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Relay delay circuit?

Discussion in 'General Electronics' 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. jasen

    jasen Guest

    | This is an ascii schematic, if the diagram appears garbled |
    | try switching to a fixed-pitch font (courier works well) |
    | pasting it into notepad works well on ms-windows. |
    | or in google groups "view source" (found under options) |
    +12--o-~ o----------+--------+----------------+--
    | | |
    | . . . .|. . . . .->|--+
    | . VCC(8) . | `3 12V relay
    | . . | 3 coil
    .----------+--RES(4) OUT(3)-- | .3 max 200mA
    | . LM555 . | |
    +--[100K]--+--TH(6) DIS(7)----+-----'
    | | . .
    | +--TR(2) CV(5)--
    | | . .
    `---|<-----+ . GND(1) .
    | . . . .|. . . .
    + | |
    ===== 10uF |
    | |
    0V -----------------+--------+-----------

    both diodes 1n4001 or similar.

    for the relay anything with a coil resistance over 72 ohms should be fine.


  3. Eric Sears

    Eric Sears Guest

    Jasen's circuit may well work ok (though I couldn't exactly understand
    the ascii circuit - but that's probably my fault).
    It may depend on how handy you are at building electronic circuits - I
    think there was a 555 timer in it.

    There may, however, be a simpler method.
    I presume you use a relay to switch on the inverter from the 12v.
    You could also use this same voltage to switch two further relays,each
    of which must have contacts rated for your mains voltage and current
    (these have 12v coils).
    The first simply switches the load between mains and the inverter.
    However, in the circuit to the inverter, are the contacts of a further
    relay, which is time delayed, and is switched by the 12v.

    The time delay is formed by using about a 200 ohm coil (lower may work
    but will require larger capacitor), and a cap of about 1000uF across
    it, with maybe about 20 to 47 ohms in the pos lead to the 12v from the
    relay coil. (It might need to be less capacitance)

    When the 12v (from the regulator) is off, the inverter will be off and
    the n.c.contacts of the first relay will connect the mains to the
    When 12v "on", the first relay will switch the load to the wire which
    goes to the second relay contact. Because of the time delay, the
    output of the inverter will not be connected until relay 2 switches.
    This time can be varied by altering the values of C and R.

    When 12v switches off again, the load will be immediately switched to
    the mains by relay 1. Relay 2 may have a time delay before it "drops
    off", but that won't matter, as its is no longer connected to relay1
    (nor is there any voltage from the inverter.)

    If this isn't clear. I will try a better explanation - but I will be
    out of town for two or three days and away from the computer.

    Eric Sears.
  4. Peter Hucker

    Peter Hucker Guest

    That sounds like it should work. I was also thinking of only one relay with a cap across it which would charge almost instantly when given a 12 volt signal from the regulator, but would take a while to discharge through the relay coil. This sounded like the reverse of what I want (delay occurs at switch-OFF of invertor, but the regulator actually switches the 0V output to 12 volts when it wants the invertor off, so I could use that and the battery ground to provide a reversed signal to the relay.


    "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
  5. Eric Sears

    Eric Sears Guest

    OK Peter, thats the right idae. Just use the nc contacts instead of
    n.o. Put the cap across (WITHOUT the resistor) for almost instant
    switch-on, but slower release.

    Eric Sears
  6. I asume you know how to make the relay driver. Just trigger it with a
    555 Integrated Circuit available at any radio shack. Try google for
    applications of 555 IC Timers. Its just a matter of adjusting the RC
    value on the timer.

    Howard Epstein
  7. John Fields

    John Fields Guest

    That won't work.
    No, it isn't.

    BTW, this is USENET, not email, so please bottom post.
  8. neon


    Oct 21, 2006
    that setup is cheap what you need is a contactor which is like a relay but travel time is long with usualy has a shoottrough. to solve some of your trouble add a cap across the relay to substain the voltage during transition
    Last edited: Aug 9, 2007
  9. sheldonstv


    Jul 17, 2007
    heres a simple 555 circuit to give you an idea how to alter it to suit what you need

    Attached Files:

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