How to wire a dimmer switch?

Bill Banks · Apr 21, 2011

I have bought a small red bulb to try out developing Ilford Ortho+ by inspection and I want to wire it up through a dimmer switch to minimise the risk of fogging. But I can't get it to work!

The dimmer switch has 3 terminals marked L1, L2 and Com. All I need is a simple circuit with switch, bulb holder and mains plug. Can anyone help?

Bill

mgb74 · Apr 21, 2011

Looks to me like you have a 3 way dimmer switch (i.e. 2 switches controlling a single light). Here's the first reference I found googling "3 way switch single light": http://www.ehow.com/how_6196311_wire-three_way-switch-single-pole.html

mr rusty · Apr 21, 2011

I have to say, if you don't know how to wire a dimmer switch, you shouldn't be attempting it - messing with mains can have unforseen consequences!. Having said that, its a 2 way switch. common is the main feed and L1 and L2 switch alternately when the switch is pressed in and out, so wire neutral to one side of the bulb and live to the other with the dimmer switch in series in the live feed connected to common and either L1 or L2 - it doesn't matter which.

N467RX · Apr 21, 2011

mr rusty said:
I have to say, if you don't know how to wire a dimmer switch, you shouldn't be attempting it - messing with mains can have unforseen consequences!

Totally agree. Since you're in the UK, I assume you have 230V, and I would think twice the idea of messing with the wiring. I've had my share of incidents with circuits, and if you don't know what you're doing it can be dangerous. And AC works slightly different to DC (which is a bit easier to work with, at least in my opinion). Also, I hope you bought an incandescent bulb, as most CFLs won't work with dimmers.

tkamiya · Apr 21, 2011

Hi....

Dimmers are just like switches except it's not ON and OFF. There are infinite "in between" between fully on and fully off. As already stated by previous posters, you appear to have a 3 way dimmer. You have a choice. You can either cap one of the "L" terminals or you could buy a regular dimmer. They'll both do the same thing.

Imagine a simple circuit. You have 2 wires coming from your plug. They both connect to a light socket. It's always ON. Now, imagine cutting one of the wires and splicing in a switch. You can turn it on and off. Now imagine, replacing that switch with a dimmer. You can adjust it from off to on and in between. There you have it. Typically COM (common) goes to the plug side and the other part you called L1 and L2 goes to the lamp1 and lamp2.

Now, you are dealing with a house-hold voltage. While it is not lethal in most situations, you must take special care since you'll be using it in darkroom where you can't really see and you have water/fluid. It CAN kill you in these situations. You'll need to be careful with wiring, take care to ground anything metal (if you use metal case for it), and depending on situations, use GFI (ground fault interrupter).

If you aren't sure of what you are doing, I caution you and hope you get some local help. You may even be able to get some help from a place you purchased your parts. I do have an electrical license but I'm not able to see what you have exactly and I'm guessing what you have from your descriptions.

But in a nutshell, what I said above is how you'd wire it.

Bill Banks · Apr 22, 2011

Thanks, everybody. I had already tried exactly what you have described and will now conclude that the old dimmer switch I found in my garage is just faulty! I was just wondering if I was missing something. Re-reading my original question, it does look a bit as though I shouldn't be messing with wires doesn't it? I'm fine - honest!

Bill

Steve Smith · Apr 22, 2011

tkamiya said:
Imagine a simple circuit. You have 2 wires coming from your plug. They both connect to a light socket. It's always ON. Now, imagine cutting one of the wires and splicing in a switch. You can turn it on and off. Now imagine, replacing that switch with a dimmer. You can adjust it from off to on and in between. There you have it. Typically COM (common) goes to the plug side and the other part you called L1 and L2 goes to the lamp1 and lamp2.

No.

To use as a simple switch, use connections COM and L1. Ignore L2 as this is only used when you use a second switch for two way switching. It is not for a second lamp.

Two way switch wiring (you don't need this!): http://www.electronics-project-design.com/images/LightSwitchWiring.GIF

Steve.

tkamiya · Apr 22, 2011

Steve,

I believe I said that in preceding paragraph.

Steve Smith · Apr 22, 2011

Yes. It was suggesting that L1 and L2 referred to Lamp one and Lamp two which I was disagreeing with.

Steev.

mgb74 · Apr 22, 2011

N467RX said:
Totally agree. Since you're in the UK, I assume you have 230V, and I would think twice the idea of messing with the wiring. ...

Out of curiosity, what is the typical amperage on a household circuit in the UK?

N467RX · Apr 22, 2011

Couldn't find a straight forward answer, apparently in Europe they have all sorts of different plugs and standards (I've only used Euro power outlets in The Netherlands), but in the UK they seem to be 2.5A, 5A or 15A

Steve Smith · Apr 22, 2011

N467RX said:
but in the UK they seem to be 2.5A, 5A or 15A

In the UK the standard power outlet is 13 amps and has been for many years. The sockets are wired to a circuit with a much greater capacity but the plugs fitted to the appliance are fused at 3A, 5A, 10A and 13A depending on their power requirements.

In the olden days we used to have three different sizes of round pin plug rated at 3A (could have been 2A) 5A and 15A. These plugs were not fused but were protected by fuses at the distribution point (fuse box)

http://en.wikipedia.org/wiki/BS_1363

Steve.

N467RX · Apr 22, 2011

So are the sockets standardized to that shape now?

Steve Smith · Apr 22, 2011

N467RX said:
So are the sockets standardized to that shape now?

Yes.

There are some current uses of the older round pin plugs. The 15A plugs are used for theatre lighting and the small 2A plug is often used to connect central heating pumps. I have also fitted these smaller sockets to a room to allow table lamps to be connected but switched on by a wall mounted switch.

But apart from old houses which have not been modernised, you will always find the BS 1363 13A sockets in the U.K.

http://en.wikipedia.org/wiki/BS_546

Steve.

mgb74 · Apr 22, 2011

So, at 13amps, your circuits are delivering almost twice the watts as here in the US (where you'll generally find 15amp or 20amp circuits).

Steve Smith · Apr 22, 2011

mgb74 said:
So, at 13amps, your circuits are delivering almost twice the watts as here in the US (where you'll generally find 15amp or 20amp circuits).

Only if the appliance draws sufficient power. A device rated at 230 volts drawing 13 Amps would be using 2990 watts. We don't have many devices rated that high. Even a two bar electric fire would only be around 2kW.

Normal appliances plugged into a 13 amp socket such as TVs, DVD players, computers, etc. will only draw one, perhaps two amps. A washing machine's heater element might take it close to the limit.

I believe that in the US you have a different circuit connected across two phases to give a higher voltage for high power devices such as electric cookers. In the UK an electric cooker is wired in permanently via its own 30 or 40 amp circuit, as is an electric shower. These things are not plugged in.

Steve.

mgb74 · Apr 22, 2011

Steve Smith said:
Only if the appliance draws sufficient power. A device rated at 230 volts drawing 13 Amps would be using 2990 watts. We don't have many devices rated that high. Even a two bar electric fire would only be around 2kW.

Normal appliances plugged into a 13 amp socket such as TVs, DVD players, computers, etc. will only draw one, perhaps two amps. A washing machine's heater element might take it close to the limit.

I believe that in the US you have a different circuit connected across two phases to give a higher voltage for high power devices such as electric cookers. In the UK an electric cooker is wired in permanently via its own 30 or 40 amp circuit, as is an electric shower. These things are not plugged in.

Steve.

My question was in the context of potential risk when working with a circuit. 15a at 110v isn't too risky - assuming you're not well grounded (not that I'm suggesting you try it out of intellectual curiosity). At 230v and 13amps, you're twice the wattage.

We also have 220 circuits (across 2 phases as you mentioned) for some permanently wired appliances such as air conditioners, ovens, dryers, etc. They typically run at 20 to 40 amps.

Steve Smith · Apr 23, 2011

mgb74 said:
My question was in the context of potential risk when working with a circuit. 15a at 110v isn't too risky - assuming you're not well grounded (not that I'm suggesting you try it out of intellectual curiosity). At 230v and 13amps, you're twice the wattage.

Only the voltage is relevant when considering risk (assuming you mean risk of electrocution) as the high resistance of the human body is not capable of drawing much current. Just because a circuit is rated at 13 amps doesn't mean that is the current which will flow if you touch it. In reality it will be a few milli-amps.

Steve.

How to wire a dimmer switch?

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