How does a Shelly 1L work with no neutral?

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johno1234

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#315070 11-Jun-2024 12:10

This is Shelly's internal schematic:

Shelly 1L

And this is the no-neutral wiring schematic:

1L without N wiring diagram-20240528-141338.png

It seems to me that when the internal relay is closed, there will be a 0V potential difference across the L and O terminals. In that case, how does this device remain powered and not simply switch off? Is it magic?

richms

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#3247425 11-Jun-2024 12:29

Are you sure its a relay and not a triac?

Richard rich.ms

sir1963

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#3247429 11-Jun-2024 12:40

Nope it will be using a small amount of leakage current through to the load giving it the differential it needs

johno1234

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#3247432 11-Jun-2024 12:51

sir1963:

Nope it will be using a small amount of leakage current through to the load giving it the differential it needs

I read similar but still don't understand it. If the relay is closed, and the load is active, the L and O terminals are connected so can't have a potential difference. They are the only two wires going into the Shelly so how is it powered? I think there must be some sort of internal low power load and bypass?

Clearly I am missing something!

If it was a triac as richms mentioned, then there would be the small forward voltage drop, but that would still not be sufficient to power the ESP chip.

johno1234

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#3247433 11-Jun-2024 12:53

BTW I've read that the 1L does click loudly when switching so would appear to be a regular mechanical relay not a solid state device.

mkissin

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#3247434 11-Jun-2024 12:54

If you read the specs carefully, there's a 20W minimum load without the neutral. That'll be to guarantee a minimum level of current flow, so they're probably deriving internal power from the voltage drop across a shunt or current transformer type thing.

johno1234

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#3247442 11-Jun-2024 13:21

mkissin:

If you read the specs carefully, there's a 20W minimum load without the neutral. That'll be to guarantee a minimum level of current flow, so they're probably deriving internal power from the voltage drop across a shunt or current transformer type thing.

I think I almost understand now.

This guy opened one up - definitely a relay inside:

https://www.youtube.com/watch?app=desktop&v=KyUSPOKIa2s

at 5:24 you can see what does look like a shunt inline with the load (the right hand two pads are L and O terminals)

So it is taking a parasitic voltage drop across an internal shunt when the switch is closed... and therefore needs a minimum current to be flowing hence the bypass requirement for <20W loads.

Thanks!