lighting circuit help

Hello, I was wondering if any here could help with a bit of info. I've been asked to help wire up a kinda chandelier "fallen on the floor" lighting display. Basically it's going to be a mess of bulbs, fixtures and structure (copper tube, glass, etc...) on the floor. The "look" has not yet been completed by the designer and I'm just trying to be ready for whatever is thrown at me. It will mostly be made of standard

25 - 40w bulbs that I will wire up to a variac (silent running is key) to control brightness. The designer also has her eye on some lower voltage bulbs she would like to mix in the display. Is it ok to add in parallel to the 120v lights a series of (lets say) ten 12v bulbs? Would the 12v bulbs on the neutral side be dimmer than those on the hot side? As this display will be frequently turned off and on (a few times a day for a few months) I would like to add an inrush current limiter to extend bulb life but am unsure of the placement. Right at the variac on the hot or neutral side? Or multiple locations? The display will use upward of 1000w and possibly more. Go big or go home! LOL!

Thanks!!! DH

You means series, I think.

I'm not sure what would happen (I've never tried it). I know that bulbs are very low resistance until they heat up. This causes a surge of current which quickly heats the bulb. Once it's at temperature, the resistance is such that it limits current to whatever is required for the power rating. If your 12V bulb is in series with the big bulb, and subjected to 120V, it's possible the 12V bulb would be destroyed by the initial surge of current. Once the bulbs are at temperature, though, what'll happen is that the voltage across the 12V will be affected by the relative wattages of the bulbs. Consider two 100W bulbs, one 12V, one 120V. The resistances will be

120^2/100 = 144 for the 120V bulb, and 12^2/100 = 1.44 for the 12V bulb.

Thus, the voltage across the 12V bulb would be

120 * 1.44/(1.44 + 144) = 1.18V

For a 100W 120V bulb and a 10W 12V bulb, we have

120 * 14.4/(144+14.4) = 11V, which is closer.

No. Elements in a resistive circuit are generally independent of where they are in the circuit. It's possible the non-linearities of the bulb heating will muck with this, but it'll only be a factor at startup. Thus, it may affect whether the 12V bulb will survive.

As this display will be frequently turned off and on (a few

Again, it shouldn't matter. These devices can be seen as a resistance which gradually diminshes with time. Initially, the resistance is much greater than the resistance of the bulbs, and gradually subsides. Thus the voltage divider which is created causes most of the voltage to be across the limiter. However, again, non-linear effects may be an issue. This device may also enable your 12V bulb thing.

However, if you are using a variac, why not just use it to slow start the whole system? That would protect your 12V bulbs.

The

-- Regards, Robert Monsen "Your Highness, I have no need of this hypothesis." - Pierre Laplace (1749-1827), to Napoleon, on why his works on celestial mechanics make no mention of God.