Electrical – Smart Light Switch power supply solution for 2-wire (without neutral) scenarios

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Considering a smart switch based on a power greedy WiFi ESP8266 (800mA spikes and 100mA on idle, @ 3.3V), there is the common situation to have the neutral wire missing from the wall switch box (left with 2 hot wires, maybe some earth wire that I don't want to consider in any way). Powering the electronics in the smart switch from the 2 hot wires will draw current through the light bulbs, making them either flash or light up (maybe except for some incandescent ones, but I can't rely on the light bulb type in any way; all usual types ranging from 1W to 100W, 110/230VAC should be considered).

At this point you might have already guessed the problem: what kind of smart switch design can bypass this issue?
Some suggestions I read were to use an extra capacitor in parallel with the light bulb so that the current drawn by the switch won't get through it. Although I'm a beginner with electronics and I don't fully understand how exactly should that work, or how to scale the capacitor (maybe someone could explain me), I only wonder why didn't the LED/CFL bulbs manufacturers just include this feature in their products, since this seems to be an universal issue. Are there any reliable solutions to this problem?

Best Answer

What kind of smart switch design can bypass this issue?

I have not seen a Wi-Fi switch that doesn’t need neutral. Perhaps this can be done, but I guess the power consumption is such that manufacturers choose different protocols.
There are popular generic RF (433 mhz, etc.), z-wave and ZigBee switches that work without neutral. All those protocols are designed around low-energy standards. Some can be used with a generic X to Wi-Fi gateway which is powered from mains.

Why do some bulbs require a capacitor in parallel for non-neutral-wire switches to work?

The switch requires some current to pass even in “off” state. If the bulb doesn’t let the current pass or if it starts to flicker as a result of this current, then you will be instructed to install a capacitor in parallel to the bulb.

Why does it solve the problem?

The capacitor forms a capacitive dropper (together with some components in the switch) that bypasses the bulb. A capacitor in an AC circuit forms a current limiter because when current flows one way the capacitor lets it pass until the capacitor is charged in one polarity and then when the current is reversed the capacitor discharged and lets the current flow the other way. The amount of current depends on the capacity of the capacitor and the frequency of the AC.

Also see this relevant discussion.

Why isn’t this standard in all bulbs?

Because it would waste energy. Unless you need some current to pass constantly to power this special kind of switch you shouldn’t install such a capacitor with the bulb.