The 7864 transformer had, in addition of the main regulated outlet for controlling trains and a fixed 12v DC for plugging in remote controls and lights, a third socket available which provided 12v AC.

This third socket was blocked with a plastic plug, as seen on about every photo of it I can find, and I've never seen any reference of it being used in an official, LEGO-endorsed way. Even the official instructions bundled with it don't mention the port, treating as if it doesn't even exist at all.

So I wonder, what's the story behind this? Did LEGO have something else in mind with it, which was later abandoned? Or did they just toss it in because they had it internally available?

Retro-compatibility could also have been an answer, but the two AC socket on the blue transformer are apparently 16v (for switches) and 3,8v (for lights), so I'm thinking that's not it either.

  • Is it possible it was for use with other sets, such as the LEGO #4561 Train Set Controller.
    – Jenny
    Commented Sep 28, 2012 at 22:40
  • I think it's unlikely due to the fact the 7864 was released in 1980 and the 4561 wasn't released until 1999. Also 7864 is 12v and 4561 is the later 9v system.
    – mcqwerty
    Commented Sep 29, 2012 at 0:22
  • Agreed, 4561 is 9v era so it wouldn't make much sense
    – Joubarc
    Commented Sep 29, 2012 at 10:07

3 Answers 3


I had one of those, and afaik it was just used for the lights. I had like 2 light poles in my set and they where used for that. lights is about the only thing that could take AC current instead of DC in lego. Those weren't LED's at the time. I just can't find any pics online from the ones I have(had...).

Edit: Found it, it was powering this light brick: http://guide.lugnet.com/set/1140

  • 1
    The socket could power lights, indeed, but as far as I can remember the official way to power lights was through the extension port of the transformer (as seen in the instruction manual for set 7867). My question is not really what one can do with the socket, but why did LEGO put it in there in the first place since there seemed to be no official use for it (unless someone can point to a link showing otherwise), and considering it was capped as to say "don't use this, folks".
    – Joubarc
    Commented Oct 29, 2012 at 10:42
  • It was capped because it's AC and putting your trains on it would backfire. I found the set I have, there where 4, not 2 : guide.lugnet.com/set/7867
    – Glenn Plas
    Commented Oct 29, 2012 at 10:43
  • But I see your point, indeed the manual doesn't tell you to use it in the front but on the side, I do remember my dad (the engineer) telling me it was ok to use on the lights, knowing the plug wouldn't survive my curious hands :)
    – Glenn Plas
    Commented Oct 29, 2012 at 10:45
  • 1
    Unless your dad worked for LEGO, he doesn't count as official :-)
    – Joubarc
    Commented Oct 29, 2012 at 10:49
  • Cheers! Didn't know this gem existed in the SE family till today. Wish I had more time to play with my mindstorms sets.
    – Glenn Plas
    Commented Oct 29, 2012 at 11:00

I think it's for test purposes if the fixed and/or regulated 12VDC outputs don't work anymore. This output is fixed 13VAC so it might be directly connected to the secondary coil of the transformer before it is rectified by a diode bridge. With this output it is very easy to test if the transformer is broken or the electronics without opening the unit.

  • That sounds like a possibility, any further evidence? Also, from having opened mine when i was a kid (bad idea), I seem to remember there wasn't much electronic in it - the regulation was done mechanically, by connecting to different sections of the transformer coil.
    – Joubarc
    Commented Oct 10, 2019 at 11:51

The AC output might be useful for blue era switch points. I guess this is how the transformer is built:Lego transformer 7864

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