Voltage Reg Ac in / Dc out circuit design help

Discussion in 'The Projects Forum' started by ABC-RICK, May 8, 2009.


    Thread Starter New Member

    May 8, 2009
    I need to design a battery charger to keep a car battery charged.

    The input voltage I’m working with averages about 20vAC with an occasional spike to 90.
    The output, of course, should be the standard 14.7vDC or there about and smart enough to know how to detect switching between 2 amps when it’s charged and 8-10 amps when it needs a charge. It could even slope its output as needed. I don’t care!!
    I’m sure a simple rectifier would get me to DC but I don’t know how to protect from the spikes and I have no idea how to detect the current state of the battery to know how to regulate the current.
    Can anyone please help with a basic design to get me going in the right direction?

    Thanks in advance....

  2. Darren Holdstock

    Active Member

    Feb 10, 2009
    This would be quite a major project, made more interesting by the input ranging from 20 V to 90 V AC. This would require a switched-mode current source with a very wide input range, wider than most SMPS ICs can handle. There are ways around this, at the price of extra complexity.

    The charge status of a lead-acid battery is determined by monitoring the open-circuit voltage - the wonderful BatteryUniversity has all the details.

    You might want to investigate an off-the-shelf wind turbine charge controller; they should do the job if they can handle the high input voltage.
    Last edited: May 12, 2009
  3. StayatHomeElectronics

    AAC Fanatic!

    Sep 25, 2008
    Depending on what the "spikes" are in duration and frequency of occurrence makes a difference. Here is a application note of transient voltage suppression that may help you decide. They have some nice general graphs of how the devices respond to voltages and currents.

    Last edited: May 12, 2009
  4. Darren Holdstock

    Active Member

    Feb 10, 2009
    Oops me, SaHE has a good point - I may have mis-read the spec as having a sine input that occasionally rises to 90 V AC, rather than 20 V AC with the odd 90 V spike. If it's the latter, then that does make life easier, and the TVS devices SaHE suggests could work a treat (as he said, depending on the nature of the spikes).