# 2 Way Switching Circuit - Confusing Voltage

#### self builder 123

Hi

I have read all the info regarding 2 way switching on this web site. but I am afraid that I am still confused, hence the post.

I have a circuit in the house which contains 5 in number 12V halogen lamps powered via 5 LV transformers. There are 4 switches on the circuit - 2 x 2 way and 2 x intermediate. The circuit works fine.

I now want to remove one of the 2 way switches and replace it with a 2 way dimmer.

At the back of the 2 way switch there are 2 wires going into the COM terminal which are always live - this must be the power in ?

There is a red wire connected to 1 Way ( which I understand is L1 ) and and yellow wire with a red band of insulating tape connected to 2 Way ( which I understand is L2 ). I understand that both of these wires will become live at some point.

When I place a multimeter from the earth on the metal backbox to COM it always reads 240Vac which makes sense as this must be the main feed into the circuit.

With the switch toggled in the up position the voltage on 1 Way is 0Vac, while the voltage on 2 Way is 240 Vac. This seems to make sense.

When the switched is toggled into the down position the voltage on 1 Way is 240Vac, however I expected the voltage on 2 Way to be 0 Vac, however it measures 50Vac ???

Is this normal ? Where is this nominal voltage coming from ?

I want to replace the 2 way switch with a 2 way dimmer, but I am nervous about how to connect in the 1 Way ( L1 ) and the 2 Way ( L2 ) to the dimmer ?

Can anyone help ????

Cheers.

#### sparkticus

When the switched is toggled into the down position the voltage on 1 Way is 240Vac, however I expected the voltage on 2 Way to be 0 Vac, however it measures 50Vac ???

Is this normal ? Where is this nominal voltage coming from ?

I am assuming that by 1way and 2way you mean L1 and L2.

If you toggle the other two-way switch or one of the intermediates then L2 will change from 50VAC to 240VAC.

The 50VAC that you measure is most likely a potential difference (a voltage) between neutral (via the transformers) and earth which in reality is probably less than 50VAC. If you measured the voltage with a lower impedance instrument it would probably read much lower than 50VAC.

In any case, it is nothing very unusual.

#### sparkticus

I want to replace the 2 way switch with a 2 way dimmer, but I am nervous about how to connect in the 1 Way ( L1 ) and the 2 Way ( L2 ) to the dimmer ?

Can you post a picture of the back of the dimmer?

#### Spark123

It sounds like capacitive coupling of the cable, it is quite common where two conductors run in close proximity. You can measure it with a high impedance volt meter but falls off to zero volts when a load is applied.
I'm assuming the lights were off when you carried out the first measurement as this would cause the wires to read 240v and 0v as you say, with the second measurement if you switched the lights on? If so then the L2 switch wire is left floating and you will get measurements like above. If you switched the lights back off again by another switch you'll likely see L2 also drop to 0v as it is now indirectly connected to neutral via the internals of the transformers.
The above scenario is perfectly normal and nothing to worry about.
Simply, the dimmer switch connects just as the old switch. L1 to L1, L2 to L2 and com to com (often denoted by an arrow and squiggly line.)
The only thing you need to make sure of is the dimmer switch is matched to the transformers as some have leading edge switching, some have falling edge switching. Some also need to be de rated when used with halogen lighting too.

#### self builder 123

Hi many many thanks.

I will post a picture later this evening.

#### ban-all-sheds

I now want to remove one of the 2 way switches and replace it with a 2 way dimmer.
You do realise that if you turn the light on from any of the other 3 locations it will come on at whatever brightness the dimmer is set to, and that to change it you'll have to go to the dimmer?

Will that be OK?

#### self builder 123

Yes that will be fine.

Sorry I can not find my camera, however based on your posts I was able to install the dimmer and everything is working fine.

Many thanks to you all.

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