12VAC lights compatible with 12VDC supply?

topic posted Sat, August 21, 2010 - 6:27 PM by  Brad
Just bought a 12VAC Xenon light for my VW vanagon. Thing is, I didn't realize it was AC at the time (thought it was DC!). In my disappointed state I hooked it up anyway, and lo and behold it lit up when connected to 12VDC.

I'm wondering if there's some hidden downside here. Am I losing efficiency by using DC? Will my bulb burn out any faster? The bulb just says "12V20W" on it. Doesn't specify AC or DC. Any help would be appreciated.
posted by:
  • B
    offline 124
    What leads you to believe that the bulb is an AC bulb?. If it is an automotive bulb it is DC. I have never heard of an 12V AC system for autos that were not some for of homebrew, experimental. It might actually get more life as it doesn
    t have the 60 Hz constant thermal shock. But it may also burn hotter as it has a higher average power going through it with a DC source than an AC source.
    • The reason I'm led to believe that it's a 12VAC light is the packaging. Also, the system comes with a step down transformer, which has "11.5VAC Output" listed on it. It's actually not meant to be an automotive light. I'm installing this in the kitchen area as a part of a remodel/renovation. It's actually for home use. I saw 12V on the package and figured they were using a DC-AC converter to run 12VDC in the home (for some reason).

      So it sounds like I should be okay, yes? No unseen inefficiencies or death traps?
      • B
        offline 124
        Wow this is confusing.

        What is the name of the product you bought?

        I'm a little confused by what ou are saying, in particular you have a 12VDC input a transformer that says 110 VAC outlet and is labeled a 'step down' transformer? I don't see the step down here?

        Is it a 100V bulb with a inverter attached? Taking DC to AC? Did you remove the bulb from the system and wire it in separate? Is it a 110 device that has a transformer for 12V to feed the bulb. You left out a lot of details in your first post and now things are even muddier.

        If It's 'step down' transformer' with 110VAC INPUT to 12V output it might have a bridge rectifier built in and be supplying unfiltered DC to the bulb.

        Wouldn't it be cheaper to buy a plain old bulb without the transformer?
  • Hold on, stop.

    Here's the deal: If the bulb is any kind of incandescent, then it doesn't care about AC or DC. Xenon bulbs are incandescent.

    In order to keep the packaging and design simple, low voltage incandescent lights often come with a transformer that puts out low-voltage AC.

    When you need to start worrying about AC vs. DC is when you get to fluorescent or LED fixtures.
    • im rq'ing an exception for the caveat to LEDs

      because the issue is one of common practice in marketing,

      which applies also to any 12 volt appliance.


      The automotive strip lighting I purchased (about 20$) was provided with a 12VDC cigar lighter plug adapter.

      Intending to use it as under-cabinet lighting, I planned to trim the connector off and connect it directly to the 12V buss in my RV.

      The error led to "letting the smoke out of the manufacturers package"...

      diagnosis?? >> the lighter-socket plug contained a voltage regulator.

      and YES!, I checked polarity before I hooked up.


      I would prefer to NOT trust the 12vdc market _not_ to package a device that will eventually FAIL from constant higher temperatures associated with a DC 12V rail, compared to the sustained RMS of 11.5V AC, which is like . . . 8 Volts effective.

      I suppose you could buy two of the lamps, run one at 12VDC and the other with the provided .phantom. AC,

      and measure with an infrared thermometer.

      my 0.02$
      • B
        offline 124
        Yes that is what I was driving at when i asked about the AC. If it has a full wave bride the RMS is over 8 but if your running AC then you're right the RMS is like 8V and it will run hot. Will it be hot enough to burn out? Probably not. Will it shorten the life? Maybe. Or will it be a wash if the thermal shock of the ac is eliminated but with a higher voltage so the bulb runs hotter. A lot hotter. The details are starting to come out and there is still missing information.

        But if you really just want a fast and dirty answer without checking then the real answer is.

        If it didn't kill you or burn up yet keep running it.
      • As you said, there is a regulator of some sort in the lighter plug (likely a current limiter) and that by bypassing the plug, you bypassed the regulation. It's poor design, but it does happen, and in this case has nothing to do with AC vs. DC.

        Often, 12V LED lights actually want *exactly* 12V, or even something as low as 10V, depending on the LEDs used therein. A regulator is needed in order to keep the power from a car's supply (which may easily wander anywhere from 10.5 to 13.8 under *normal* circumstances) from going over. In fact, many LED lights and other 12V devices come with a warning to that effect if you read the data sheets.

        It is, however, bad design. The regulator should be in the device, not in the plug. That's my opinion, anyway.
        • B
          offline 124
          It's not a bad desig. You put the regulator where it fits and can disapate any heat without hurting anyone. That is if it gets pretty hot. You just have to check the entire *system* and not just assume that the individual componets will play the way they look at firrst glance.

          I never trust those automotive 12 V plugs. They are really big and you can hide a lot of things in them. And a voltmeter goes a long long way to answering questions before you cut the cord and rewire.
          • 12VDC cigar sockets demand caution, anyway you look at it.

            Someone lost a dime in the plug-hole of an Opel GT I was given,

            when in an effort to keep the auxillary systems powered resulted in a jumper over the fuse block,

            about 10 percent of the harness turned to molten plastic,

            and cut gouges in the insulation of about half the harness.

            I admit freely I enjoy spades and ring terminals, but Dear God...
            • B
              offline 124
              With the multitude of connectors available why the auto industry still uses connectors that fit in a hole designed to lite a cigarette is beyond me. And it's not that there are no connectors that aren't good enough for automotive use. There are hundreds of thousands of mill spec connectors out there that can be mass produced. I truely despise those cigarette lighter connectors.
          • If the system has a point that comes apart, then there needs to be some indication that the part isn't replaceable with something generic. There is the message "Use only with Acme power supply", but this is so badly overused (to drive sales of overpriced wall warts) that for it to appear on a device is essentially noise.

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