How is this switch powered? I thought all circuits needed a neutral?

[DanMcLiverpool]

Usual problem with smart switches - no neutral at the switch, been plenty of threads on it before.

Fortunately managed to find some switches that don't need one. So the circuit's permanently 'complete' since the switch is permanently 'on', and actually power is going to the light, but not enough to actually glow.

My question is this: I'm not an electrician so I'm new to this, but I thought all circuits needed a neutral to be 'complete' and provide power, due to the potential difference between the live and neutral. Yet this switch intercepts only the live, so how does this device not need neutral? Or more to the point, how is it getting power?

Thanks in advance, I know I'm missing something basic here, as I say I'm just still in the process of working all this out.

 

[Wilko]

Hi - yes the circuit is completed via the load’s impedance. If the lamp was burnt out the switch wouldn’t work either. But the switch has nothing to switch then so no problem.

By Ohm’s Law, with a very very small current flow through the internals of the switch’s circuitry there is very little voltage drop across the load, which itself is connected to N and completes the circuit.

 

[DanMcLiverpool]

Thanks for the reply mate, appreciate it!

So is my understanding correct that a device that needs power, typically, has to sit between a live and neutral and the voltage (pd between live and neutral) will give the power there? But in this case it's the current that powers it? I may have missed the point here sorry.

Just to be clear, I'm not asking from a practical point of view - I get that the switch will work, I just have an obsession with the 'why' of things and I'm new to this.

Thanks again, nice one.
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Basically, this contradicts the (VERY) little I know about electrics. I thought the premise was that a device needed a live and a neutral, this doesn't.

 

[Wilko]

Thanks for the reply mate, appreciate it!


So is my understanding correct that a device that needs power, typically, has to sit between a live and neutral and the voltage (pd between live and neutral) will give the power there?
Yes

But in this case it's the current that powers it? I may have missed the point here sorry.
Remember that Power (in Watts) is the product of V and I. So the very small amount of power consumed by the switch in standby mode comes from a very small current flow and a voltage likely to be close to 230V.

Just to be clear, I'm not asking from a practical point of view - I get that the switch will work, I just have an obsession with the 'why' of things and I'm new to this.

Thanks again, nice one.

Hi - I’ve answered a bit more in the post. Good on you for being interested.

 

[telectrix]

it's similar to the "finfme in the dark" switches used to be from MK. with only a L and a Sw/L at the switch, an indicator light is wired across the switch.the light illuminates with the switch in the Off position, as the Sw/L is close to 0V through the load.

 

[marconi]

A no neutral switch still has some electronics inside it which need to be powered by a constant low dc voltage - say 5Volts. The general idea is to include a bridge rectifier ( the diamond in blue with ~ and + and -) in the circuit comprising of the switch S and lamp L as shown above. The low voltage smoothed dc is from the red and black stabilised by the shunt power zener diode and capacitor (shown in yellow). The lamp current always flows through the bridge and power zener diode. This will be a high current when the switch is closed so the lamp will illuminate. When the switch is open the current is small and not enough to illuminate the lamp and of a magnitude determined by the impedance of the series capacitor and resistor (in blue) across the switch - still enough to cause the power zener to conduct and produce a 5V low current supply for the electronics. In practice the switch S may be a relay, a power transistor or a silicon controlled rectifier (thyristor) under the control of the switch electronics.
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By this method a constant low voltage dc supply is produced in the switch when wired only as a normal switch with line and switched line. without this trick a neutral would have to be provided at the switch to power a low power from L and N an ac to dc converter for the electronics.

The ring type doorbell uses a similar trick when it replaces an ordinary bell push.