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Periodic transformer failure

Discussion in 'Beginner Electronics' started by vMike, Oct 5, 2006.

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  1. vMike

    vMike Guest

    I have a 120v to 24vac transformer for a lawn sprinkler system that has
    failed about 3 times since I purchased the house. This time I took the
    windings apart and found the break in the 120v winding. I live in a
    lightening prone area. Is this failure most likely from power surges or is
    there some other possible cause.

    Thanks.

    Mike
     
  2. Guest

    What do you mean it failed about 3 times? Do you mean you replaced it?
    What happened the other times? How do you know it failed? All that info
    is necessary.

    -tg
     
  3. PeterD

    PeterD Guest

    It helps to ask a clear question... <g>

    Let's say lightning. Then a surge strip might be a good idea.

    Let's say 'bad engineering'. THen a better transformer might be a good
    idea.

    When off how warm/hot does it get?

    (There should be little or no heat from an unloaded transformer)


    When on how warm/hot does it get?

    (the temperature rise should not exceed the point where you cannot put
    your hand on it without saying: "OUCH" damn, that's hot.)

    Also check your line voltage adn the specified voltage of the
    transformer: do they match, or is the transformer rated for a lower
    line voltage (bad).
     
  4. default

    default Guest

    Lightening might be a culprit. Lightening may manifest as an open
    where the connection to the winding is made or an internal short.

    A surge protector and a good ground will do wonders for lightening
    problems -

    You said you took the winding apart? If the transformer were dying
    due to over current/heat, you's see charred windings.

    The other good choice is a massive momentary overload that is enough
    to open the winding but isn't present long enough to char the
    insulation on the magnet wire. Shorted wire out to a valve - or
    compromised insulation inside the transformer itself.

    Most of the lawn timers I've seen have fuses. Check that to see if it
    is the correct size.
     
  5. vMike

    vMike Guest

    Yes I have replaced it 3 times. I took the last one apart to see where the
    failure was. I also have now found there is a big voltage drop on one of the
    zones... from 24 to 15. So the problem appears to be in that zone. Could be
    the box, the zone solenoid or the wiring to the solenoid. Hoping it isn't
    the last one!!
    Mike
     
  6. vMike

    vMike Guest

    The fuse was over sized. Probably because it was blowing. See my response to
    previous poster.

    Mike
     
  7. w_tom

    w_tom Guest

    Lightning seeks a conductive path to earth. A most common source of
    lightning strikes is to AC utility wires. Incoming on AC mains, into
    house, and out to earth ground via transformer.

    Transformer provides galvanic isolation. But if that surge is too
    large, then galvanic isolation will be overwhelmed. Transformer
    primary (120 volt) and secondary (12 volt) conduct the surge to earth.

    Do you think a power strip protector will stop what three miles of
    sky could not? That is what another here has recommended. Real world
    protection is about earthing that surge before it even enters a house.
    Same one protector that is sufficiently sized to remain functional
    after each lightning strike. Same one protector that also protects
    everything inside a building.

    All appliances already contain internal protection - just like the
    transformer. Protection inside any appliance might be overwhelmed if a
    direct lightning strike is not earthed before entering the building.
    It is called 'whole house' protector. Effectiveness defined by quality
    of earthing at the service entrance (where AC electric, telephone, etc
    enter the building).

    Effective 'whole house' protectors are sold in Lowes, Home Depot, and
    electrical supply houses, Responsible manufacturer brand names are
    Cutler-Hammer, Leviton, Intermatic, Siemens, and Square D. A dedicated
    earthing wire short to that earth ground rod means lightning need not
    find earth via that transformer.

    Building earthing electrode must meet and exceed post 1990 National
    Electrical Code requirements. Essential is an earthing connection that
    is less than 10 feet, no splices, no sharp bends, separated from other
    wires, etc. A protector is nothing more than a connection device to
    protection - that earth ground electrode. Those AC electric wires -
    the most common path for surges that would blow through a transformer.
     
  8. bud--

    bud-- Guest

    The best information I have seen on surge protection is at
    http://www.mikeholt.com/files/PDF/LightningGuide_FINALpublishedversion_May051.pdf
    - w_tom provided the link to this guide
    - the title is "How to protect your house and its contents from
    lightning: IEEE guide for surge protection of equipment connected to AC
    power and communication circuits"
    - it was published by the IEEE in 2005
    - the IEEE is the dominant organization of electrical and electronic
    engineers in the US

    A second guide is
    http://www.nist.gov/public_affairs/practiceguides/surgesfnl.pdf
    - this is the "NIST recommended practice guide: Surges Happen!: how to
    protect the appliances in your home"
    - it is published by the National Institute of Standards and
    Technology, the US government agency formerly called the National
    Bureau of Standards
    - it was published in 2001


    Both guides were intended for wide distribution to the general public
    to explain surges and how to protect against them. The IEEE guide was
    targeted at people who have some (not much - should be easy for anyone
    here) technical background.

    Both say plug-in surge suppressors are effective.

    For complicated equipment, all interconnected devices, like a computer
    and printer, need to connect to the same surge protector. If a device,
    like a computer, has external connections like phone or LAN, all those
    wires have to run through the surge suppressor for protection. This
    type of suppressor is called a surge reference equalizer (SRE) by the
    IEEE (also described by the NIST). The idea is that all wires connected
    to the device (power, phone, CATV, LAN, ...) are clamped to a common
    ground at the SRE. The voltage on the wires passing through the SRE are
    held to a voltage safe to the connected device. The primary action is
    clamping, not filtering or earthing.
     
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