Garden Lighting ac/dc

[pegasus]

The output of my garden lighting transformer is 12 volts ac.

Is there any technical reason reason why ac is used as opposed to dc....or is it just the case of why not. I guess ac transformers are slightly cheaper than dc variants.

The bulbs are also marked as 12 Volts 7 watts. This would equate to a resistance of 20 ohms ?? (using the formula V squared divided by P)

When measured with a DVM the reading of the lamp is only 2 ohms.

I am surprised at such a large difference.

Can anyone explain how the bulb markings are calculated?

 

[Diyisfun]

A transformer will not work on DC.
The filament resistance you measured was cold, when the light is on it is hot

 

[ban-all-sheds]

A transformer will not work with a DC input, and will not produce a DC output.

A power supply (which is what most ELV "transformers" actually are) could easily be designed to output DC, but they would cost more.

 

[mapj1]

note also the resistance of a lamp filament is almost linearly proportional to absolute temperature (look up 'debye model' on wikipedia if you want the low down.) So at ~300K (room temp) it is 8 to 10 times lower than at opertional temperatures of 2700 to 2900K.
Actually this cold inrush is responsible for the death of many badly designed dimmers, as hotter running lamps like quartz halogen have a greater ratio between hot and cold, and running a lamp at 'half mast' causes the dimmer to see a lower load resistance for the fraction of the time it is on. (a dimmer works by slicing up the voltage, so the full voltage is applied, but only for a fraction of the time - the thinn slices of very high current can be injurious to some designs.)

And for filament lamps AC tends to even up filament wear, while with DC any metal that vapourises tends to drift to the negative end and blacken the glass.