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240v Delay Off Timer

Discussion in 'Electronic Design' started by [email protected], Nov 3, 2006.

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  1. Guest

    I am looking to use a 240v delay off timer to shut down a central
    heating circulation pump a few minutes after the thermostat has closed
    down the burner. I am looking for a timer that I can delay the pump
    stopping for between 3 and 10 mins after the burner has shut down.
    Grateful for any suggestions.
  2. Guest

    As it happens I'm making 1 for my own central heating system, should be
    ready next week. Eail me if you live in england and are interested.
  3. "off delay" time delay relays area a lot less common than
    "on delay" relays, because either they need a separate power
    connection to keep the timer running after de-energization,
    to an internal energy storage to do the same thing. I think
    you would be better off to try to use an "on delay" relay
    with a normally closed contact to run the pump. Then you
    need a normally closed contact to energize it when the
    burner shuts down. Of course, this means that the time
    delay relay will be continuously powered when the burner is off.

    But if you Google "off delay" "time delay relay" you might
    find the one you are thinking about, now.
  4. Off the shelf item.

    + Required crap appended to avoid restrictions imposed by brain +
    + damaged idiots.
    + Server Response: '441 Posting Failed (Rejected by POST filter)', +
    + Port: 119, Secure(SSL): No, Server Error: 441,
    + Error Number: 0x800CCCA9
  5. Elektor once published such a circuit. It was placed between the mains and
    the heating unit and monitored the thermostat. So it could switch off the
    heating - including the pump - but without loosing its own power. When the
    requested temp was reached, the thermostat signaled the heating unit which
    switched off the heater but not the pump. The circuit kept powered the unit
    for some minutes then switched it off. It kept monitoring the thermostat so
    when it required heating again, the unit was switched on. An extra feature
    is a 24h timer which switched on the unit for about 15min when it had not
    been running for a whole day, so to prevent the pump to get stuck.

    It's an old design, maybe over 20 years and not usefull anymore as modern
    heaters have this provisions already build in.

    petrus bitbyter
  6. Guest

    Since you still need the control coltage to keep the blower riunning
    this is not an issue. The LV transformer is always powered. The easy
    way is to use an SSR to drive the blower and a capacitor feeding a
    darlington for the time delay. We had this discussion a week or so
  7. Ken Moffett

    Ken Moffett Guest

    wrote in

    These are "Delay-on-break" timers. You would use one of these to control a
    relay that will handle the voltage and current of your pump.

  8. ehsjr

    ehsjr Guest

    View in fixed font:

    I'm assuming the thermostat switches 24 volts, not 240.
    If the timing is not critical, you can run the 24 VAC
    from the closed contact of the thermostat through a
    rectifier to charge a cap, and connect that to a comparator.
    The output of the comparator operates a relay through a
    driver transistor. When the thermostat calls for the burner,
    the thermostat contact closes and the burner runs. When the
    'stat contact opens. the comparator output continues to
    hold the relay energized, until the cab discharges below
    the reference voltage, then the relay drops and the circulator
    motor shuts off. You need an always on DC supply to power.

    | |
    [CAP] [R1]
    | |
    24VAC--Bridge---+-[R2]-+---- Vin to Comparator input
    DC Ground ------+-[R3]-+---- Vref to Comparator input
    + 12 Vcc --------------+

    R1 and R2 are a voltage divider to set Vin below + Vcc
    You want then to be high value to get a large RC.
    I'd use 680K for R1 and 330K for R2 and 1000 uf for
    the cap to give you about 11 volts at Vin when the
    burner is on. That will give you plenty of discharge

    R3 & R4 set the reference voltage to the comparator.
    A pot here can be used instead to vary the timing.

    The comparator output operates a relay driver, that
    relay gates the 24 V control voltage to the circulator.
    See below

    Original wiring:

    24vac ----o o---------+------------+
    | |
    [Burner] [Circulator]
    | |
    24vac -----------------+------------+

    New wiring:

    Tstatcontact "10 minute" relay contact
    / /
    24vac ----o o---+------+----o o----+
    | | |
    | [Burner] [Circulator]
    | | |
    24vac --------------+---+------------+
    | |
    | |
    | [CAP]
    | |
    +-----+ etc

    Pick a relay whose contacts can handle the
    circulator load.

  9. John Fields

    John Fields Guest

  10. ehsjr

    ehsjr Guest

    Oops - that last diagram was supposed to be:

    New wiring:
    | |
    | Tstatcontact | "10 minute" relay contact
    | / | /
    24vac +---o o---+------+ +-----o o
    | | |
    | [Burner] [Circulator]
    | | |
    24vac --------------+---+------------+
    | |
    | |
    | [CAP]
    | |
    +-----+ etc

  11. In my country - and I guess in many others too - it is not allowed to modify
    the circuit or wiring inside the heater enclosure. You may run into safety
    and/or assurance trouble if you do.

    petrus bitbyter
  12. John B

    John B Guest

    Go here:

    and search for H3DS. It does just what you need. Available from RS
    Components, Farnell and many others.
  13. ehsjr

    ehsjr Guest

    The tstat contact, 24 volt transformer and circulator
    relay are outside the enclosure. There is no mod to
    anything inside the enclosure. I can't say whether
    that applies to 100% of heating units using circulators
    in the world, but it does not make sense to me that the
    circulator or its low voltage (control) or high voltage
    wiring would be inside.

  14. Well,

    Things may differ from place to place. I never saw a heater with
    transformer, pump or relay outside the enclosure. FAIK it's due to local
    safety regulations to make sure the gas burner wil not work without a
    working circulator pump. The elektor article I mentioned before warned
    explicitely not to change anything inside the heater enclosure. But of
    course, there will be millions of heaters but the few I ever saw. Besides,
    newer heaters have these facilities build in already. That's to say, the
    ones you can buy over here.

    petrus bitbyter
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