If you wire them in series you double the total resistance, and halve the total power. each bulb will have 115V across it and so will glow 1/4 as brightly, P=V^2/R (together their power adds to 1/2 the original power)

So to answer your question each bulb will glow approximately 1/4 of what it did before at the same setting on the dimmer.

This is only approximate of course because the tungsten filament will have a lower resistance when it is cooler, but you will not notice it by eye

Thanks, guys.

The lights are for a concealed display in a kid's bookshelf, and we wanted the lighting to be dimmable.

Not only that, but we'd prefer that they go off completely (or almost so) when they're dimmed, so that they also act as a night light.

We can't test the gear because we haven't got it yet. It's been purchased by an out-of-town relative, which is why we feel obliged to use the stuff. So, suspecting that the fully dimmed display might be too bright, I wondered about the series option.

If it had been left to me I would got some LED lights and a suitable LED-type dimmer, but we're stuck with this ahem, gift.

From what you've said, then, my suggestion is feasible? It's just that I wasn't sure if the arrangement might affect the dimmer in some way.

Great. Thanks for the tips :-)

I forgot: A couple more related questions:

What's the smallest 240v incandescent lamp available in NZ? Like, we're apparently being given two 25w lamps for the above project, but I wondered if there was a lamp with an output of less than 25w?

If so, would they have a smaller diameter base and bayonet fitting?

Although the lamps and fittings could also be edison thread, I guess.

e12 nightlight bulbs go down to 5-7 watts, but since they have such small filiments will get very flickery when dimmed, is that is an issue for you?

At these power levels there is often not enough power for the dimmer to operate reliably and that can cause more flicker and instability.

Plenty of decorative e14 and e27 based lamps at the 15 watt range too, look for the vintage styled ones, they run so cool that they take heaps of power and make stuff all light, usually a really dim orange glow.

But this could all be done with a cheap LED PWM dimmer and short length of LED tape for a few bucks off china and not have the instability issues that incandesent lamps have when on a cheap phase angle dimmer.

At the lower edge you will find the lights drop out and dont come back till you raise the dimmer quite a bit till it has enough power to get the filiment hot and therefore resistance up, then you can drop the brightness down untill it drops out again next time there is a brief brownout on the power or something.

I would get over the feelings and give the useless dated technology back and re-buy things to suit.

 richms: Your suggestion re a 'cheap PWM dimmer' and an LED strip sounds like a good idea.

But where could I find these items?

Dimmer - http://www.aliexpress.com/item/12V-24V-DC-8A-Balck-Single-Color-LED-Dimmer-Switch-Brightness-Controller-for-led-lamp-strip/32401644680.html

Strip - http://www.aliexpress.com/item/LED-strip-5050-SMD-12V-flexible-light-60LED-m-5m-300LED-White-White-warm-Blue-Green/1830137201.html

Also available on trademe if you can face their dire search. Those are not the cheapest sellers on aliexpress, just the first I found with a search.

Some sellers will sell you a single meter of the strip but its not a hell of a lot cheaper than a 5m roll from a seller who doesnt waste time cutting and packing the stuff.

Use a generic 12v wallwart you already have or get a locally sourced 12v LED power supply so you are all legit.

richms: Use a generic 12v wallwart you already have or get a locally sourced 12v LED power supply so you are all legit.

Thanks, mate.

Um..'generic 12v wallwart'? What's that?

And a '12v LED power supply'. Wouldn't those also be available from Ali? Or don't they meet our specs\regs?

geekIT:

 

Thanks, mate.

 

Um..'generic 12v wallwart'? What's that?

 

Something that plugs in and outputs 12 volts DC, check ones that came with old routers and cordless phones etc.

geekIT:

 

And a '12v LED power supply'. Wouldn't those also be available from Ali? Or don't they meet our specs\regs?

 

Inline thing if you want to hide it in the roof or similar, ali ones will not be approved for fixed wiring and some are really crappy.

http://ledworld.co.nz/index.php?dispatch=products.view&product_id=30711 would be one. see how they have added the tick sticker to it, means they are saying it is approved and stuff.

OK, so I would need:

1) PWM dimmer

2) LED light strip

3) Power adapter\transformer\wall wart. Is this also known as a 'driver'?

As you suggested, I wouldn't want the power adapter lying about, so I guess creating a wall box with a removable panel would be a good move.

Does the above sound right?

An LED driver may be made for constant current use, which will not work with a PWM dimmer since it is turning on and off really fast. Check its a constant voltage output if you are getting an LED driver type power supply.

Other than wire, that is all you need. Check the watts per meter of the LED strip and that it is not more than your power supply will do and you are good to go.

richms, thanks again.

RE the 'driver' gizmo, I only mentioned the term because I thought it was the same thing as a wall wart.

Do I need a 'driver', then? Or just a wall wart?