Answers.Body: <p>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...).</p>
  
  <p>Edit: Found it, it was powering this light brick: <a href="http://guide.lugnet.com/set/1140" rel="noreferrer">http://guide.lugnet.com/set/1140</a></p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2012-10-29T10:42:26.617Z
Answers.Text: 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](http://www.peeron.com/scans/7867-1/4)). 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".
Answers.UserDisplayName: Joubarc
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2012-10-29T10:43:44.837Z
Answers.Text: 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 : http://guide.lugnet.com/set/7867
Answers.UserDisplayName: Glenn Plas
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2012-10-29T10:45:50.000Z
Answers.Text: 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 :)
Answers.UserDisplayName: Glenn Plas
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2012-10-29T10:49:50.463Z
Answers.Text: Unless your dad worked for LEGO, he doesn't count as official :-)
Answers.UserDisplayName: Joubarc
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2012-10-29T11:00:19.083Z
Answers.Text: Cheers!  Didn't know this gem existed in the SE family till today.  Wish I had more time to play with my mindstorms sets.
Answers.UserDisplayName: Glenn Plas
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2012-10-29T10:35:57.387Z
Answers.LastActivityDate: 2012-10-29T10:35:57.387Z
Answers.UserDisplayName: Glenn Plas
Answers.Body: <p>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.</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2019-10-10T11:51:50.630Z
Answers.Text: 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.
Answers.UserDisplayName: Joubarc
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2015-01-10T22:27:42.977Z
Answers.LastActivityDate: 2015-01-10T22:27:42.977Z
Answers.UserDisplayName: Niels
Answers.Body: <p>The AC output might be useful for <a href="https://bricks.stackexchange.com/questions/8526/wiring-diagram-of-this-blue-12v-switch/8528#8528">blue era switch points</a>. I guess this is how the transformer is built:<a href="https://i.sstatic.net/7syHf.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/7syHf.png" alt="Lego transformer 7864" /></a></p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2021-10-10T08:57:20.343Z
Answers.LastActivityDate: 2021-10-10T10:16:14.550Z
Answers.LastEditDate: 2021-10-10T10:16:14.550Z
Answers.UserDisplayName: Niels
Body: <p>The <a href="https://brickset.com/sets/7864-1" rel="nofollow noreferrer">7864 transformer</a> 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.</p>
  
  <p>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 <a href="http://peeron.com/scans/7864-1/2/" rel="nofollow noreferrer">official instructions bundled with it</a> don't mention the port, treating as if it doesn't even exist at all.</p>
  
  <p>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?</p>
  
  <p>Retro-compatibility could also have been an answer, but the two AC socket on the <a href="https://brickset.com/sets/742-1/12v-Transformer" rel="nofollow noreferrer">blue transformer</a> are apparently 16v (for switches) and 3,8v (for lights), so I'm thinking that's not it either.</p>
  
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2012-09-28T22:40:24.610Z
Comments.Text: Is it possible it was for use with other sets, such as the LEGO #4561 Train Set Controller.
Comments.UserDisplayName: Jenny
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2012-09-29T00:22:36.760Z
Comments.Text: 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.
Comments.UserDisplayName: mcqwerty
Comments.ContentLicense: CC BY-SA 3.0
Comments.CreationDate: 2012-09-29T10:07:29.940Z
Comments.Text: Agreed, 4561 is 9v era so it wouldn't make much sense
Comments.UserDisplayName: Joubarc
Tags: <trains><12v>
Title: What was the 12v AC socket for on the 7864 transformer?
UserDisplayName: Joubarc
ThreadText: # What was the 12v AC socket for on the 7864 transformer?
  
  (Asked by: username_0 on 2012-09-10)
  
  The [7864 transformer](https://brickset.com/sets/7864-1) 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](http://peeron.com/scans/7864-1/2/) 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](https://brickset.com/sets/742-1/12v-Transformer) are apparently 16v (for switches) and 3,8v (for lights), so I'm thinking that's not it either.
  
  **Tags:** trains, 12v
  
  > **Comments:**
  > **username_1**: Is it possible it was for use with other sets, such as the LEGO #4561 Train Set Controller.  
  > **username_2**: 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.  
  > **username_0**: Agreed, 4561 is 9v era so it wouldn't make much sense  
  
  ---
  
  ## 3 answers
  
  ### Answer by username_3 on 2012-10-29. Score: 5
  
  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>
  
  > **Comments:**
  > **username_0**: 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](http://www.peeron.com/scans/7867-1/4)). 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".  
  > **username_3**: 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 : http://guide.lugnet.com/set/7867  
  > **username_3**: 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 :)  
  > **username_0**: Unless your dad worked for LEGO, he doesn't count as official :-)  
  > **username_3**: Cheers!  Didn't know this gem existed in the SE family till today.  Wish I had more time to play with my mindstorms sets.  
  
  ---
  
  ### Answer by username_4 on 2015-01-10. Score: 2
  
  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.
  
  > **Comments:**
  > **username_0**: 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.  
  
  ---
  
  ### Answer by username_4 on 2021-10-10. Score: 1
  
  The AC output might be useful for [blue era switch points](https://bricks.stackexchange.com/questions/8526/wiring-diagram-of-this-blue-12v-switch/8528#8528). I guess this is how the transformer is built:[![Lego transformer 7864](https://i.sstatic.net/7syHf.png)](https://i.sstatic.net/7syHf.png)
  
  
  ---
  
  