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Fuse blown in 18" flat screen..

Discussion in 'Electronic Repair' started by ICT User, Oct 24, 2007.

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  1. ICT User

    ICT User Guest

    Hope I have the right group here..

    I have an 18" flat screen monitor which stopped working - I opened it
    up and replaced the fuse inside. The fuse in there was a t3.15ah 250v
    but the one I got from a local shop was a t3.15al 250v which i didnt
    realise until after I had put it in and switched on. It blew the fuse
    to bits. Can anyone tell me the difference between the ah and al and
    if this would be why it blew instantly or should it still work.


  2. I'm not sure of the difference, but the current rating is the same.
    If it was blown to bits, doubt your problem was simply a bad fuse.
    It sounds like there is a short in the power supply.

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  3. Guest

    Well, the *ah* fuse is a time-delay and the *al* fuse is an instant.
    This means that the *al* fuse will very most likely blow at start very
    most likely every time.

    However "blew ... to bits" is indicative of something more serious
    than the wrong fuse. So: find the correct Time Delay fuse and try it
    ONCE (1-Time). If it holds, move on with caution. If it blows again,
    seek professional help.

    Peter Wieck
    Wyncote, PA
  4. ian field

    ian field Guest

    If the previous fuse blew to bits I would *definitely not* try just one
    more! This could be more likely a S/C bridge rectifier or any suppression
    capacitors associated with it as the chopper circuit usually has current
    sensing resistors that tend to 'protect the fuse'.
  5. Arfa Daily

    Arfa Daily Guest

    I don't think that is right. It's the "T" at the start which determines
    whether or not a fuse has a "T" ime delayed action, and the "H" or "L" at
    the end specify whether it has a "H" igh or "L" ow breaking capacity - that
    is the maximum current that it can safely interupt. The fuse in the front
    end of a SMPS is usually a "T" rated HRC type, often with a ceramic body to
    prevent potential shattering. The "A" preceding either of these letters is
    merely for "A" mps. So the OP's original fuse of T3.15AH, is a high breaking
    capacity time delayed 3.15 amp fuse, which is exactly what I would expect in
    that position.

    As far as I am aware, low breaking capacity fuses are typically rated 35A
    and high breaking capacity ones at 1500A. I think all combinations are also
    available, so you can get a "T" rated fuse in an "L" or "H" version, and
    likewise for an "F" rated type.

    FWIW, an "L" specified fuse should still work ok and should not vapourise
    under normal circumstances. The fact that it does, would indicate an
    additional problem which is causing a current draw which exceeds the fuse's
    *rating*, which is the only important factor as far as the fuse failing is
    concerned. As long as they are both "T" rated types, the A or H is of no
    consequence, except with regard to safety from the glass shattering. As
    others have said, the problem is likely to be a bridge diode, line-side
    filter cap, or possibly the switching element itself, depending on design.
    When the problem is sorted, the fuse should be replaced with one of correct
    specification, to maintain the monitor's safety approvals.

  6. ICT User

    ICT User Guest

    Thanks for your replies. I guessed that it would be more than the
    fuse, wanted to check before I tried again - it did scare me when it
    blew and it also knocked out the trip switch. When it originally
    blew it didnt seem to make any noise, i was using the monitor at the
    time and it just went off, which is why i just replaced the fuse
    inside. As im not an expert in electronics I may just bin it or try
    and find a local electronic shop that might be able to fix it as its/
    was a great monitor.

    Thanks again.

  7. James Sweet

    James Sweet Guest

    Thankfully you didn't do as some, and keep putting a bigger fuse in until
    the monitor is completely destroyed.

    It'll likely be a simple and inexpensive repair at this point.
  8. b

    b Guest

    sometimes fuses open up, in that case trying a replacement is often
    all thats needed. BUT
    fuses which go blackened or blow apart indicate a dead short very near
    the point where the mains enters the device . measure the rectifier
    diodes out of circuit. Then proceed towards the chopper.
    if it is a CRT then remove the degauss posistor.

    in future, to avoid surprises like that, connect the mains in series
    with a 100watt light bulb to protect the device. if the bulb shines
    then the short is still present.

  9. ian field

    ian field Guest

    Suggesting a current limiting lamp requires a little more explanation.
    Charging of the mains reservoir cap will cause the lamp to light for a short
    period, which should not be confused with a fault. In the case of a CRT
    display the degauss posistor can take a couple of minutes to heat up with
    most of the supply dropped across the lamp, the slow risetime of the supply
    can seriously confuse the PSU - possibly tripping the UVLO shutdown, its
    best to remove the posistor for this test. In the event that the PSU starts,
    this will draw more current through the lamp and increasing the 'lost'
    voltage - this can also trip the UVLO so the PSU will probably pulse or
    "breathe" a few times and then shut down.
  10. A sigificant number of LCD monitors can be revived by replacing all the
    cap's on the power supply board.

    Use low ESR high temp rated caps from a reputable manufacture

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