Help with a multi-bulb idea...

[jugg2]

NOTE: This may belong in fixed lighting, but it is a question about icandesent bulbs so I posted it here.

I was thinking about modifying a flood/work light, you know the 500 watt deals from home depot. If I were to use 10 12V bulbs in series would that work when plugged into 120V AC? I know the AC part is not an issue, but I do not know if the voltage drop over each bulb will keep them from blowing, or make them way underdriven.

Any info is appreciated! Thanks guys.

 

[Timothybil]

From a purely resistance viewpoint, it should work. Whether it's one bulb with sufficient resistance to drop 120v, or ten bulbs in series each having sufficient resistance to drop 12v, the result is a 120 volt drop. There might be an issue with AC vs DC, since I seem to recall the fact that AC calls for some engineering changes to handle the 60 cycle heat/cool thermal vibration vis a vis DC steady state. I think it revolves around a different alloy to avoid fatigue and different physical support.

Of course, you can overcome all of this just by putting in a half or full wave rectifier in line with the bulbs. AC, become DC!

 

[Mr Happy]

NOTE: This may belong in fixed lighting, but it is a question about icandesent bulbs so I posted it here.

I was thinking about modifying a flood/work light, you know the 500 watt deals from home depot. If I were to use 10 12V bulbs in series would that work when plugged into 120V AC? I know the AC part is not an issue, but I do not know if the voltage drop over each bulb will keep them from blowing, or make them way underdriven.

Any info is appreciated! Thanks guys.

It will work fine electrically, but there might be an issue with safety.

The simplest problem is with how you mount the bulbs. Your bulb holders and wiring must be rated for 120 V rather than 12 V. You should not, for example, use automotive bulb holders.

The slightly less expected problem might occur when one of the bulbs blows. Although the bulb has only 12 V across it when it is working, it will suddenly develop the full 120 V AC mains voltage across it when it reaches the end of its life and fails. At this point there is a risk of a very hot electrical arc forming at the point where the filament broke which might shatter the glass bulb and send hot sharp fragments everywhere, as well as perhaps damaging the light fitting and/or blowing the circuit breaker.

Mains rated 120 V AC bulbs are designed to withstand the arcing problem, and may even contain a special fuse inside them to prevent tripping the breaker. If you run a string of 12 V bulbs, then as well as ensuring the insulation and bulb holders are rated for 120 V, you should also include a dedicated fuse in the circuit.

(Incidentally, this series connection is exactly how Christmas tree lights work, but they are designed from the outset for mains voltage.)