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AC timer circuit

Discussion in 'Electronic Basics' started by Dave, Nov 16, 2005.

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  1. Dave

    Dave Guest

    I would like to build a device which turns an AC power outlet for a
    predetermined (ideally adjustable or programmable) time. I will use it to
    power a battery charger on my boat. I've got a fully automatic battery
    charger, but not a terribly expensive one. This charger charges the battery
    until a "full" indicator comes on, at which time charge voltage shuts off.
    However, the sensor that detects "full" is a simple voltage sensor, which
    appears to toggle at about 14.5VDC. Well, as the battery settles down after
    charging ceases the voltage drops over 10 minutes or so until it reaches
    14.4VDC, at which time the charger turns on again. As the boat is
    unattended, I'd rather not have the charger turning on and off indefinitely.

    Instead, I'd like a simple box with one button on it. I push the button, AC
    circuit that powers the charger is energized for, say, 8 hours, then shuts
    off. I have seen homemade versions of this device at a high-tech company I
    worked at, where the lab manager built a bunch and hooked up all of the
    solder stations to them so that when people forgot to shut them down, it
    would happen automatically.

    Any help greatly appreciated.

  2. Guest

    What you want is a monostable 55 timer circuit. When the circuit is
    triggered it will activate a relay for a predetermined time. This page
    has a good schematic:
    You will need to change some of the values to get the time period to
    suit your needs. This circuit can also be brought as a kitset from most
    electronics stores if you don't feel comfortable building it from the
  3. John Fields

    John Fields Guest

  4. John Fields

    John Fields Guest

    How about, instead, a box that when you press the button the charger
    starts and when the battery gets to 14.5V the charger is
    disconnected from the mains and stays disconnected until you press
    the button again?

    Maybe even disconnects the battery from the charger so it can't leak
    back through the charger's output circuitry?
  5. Guest

    look at Nuts & Volts issue September 2004, page 32. This circuit is
    exactly what you want.
  6. spudnuty

    spudnuty Guest

  7. Ralph Mowery

    Ralph Mowery Guest

    You can go to a store like Lowes and get a water heater timer. There are
    also many other timers that can be bought for not too much. The water
    heater timer will probably have to be set to come on every 24 hours for a
    short time.
  8. JeB

    JeB Guest

    I'm probably being helpful but I'd spend the money on a better
    charger ... I think a decent unit will charge at a predetermined
    rate then drop to a small trickle charge to keep it topped off.
  9. Bob Monsen

    Bob Monsen Guest

    Buy a better charger. Hacking something like this together is a recipe for
    disaster, particularly in an unattended setting. Also, your battery will
    die if you let it self-discharge for too long; you need to keep lead acid
    batteries charged.

    One idea would be to get a weekday timer, and set it up to charge every
    friday (before you go out).

    Another idea would be a solar panel charger, like those dashboard auto

    Bob Monsen

    There once was a man from Hornepayne,
    Who tried to transform the whole plane,
    It bent a meridian
    So it wasn't Euclidean,
    And frustration drove him insane.
    - Anonymous
  10. Vey

    Vey Guest

    Timing is not the best way to charge a boat battery.

    I use a solar panel to keep my battery up.
    It will not charge enough to over-charge (and boil).
    It charges enough to keep it up, but every 6 months or so, I put a real
    charger on it, check the water, etc.

    If you don't see your boat at least every six months, you need to sell it.
  11. Dave

    Dave Guest

    Both options are better than mine, although as other posters have noted,
    probably the best thing for me to do is just upgrade my charger. I know
    very little of timing circuits other than that they exist, hence my simple

    just out of curiosity, why did you say that the poster who advocated the
    "555 monostable circuit" or some such was insane?

  12. Rich Grise

    Rich Grise Guest

    Or any ordinary coffepot timer, albeit they're probably not rated for
    outdoor use. :)

    Good Luck!
  13. Rich Grise

    Rich Grise Guest

    Probably because suggesting a 555 circuit in this application is insane. ;-)

  14. John Fields

    John Fields Guest

    The best you can get out of a bipolar 555 depends on the bias
    current of the 555's input comparator, the leakage current of the
    timing cap and the resistance of the timing resistor. If you're
    lucky you can get maybe half an hour out of the thing on a good day,
    but there's _no way_ You'll get 8 hours out of it. The bipolar 555
    and the CMOS 7555 have the same typical trigger current, 50pA, so
    it's no better for the 7555. It's the capacitor's leakage current
    that'll kill the long timeout for both of them.
  15. Ralph Mowery

    Ralph Mowery Guest

    You can go to a store like Lowes and get a water heater timer. There
    Yea, today the local paper had some timers on sale at Lowes for $ 7.95
    reduced from $ 9.95.. Some things are just too inexpensive to reinvent.
    Unless one has a well stocked junk box the price of the parts are more than
    the whole thing can be bought for.

    I used a water heater timer to keep a standby battery charged on a remote
    repeater site. The charger was a 10 amp charger that would cut to a
    trickle. Even at this the battery was not used much so I set the charger
    for about an hour each day.
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