connecting two 15V AC adaptors together?

Discussion in 'General Electronics Chat' started by dentaku, Mar 4, 2014.

  1. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    I have two 15V AC "wallwarts" that my meter measures at 18.23V AC with no load (I seem to remember it was 16.something when connected to the DSL box it came with) and produces a slightly bumpy sine wave at around 48V peak-to-peak on my old scope.

    Question:
    Is it possible and safe to connect the tips of these two identical adaptors and make something that's essentially a 15V, 0V, 15V center tapped transformer
    similar to the way you can connect two 9V batteries together to get -9,0,+9?

    In other words connect the center pin of one connecter to the outside sleeve of the other to create a 0V point and the other two points would be 15 and 15.
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    That works...........................
     
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  3. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    I'll have to do an experiment and see if I can full wave rectify the output of this add some big caps and get at least DC -15V, 0V +15V. It should be closer to 20V though.
     
    Last edited: Mar 4, 2014
  4. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    I just did a test with only ONE of these AC adapters, a 3n257 "GLASS PASSIVATED SINGLE-PHASE BRIDGE RECTIFIER" and a 1000uF 25V capacitor and I got 23V DC out of it just fine.

    Then I thought, could I just build two of these then connect to + of one to the - of the other and do all of this on the DC side of the circuit (exactly like snapping two 9V batteries together) instead of connecting them together on the AC side?

    In other words...
    Would there be any advantage to connecting them together after rectification and smoothing caps when it's DC instead of right away when it's still AC?
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,061
    3,823

    Yes, you can do that.

    You should make one test before you put this together. Make sure these are really isolated power applies. Check for continuity between 120 VAC plug and each conductor on your secondary (sleeve and tip). If all four measurements (each AC prong to each secondary conductor), then they are isolated and you can connect them.
     
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  6. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    +1
    Don't assume, measure.

    There is a small efficiency advantage to adding the voltages before rectification instead of after. You lose voltage and thus power across each rectifier.
     
  7. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    Yup, I just measured it and it seems to be completely isolated.

    I was wondering about that because this adapter doesn't have screws that would make it easier for me to check how it's built.
     
  8. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    Aha, that's good to know.
    I never expected this to be able to supply much current in the first place so I don't want to do anything to make it worse.

    This is just an experiment for my own education anyway.
     
  9. dentaku

    Thread Starter Member

    Jul 29, 2013
    45
    2
    I tried it out last night with the two AC adapters and it works.
    I then tried it with a small center tapped transformer and that worked well too obviously.

    I've never built anything that plugs directly into 120V AC like this. I've pretty much only experimented with battery powered stuff.
     
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