Connect with us

Replace AA batteries with AC adapter in parallel?

Discussion in 'Electronic Design' started by tinkerer, Feb 11, 2006.

Scroll to continue with content
  1. tinkerer

    tinkerer Guest

    I have an automatic fish feeder that periodically runs out of batteries
    and I don't notice. I would like to replace the 2 AA batteries with
    aligator clips to the terminals, and hook up to a 3V ac adapter to keep
    the unit running. Can I just cut the wires and hook up the clips to
    the terminals being careful about polarity? And then can I take a
    radioshack battery pack in parallel to act as a battery backup?

    Thanks for any advice
     
  2. Jim Thompson

    Jim Thompson Guest

    What are you using for an automatic fish feeder? I'm finally going to
    be gone to a wedding for a few days and the lid on my reef tank is too
    heavy for my adult daughter to lift :-(

    ...Jim Thompson
     

  3. First of all, this belongs on and No, you
    can't replace batteries with an AC source. How can you be careful about
    polarity with 3 VAC? If you meant a 3 VDC source, "maybe". You need to
    know the current required, and if it needs to be regulated.

    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  4. Yes re the adapter; no re hooking a battery pack in parallel.

    You can make the "battery backup" work if you use diodes to ensure that the
    batteries don't try to power the adapter or vice versa, like this:

    ' BAT + -------->|-------o------ + TO FISH FEEDER
    ' 1N5818 |
    ' ADAPTER + ---->|-------'
    '
    ' BAT - -----------------o------ - TO FISH FEEDER
    ' |
    ' ADAPTER - -------------'

    Use Schottky diodes, like 1N5818, because they have lower voltage drop.
    Note that you'll only need the diodes on the + side; on the - side you can
    just connect the adapter and battery pack directly.

    This will work as long as the voltage produced by the adapter is a bit more
    than the voltage produced by the batteries. Otherwise, the batteries will
    win and they will power the fish feeder until they are drained enough that
    the adapter voltage is higher again.

    To work around that, you might want to use a 4.5V adapter, and two regular
    diodes (1N4004) rather than a Schottky diode in series with the adapter.
    Each regular diode has a drop around 0.7V, and the Schottky will drop about
    0.3V. So you get 3.1V from the adapter versus 2.7V from the batteries,
    guaranteeing that no current will flow from the batteries until the AC
    fails. But that might not be necessary; the 3V adapter probably puts out a
    bit more than 3V depending on load, so you might just be fine with the
    circuit I drew above.
     
  5. tinkerer

    tinkerer Guest

    Thanks for the help everyone! Thanks Walter for the detailed diagram!
    Sorry I posted this in the wrong group everyone. Yes, I meant an AC
    plug to DC adapter output of 3V. As far as Jim's question as to the
    kind of fish feeder, I am using an eheim feeder for my salt water tank
    (http://www.bigalsonline.com/catalog/product.xml?product_id=22595;category_id=3005).
    It has been reliable in despensing flake or pelet food, and it has
    moisture control to prevent clogging. My only problem is that it is
    easy to let the battery run out but still see food in the feeder, and
    not notice it is not actually dispensing.
     
  6. tinkerer

    tinkerer Guest

    Thanks for the help everyone, sorry about the wrong group! Thanks
    Walter for the detailed diagram, I am going to try that! Yes, I meant
    an AC to DC converter, giving an output of 3V. As far as Jim's
    question as to the kind of fish feeder, I am using an Eheim feeder for
    my salt water tank
    (http://www.bigalsonline.com/catalog/product.xml?product_id=22595;category_id=3005).
    It has been reliable in despensing flake or pelet food, and it has
    moisture control to prevent clogging. My only problem is that it is
    easy to let the battery run out but still see food in the feeder, and
    not notice it is not actually dispensing.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-