VAC/VDC confusion.

Discussion in 'General Electronics Chat' started by nonsuchpro, Jun 16, 2014.

  1. nonsuchpro

    Thread Starter New Member

    May 31, 2013
    Hi all.

    I recently acquired a piece of gear that says it requires 12 VAC 1A.
    I didn't look at the requirement correctly. I just saw the 12 and didn't read further. So just like every other piece of gear I have with wall wart, I gave it 12VDC. Plug it in, it lit up and I've been using it like this for about a month now with no issues what so ever. The other day I did a bit of rearranging and unplugged it and other gear I was moving. Upon plugging the unit back in, in the corner of my eye I saw 12 VAC 1A. I did a double take and then looked at the wall wart I'm using for it. It said 12 VDC. :eek:

    So my confusion is obvious. What is going on here? The only thing I can think of is it's either a misprint on the unit OR... I'm assuming it has an internal rectifier for the VAC and since I'm using a VDC wart with a rectifier, it's just getting double rectified? After thinking about it I can see it working this way and not the other way around... i.e give VAC to a VDC unit.

    What do you think?

    Thanks in advance!
    Last edited: Jun 23, 2014
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Presumably your piece of gear has a rectifier/ filter at the power input point and is oblivious to the input power type it receives. The outcome suggests there is no isolation transformer in line with the 12V ac input.
  3. crutschow


    Mar 14, 2008
    Which is common since you can't readily get 12VAC without a mains transformer so there's no reason to add another transformer.
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
    And 12VDC will properly pass through a bridge rectifier internal to your equipment.

    Your inattention to detail got by disaster this time and maybe the equipment was cheap and disposable anyway so releasing its smoke is of no consequence. But.... my point is with electronics and many other things in life, attention to details has many advantages like preventing the destruction of needed/valuable equipment, not needlessly causing fire and/or inconvenience etc.

    I have said before "Smoke and electronics are friends as long as you keep the smoke inside where it belongs." You lucked out this time.
    nonsuchpro likes this.
  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    In more ways than one. With two diodes instead of 1, 12 V AC can be turned into bipolar power rails, such as +/-12, for opamps. That your audio is not grossly distorted means that the device probably is not doing this. This time.

    Last edited: Jun 16, 2014
  6. ScottWang


    Aug 23, 2012
    Assuming that the plug only two pins, so internal need a full bridge rectifier, normally it won't harm anything at all, the thing could be happen is the load not enough power to work.

    When you used a Vdc to a Vac plug that it just like a dummy avoid circuit using full bridge rectifier, the voltage will be as:
    1. When 12Vac input, the Vout = 12Vac * 1.414 = 16.968Vdc ≒ 17Vdc
    2. When 12Vdc input, the Vout = 12Vdc - (0.7V*2) = 10.6Vdc.
    The original need 17Vdc, but you used 12V to input, and the device still working fine, so the device has a big range to supply the power for it.

    Maybe the load had a regular IC for the filtered voltage.

    1. When 12Vac input, the Vout = 12Vac * 1.414 - (0.7*2) = 16.968Vdc -1.4Vdc ≒ 17Vdc -1.4V = 15.6V
    Last edited: Jun 16, 2014
  7. wayneh


    Sep 9, 2010
    Another unmentioned "danger" is that all power to the device now passes thru only 2 of the 4 diodes in the device's internal rectifier (assuming it has one). The rectifier diodes should be over-rated and handle this without a problem, but on the other hand rectifier failure is not uncommon under normal usage, so placing a 2X stress on the diodes may shorten the service life.
  8. #12


    Nov 30, 2010
    Scott Wang hit the point I was considering. 12 VAC rectifies to about 15.5 volts because of the peaks on the AC wave. You are running the internal circuits at about 10.6 DC volts.

    This means it won't have the quality it would have at the "correct" voltage. 4 instance, if it's a transmitter, it won't transmit as far. If it's an audio amplifier, it won't amplify as loud.

    Please get it an AC adapter soon so it won't put all the current stress on 2 diodes instead of 4 diodes (wayneh). Meanwhile, feel some gratitude. You really lucked out!
  9. ScottWang


    Aug 23, 2012
    I forgot to subtract the forward voltage of two diodes 0.7V*2=1.4V.
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
    Another typical thing in audio gear is that mixers and similar devices multiply the AC input to get 48V phantom supply for microphones.
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    While you may be able to get away with using a DC supply on an AC input don't ever stick an AC supply on a DC input.

    It probably does not have those funny diodes.
  12. bertus


    Apr 5, 2008

    The AC can also be rectified to + AND - voltages inside the box.
    They could have set it up to have say +9 AND -9 Volts for having a dual powersupply for the opamps.

  13. nonsuchpro

    Thread Starter New Member

    May 31, 2013
    Thanks for the replies guys.

    I did luck out. I've ordered the correct adapter. The unit is only a controller. No audio going in or out, just midi.