How does a two-wire (no neutral) smart switch power itself while the lightbulb is ON?

Thread Starter

LMF5000

Joined Oct 25, 2017
63
So, I just bought a Sonoff T2 smart switch. Basically it replaces a normal light switch with a smart switch that can be flicked on and off remotely over WiFi. This is the two-wire model, since in this house the switch boxes don't have any Neutral or Earth wires - there's just the Live wire going into the switch box (coming from the main circuit breaker), and the Live wire going out of the switch box (going to the light fixture).

The switch has no built-in power supply (battery), so it somehow draws current from the mains to power itself. Part of the installation involved putting an "anti-flicker" device in parallel with the lamp. I've disassembled this device and it's just a pair of polyester capacitors across the lamp's live and neutral wires.

So my theory for how the switch powers itself when the light is OFF is that with the switch and anti-flicker device all in series with each other across mains voltage, a small current flows through the capacitors in the anti-flicker box and the switch, and it's able to harvest that. So far it's simple.

What I can't understand is how the switch powers itself when the light is ON, since the relay clicks and allows full current to the light. If the two terminals of the smart switch are essentially shorted together via the relay, how can the switch draw any power? This switch can handle up to 15 Amps. I didn't see any low-resistance shunts in the internal circuitry, I was able to identify a bridge rectifier, transformer, large electrolytic capacitor and various diodes, transistors and ICs.

Can anyone think of a way the switch is harvesting power with lamp on? I've attached pics of the wiring diagram and switch's power board and anti-flicker device. The power board stacks onto the logic board via the 8-pin connector.
 

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MaxHeadRoom

Joined Jul 18, 2013
21,191
There are devices that operate with only two conductors, for both (small) load and also indication in the way of LED indicator.
The 2 wire proximity switch for e.g.
This operates as a normal contact switch to indicate when it is energized, the sense circuit also has to remain active and also light the on/off LED.
It operates on a very small leakage current.
Max.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
63
There are devices that operate with only two conductors, for both (small) load and also indication in the way of LED indicator.
The 2 wire proximity switch for e.g.
This operates as a normal contact switch to indicate when it is energized, the sense circuit also has to remain active and also light the on/off LED.
It operates on a very small leakage current.
Max.
But how does it draw that leakage current when the two conductors are shorted together by the relay (when the light is on)? With no potential difference between them it can't draw any current right?
 

Thread Starter

LMF5000

Joined Oct 25, 2017
63
Are the relay contacts directly across the external terninals, or is there something in series with them?
Hard to say - I've tried following the circuit from the photos (since the switch is all wired up now and a bit of a hassle to remove) but I can't really tell. The external terminals seem to both feed into the bridge rectifier, and from the relay datasheet I know the switch terminals are the two top ones, but I haven't been able to successfully trace the circuit.
 

AlbertHall

Joined Jun 4, 2014
10,226
If the relay directly shorts the terminals then I can't see any way the circuit can get any power. So I think there must be something in series so the answer to your question lies in finding out what that something is.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
63
If the relay directly shorts the terminals then I can't see any way the circuit can get any power. So I think there must be something in series so the answer to your question lies in finding out what that something is.
That's what I was thinking. Could it be the transformer? I can't really imagine it passing 15A through the primary though. What else could it be? It's not a shunt. Maybe a diode or transistor?
 

ronsimpson

Joined Oct 7, 2019
811
I do not know but:
We built a 24Vac thermostat that had this problem. When the switch is closed there is no power to live in. We opened up the switch for a fraction of a second and stole the power for that time period. (that only works if there is a load)
When the switch is open we "leaked" 1mA through the switch and lived on that. The load did not react to 1mA or to a hole in the power for 1/2 cycle.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
63
Well, I've figured it out by googling a little better. I unearthed this video explaining exactly how they did it:

Basically there's a triac in series with the relay and it blips the power off 50 times a second - just long enough for the power supply IC to harvest the energy needed to keep the power supply capacitor charged to 12V.
 
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