on this websites textbook where I noticed the image below.

03226.png

Is this the circuit for what you are describing?
 

Thread Starter

lesneypark

Joined Dec 31, 2015
47
I can think of about three ways to do it.
From what I've read and what has been said on this forum any viable solution will need to use the lamp as the neutral connection to complete the circuit. Power will only be available to the light switch when the lamp is installed in its receptacle.

Assuming that the switch consumes power continuously to keep its radio alive the switch must restrict the power flowing through the lamp to absolute minimum to prevent the lamp from illuminating when it is not required to be on. I've watched a few youtube videos for a similar switch from GE and this seems to be demonstrated in the videos.

I can see two possible solutions - adding a series resistor and using a triac, both functioning as traditional style dimmers - I'd be keen to hear your thoughts on the potential solutions.
 

WBahn

Joined Mar 31, 2012
26,156
The key to most approaches is that the module itself requires very little power. So when the switch is open you can simply put it in parallel with the switch and use the load as the neutral connection and the current draw will be so low (the module looks like a high impedance series block) that the lamp won't light. The problem is when the switch is on. Now it shorts out the module plus you can't get the voltage difference you need. You can deal with these issues by incorporating switches in both paths to alternate between the two (and you are using one of these switches (triac, FET, or otherwise) anyway to operate the load). So you reserve a portion of each cycle for the purpose of powering the module (pumping up its storage capacitor, most likely).

Another approach is to put a current transformer (or other transformer that will not be seen as a significant series load) and use that to power the module's electronics. When the light is off, you still have to use it as the neutral connection.

Yet another approach, which they use on a number of their products, is to put a battery in the darn thing. Given the current requirements for its functions, a battery might well last ten years or so.
 
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