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Timing Circuit for a battery charger

Discussion in 'Electronic Basics' started by Mitch, Sep 14, 2003.

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  1. Mitch

    Mitch Guest

    Ok, this is my first post in this group, and I've done some searching,
    but I'll still continue to do so.

    I just bought a replacement charger for my battery powered drill and
    saw combo. The previous charger crapped out, so I opened it up to see
    what it was... All it is, is 4 diodes to rectify the AC, 2 resistors,
    and an LED. The resistors only seem to run the led. So the thing is
    pretty simple.

    When I bought the new charger, I was talking to the guy in the store
    and he said that these chargers tend to "burn" up if you put the
    batteries on to charge but don't take it off in 3 hours. Not to
    mention that it isn't good on the batteries. And since it's an odd
    voltage, the batteries are 2x as expensive as common voltage batteries
    (Wish I'd known that when I bought it!) So what I would like to do is
    design some kind of timing circuit that could turn off the charger
    after 3 hours.

    I know that the battery I'm charging is 15.6V but I don't know what
    the transformer steps the AC down to. (The transformer is where my old
    one went bad, and I haven't opened up the new one) So I figured that
    if I could rig up something that plugged into the house power outlet,
    and then plug the charger into it. The timer would kill the power to
    the charger after 3 hours. I don't even need anything automatic
    really... I have no problem putting something like a momentary button
    to use for a starter switch.

    I'm handy with a soldering iron, and I collect pinball machines, so
    I'm pretty good with a schematic, and I have kind of a general idea
    what a lot of components do, but I'm no where near skilled enough to
    try and put something like this together on my own.

    If anyone has any thoughts, I'd love to hear them.

  2. Rich Webb

    Rich Webb Guest

    Why not just get a couple of X-10 components (if you're not boycotting
    them for their annoying web-adverts for their video cameras)?

    Might be overkill just for this but something like and a couple more
    modules can be expanded to a handy way to also turn off the downstair's
    lamp or the light in the garage without needing to get up out of bed.
    Not that *I'm* so lazy, nope, not me, nosiree...
  3. Mitch

    Mitch Guest

    Not a half bad idea really... and I don't like x10 ads popping up
    anymore than the next guy, but at least they usually have some hot
    chick to look at for a moment. ;)

    Anyway, I think that the biggest difference between that and what I
    would like to do is that I don't know what time I'll put the battery
    on, or when I would have to shut it off...

    So everytime I put on a battery, I'd have to stop and figure out what
    time it is, then set the timer to go off 3 hours later (I realize
    that's not exactly difficult math) And at what time does that timer
    turn the power back on? I would still have to get there in time to
    take the battery off before it came back on...

    What I'd like is something that when I plug in a battery to charge, it
    doesn't even start charging till I poke a button to tell it to
    start... Then 3 hours later, the whole thing just shuts off...

    I don't even need anything fancy like a way to set how long it runs...
    but I wouldn't mind learning how to do that either. :)

  4. the Wiz

    the Wiz Guest

    You want the following, all of which should be available at your local
    electrical supply house - maybe even Home Depot.

    one 3-wire cord with plug
    one duplex outlet
    one 4" square electrical box
    one box cover with cutouts for a duplex outlet and a wall switch
    one windup timer (something like the Intermatic FD or FF series

    Wire the timer in series with the AC to the duplex outlet.

    Plug the charger into the outlet, then turn the timer to 3 hours.

    The timer is the most expensive part, but it should be under $20. Home Depot
    has the 60 minute version (FD50MW) for $15.99 and other time ranges should be

    You may be able to build it cheaper with "electronic" parts (555 timer, PIC of
    some type, etc), but the windup timer is the most reliable *simple* device.

    More about me:
    VB3 source code:
    VB6 source code:
    VB6 - MySQL how to:
    My newest language - NSBasic for the Palm PDA:
    Drivers for Pablo graphics tablet and JamCam cameras:
    johnecarter atat mindspring dotdot com. Fix the obvious to reply by email.
  5. Joe

    Joe Guest

    Maybe you already thought of this, but the first thing that comes to my mind
    is a timer that you can buy to turn lamps on and off when you go away. So it
    looks like someone is home. It has a circular dial on it and you get a set
    of 'thingies' to plug into it at different times of the day to turn things
    on and off. You just set up the timer to have the on time preset (anytime)
    and the off time preset to 3 hours later. Then you plug the timer into the
    wall with the charger plugged into it and turn the dial to activate the 'on'
    cycle. It doesnt matter what the real time is.
    After 3 hours, it switches off, and you still have about 21 hrs before you
    need to come back and unplug it.
    I think they cost less then US $10 and may be all that's required.

  6. Mitch

    Mitch Guest

    I love this idea! Thanks!

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