12V AC vs DC

[Jim Katen]

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. 

Why? 

[Chad Fabry]

What number bulb did you get? Many of the automotive bulbs are 20 W and more.

[BADAIR]

i had the pleasure of inspecting a nice portion of the satco 343,125 sqft facility early last year

if it's anything to do with bulbs they'd know & know where to purchase the right replacement

Edited December 29, 2017 by BADAIR

[Jim Katen]

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. 

[John Kogel]

Could it be an extra bright bulb for signals or emergency flashers? They get very hot if left on. You could probably find a small 12 v running light bulb that wouldn't get as hot. Just a guess.

[Marc]

AC or DC doesn't matter.  Duty factor of that automotive bulb is likely the culprit.  John K hit this nail right on the head.

[John Kogel]

Thanks, Marc, but Chad planted the seed.

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

[Jim Katen]

Definitely not a flasher. 

Here's what I usually use: 

https://www.amazon.com/Philips-416719-Landscape-Lighting-13-Watt/dp/B008ATHFGU

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

https://www.amazon.com/Satco-S3623-Bayonet-12-Watt-Light/dp/B007ZY4NNO

 

 

[Marc]

49 minutes ago, John Kogel said:

Thanks, Marc, but Chad planted the seed.

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.

[Marc]

19 minutes ago, Jim Katen said:

Definitely not a flasher. 

Here's what I usually use: 

https://www.amazon.com/Philips-416719-Landscape-Lighting-13-Watt/dp/B008ATHFGU

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

https://www.amazon.com/Satco-S3623-Bayonet-12-Watt-Light/dp/B007ZY4NNO

 

 

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.

[Jim Katen]

None of them are halogen. The ones I usually get certainly aren't and the automotive ones cost .88 for two of them - definitely not halogen. 

[Bill Kibbel]

Did the new bulb get hot when you tried it on DC?

[Jim Katen]

I never tried it on DC. I was afraid that it would cook the wiring in my lamp, so I tossed it. 

I figured that there was a simple explanation that I was just ignorant of. Now I'm going to have to go get another and do some further testing. 

[Bill Kibbel]

Different filament length or coil diameter?

[Marc]

Jim, out of curiosity, could you check the voltage on the secondary winding of your lamp's transformer?

 

[Johnny-A]

Could it have anything to do with RMS (Root Mean Square)?

Mathematically, Peak Value = 1.414 x RMS value.

For example, the 12 VAC, 60 Hz. is the RMS value.

The peak value corresponding to this is = 1.414 x 12= just under 17 volts, and peak to peak is 34 volts vs 12 volts DC.

[Jim Katen]

I really don't know. In fact, I'd forgotten about this discussion. Now I'll have to go back to the auto store, find those lamps and start messing with them again. 

[Johnny-A]

Sorry, New member here, I completely forgot to look at the date! This old dog just got taught a new trick. I will look more carefully next time.