12VAC common to 12VDC floating

Discussion in 'The Projects Forum' started by Kelly Lee, Aug 25, 2009.

  1. Kelly Lee

    Thread Starter New Member

    Aug 25, 2009
    Hello. I'm hoping some of you may be able to help me with a project that I and another fellow are trying to do. I have a very limited background playing with electronics. Mainly relays and diodes in car audio and security systems.

    On to our project. We both own Kawasaki motorcycles that run on 12vac common ground system. That is no problem for the basic lighting system but we need to hook up a horn which runs on 12vdc. This is the only item that needs DC current.

    What can I make to handle this one and only item? I was thinking a RS# 276-1185 - 25A, 50V Full-Wave Bridge Rectifier connected to a RS# 276-1778 - LM317T 12vdc adjustable regulator and a suitable sized (?) capacitor. Of course, I would use proper heatsinks on both the regulator and rectifier.

    So, is my thinking sound or am I missing something? Would this work in a floating ground setup? Please enlighten me!
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    If it is a typical vehicle horn, which is electromechanical, probably all you need is the bridge rectifier to give unsmoothed DC.

    The voltage on a vehicle '12V' system can be anywhere from 11 to 15V, items designed for this are completely none-critical.

    The problem with adding a smoothing capacitor is that you then get the peak voltage rather than the average, so you could easily get 20V or more.

    Even then, you should not need regulation unless it's an electronic device. A mechanical horn may just be a bit louder.

    Try with only the rectifier first, at worst it may sound slightly strange or the sound change with engine revs.

    Make sure the horn is fully isolated from the bike frame! Some I've seen use one terminal and chassis return; as you have said, the DC side MUST be completely floating to avoid problems or even damage. Fuse the AC side, just in case..

    One note is that when you pick a bridge rectifier for any purpose, get a high voltage one. The commonest cause of failure is voltage spikes rather than overload and often the highest voltage part in the range is only a trivial amount more.
  3. Kelly Lee

    Thread Starter New Member

    Aug 25, 2009
    Thank you so much for your help. It is much appreciated.