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12V AC vs DC

Jim Katen

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Perhaps Marc can answer this question that has me puzzled. 

My desk lamp is an old Eagle gooseneck affair from the '60s. It runs off of a 12-volt transformer in the base and it takes 13 watt S8 bayonet lamps. 

While I was in the auto parts store the other day, I noticed some automotive lamps with exactly the same bayonet configuration, listed at 12 volts and about 13 watts. I figured that AC vs DC shouldn't make much difference with regard to a light bulb filament. When I got home and tried it, the light worked fine but it put out enough heat to cook over. 


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To the best of my recollection, it was a perfect match with regard to voltage, wattage, and base configuration. Yet it produced way too much heat. 

I don't need to use automotive lamps, the regular ones are still easily available for landscape lighting. I might even try the LED versions that have just appeared. 

Mostly I was curious why a lamp designed for DC would produce so much heat when used with AC. 

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49 minutes ago, John Kogel said:

Thanks, Marc, but Chad planted the seed. [:D]

When in doubt, E=IR, no matter what.

Hate to bust your island of sanctuary but no...E does not equal IR where reactive currents (vector quantities) are involved.  It gets more complicated but to give you the light at the end of the tunnel: Everything in the electrical field bows down to he who fully comprehends the 6 Maxwell equations.  And that is well over my head.

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19 minutes ago, Jim Katen said:

Definitely not a flasher. 

Here's what I usually use: 


Here are the automotive ones that get hot, made by Barry's friends at Satco: 




It could be that the new one is a halogen bulb.  Incandescent lamps with the halogen feature need to run hotter to keep the Halogen disassociated.

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