How do Wi-Fi controlled light switches feed themselves?

Question

There are smart light switches in the market that are Wi-Fi controlled like Plumlife (formerly ube) and Belkin WeMo. I wonder how they control the electricity that they operate on.

When I unplug my old fashioned light switch, I see that there are two wires. I believe that they work in this way.

But in smart switches, there must be some inner circuit that contains a relay inside of the smart light switch. If current goes through that circuit, that means smart light switch is closed too; therefore bulb is on all the time.

How do these products run on and control the same electric wire. If there is an external power source that feeds smart light switch like a fellow stackexchanger mentions in this question , I understand. But there is not. So what I ask is how is it possible to feed a circuit that controls it's own electricity.

Answer 1

Most likely it uses a transformer and a relay. The transformer brings it down to a DC current with usable voltage for the wifi and anything else that is needed. The relay then turns on and sends power to the bulb. The power is always going to the switch and the transformer to keep the wifi working.

I would think since the wifi uses such a low power and there are only 2 wires that there is always a small amount of power going through. It must use a current limiter to not let enough power through to turn on the light bulb as it is using the power it needs to let the wifi work properly.

Answer 2

The WeMo web page specifically says that a neutral wire is required!

Another method, which I think 'smart' thermostats use, is to have a battery to maintain power (the battery charges when the switch is off). The batteries will eventually wear out, and Belkin lists the lack of a battery as a feature.

Answer 3

The simple answer is that the switches' electronics are in parallel with the load, which it controls via a SSR or triac, solid state devices, instead of a mechanical relay.

^{simulate this circuit – Schematic created using CircuitLab}

It is in parallel because these switches normally require the neutral wire be connected, and typical house wiring has both wires going to the light switch box, then run to the light fixture.