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Relay delay circuit?

Discussion in 'Electronic Design' started by Peter Hucker, May 5, 2007.

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  1. Peter Hucker

    Peter Hucker Guest

    I need to make a relay switch on 1 second after a 12VDC line comes on, but switch off immediately the 12VDC goes off. What's the best way to do this? If you need to know why I want this, see the paragraph below:

    I have a solar power setup and wish to switch the load between the invertor output and the mains supply. Currently I have a relay with a 240VAC coil which does the switching based on the output of the invertor. It doesn't work too good - the invertor output falls a bit as the battery gets low, and the relay starts jumping back and forth. What I would prefer is to use the 12V signal from the solar regulator (which is on whenever the battery is ok (and is what switches the invertor on). However I need to wait at least 1 second before moving the load to the invertor, as the invertor takes a while to start up.
  2. Something simple like this should work. R1 C1 would be your time constant.
    A current limit resistor in series with D1 should be added depending upon
    the drive.
    12vdc>--+--|R1|-----+--|R2|---|<-- Q1
    | | |--+
    +--|<D1|----+ |
    | ---
    --- gnd
  3. Tam/WB2TT

    Tam/WB2TT Guest

    If the input goes to an open circuit, D1 won't do anything at all. For a
    more robust circuit, I would add a driver transistor to the input. This will
    invert the polarity; so, you need a pull-up resistor from its base to +12
    (or +5) , and supply ground to the input to operate the relay. Also, the
    threshold is not well controlled. I would put a 6V Zener in series with the
    source of the FET, and use the drain current to operate a PNP transistor to
    actually switch the relay. The PNP should have about a 330 Ohm resistor from
    base to emitter to ensure the FET and Zener are drawing current before
    operating the relay.

  4. Fred Bloggs

    Fred Bloggs Guest

    You will be better off using a delay-on-make solid state relay (DOM SSR)
    to do the timing and switching, a sealed unit with push-on connectors.
    Just fit it into your existing set-up like so:
    View in a fixed-width font such as Courier.
  5. John Fields

    John Fields Guest

  6. John Fields

    John Fields Guest

    Unfortunately, that won't solve the 240 volt relay chatter problem.

    This should:

    .. +----------+
    .. +----------+----------|+ ~|---------o o----mains
    .. | | | | /
    .. +-----+ - | | o----->
    .. |solar| DOM|\| | |
    .. |panel| SSR|\| | inverter | LOAD
    .. |+batt| |\| | |
    .. +-----+ - | | o----->
    .. | | | | \
    .. +----------+----------|- ~|---------o o-----mains
    .. +----------+

    There is some question as to whether he's turning the inverter on
    and off with the 12V source or whether there's a separate 12V signal
    being used to do that, which could also impact the scheme.
  7. Fred Bloggs

    Fred Bloggs Guest

    It's some kind of UVP signal for the inverter from the battery monitor.
    I suspect his problem is that his load runs the battery down, causing
    the 240VAC to drop out, then the inverter gets off-loaded, causing the
    battery output to come back up to run the inverter unloaded, causing the
    240VAC relay to pull back in - then battery output falls, everything
    drops out- ad nauseum repeat. As usual, the OP doesn't know what he
    needs...the signal he has in mind will not work for him.
  8. Fred Bloggs

    Fred Bloggs Guest

    The problem is not relay chatter- the AC relays especially have huge
    hysteresis. His problem is the highly non-linear output impedance
    increase of the battery as it is discharged under heavy load. The
    off-loaded battery voltage turns the inverter back on. He needs a fuel
    gauge type battery monitor that accounts for re-charge through the solar
    panel, and that is way over his head. Take a shot at if you want:)
  9. Peter Hucker

    Peter Hucker Guest

    If I knew what I needed I wouldn't be asking would I?? Not sure what you mean by "as usual" either - I've only been in here twice in the last year.

    The problem is the relay I currently have takes the 240V from the invertor output to trigger it, but this tends to fall slightly as the battery gets low, so the relay stutters. If I had a relay driven off the 12V signal instead there wouldn't be this problem.


    A farmer drives his tractor away from the homestead when half a mile later his brake cable snaps.
    He sees his wife on the porch and manages to catch her attention but is unable to make her hear what he is shouting.
    However, he thinks she should understand what he wants if he uses sign language.
    So he raises his hand above his head and starts to operate an imaginary pair of pliers, then opens and shuts an imaginary cupboard door and then puts the tips of his fingers together to indicate a shed roof.
    His wife waves to him and then grasps both her breasts, then grabs her crotch and lastly lifts both her buttocks.
    "Stupid woman", he mutters to himself. "Hasn't understood a thing". So he repeats his signals, but gets the same response.
    Exasperated, he walks back to the homestead ready to berate his wife. "Didn't you understand a damn thing?' he asks.
    "Yes", she says, "you wanted a pair of pliers from the cupboard in the shed. But what I was telling you was that there is a pair in the toolbox under the seat."
  10. Peter Hucker

    Peter Hucker Guest

    It's a signal from the regulator which switches on the invertor.


    An infantry brigade was training in the summer heat, learning
    methods to counter Soviet offensive tactics. That summer, the
    area had experienced an infestation of rattlesnakes. Officers
    and NCOs were given one magazine of live ammunition to counter
    this danger, as several men had already been bitten.

    So much ammunition was expended shooting, supposedly, at
    snakes that the post commander demanded that every officer
    and NCO who had shot at a snake present the dead snake as
    proof that the expenditure of rounds was justifiable.

    The next day, the post commander entered his office and spotted
    a shoe box on his desk. He opened it, revealing a sleepy and
    sluggish, but very live, rattlesnake. Inside the box were twenty
    expended cartridges, and a short note. The note said, "I missed!"
  11. Fred Bloggs

    Fred Bloggs Guest

    Don't take it personally, I am only referring to you as a generic entity.
    I don't think so. Almost any AC relay pulls in at 70% and holds until
    10% before dropout, so your assumptions are baseless unless you have the
    specifications sheet for the relay that says otherwise. The relay drops
    out because that 12V battery control signal to the inverter drops out
    and shuts it off entirely, this is my guess.
  12. ehsjr

    ehsjr Guest

    That's what I said, essentially. But it doesn't matter
    what D1 does, the problem is that C1 can't discharge
    through either D1 or R1 when the input circuit is opened.
    Also, discharging through the supply is not the best idea.

  13. Peter Hucker

    Peter Hucker Guest

    I think that sounds better....
    The invertor is running properly and continuing to power the load. The invertor does start bleeping around that point to indicate a low battery, but is supposed to cut out if it's too low. My guess is the "240V" output is nothing like what it's supposed to be at that point.


    "The Ten Commandments contain 297 words.
    The Bill of Rights is stated in 463 words.
    Lincoln's Gettysburg Address contains 266 words.
    A recent federal directive to regulate the price of cabbage contains 26,911 words." -- Atlanta Journal
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