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Basic fuse question

Discussion in 'Electronic Repair' started by Dan, Oct 18, 2013.

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  1. Dan

    Dan Guest

    I need to replace a PSU fuse in an LCD monitor. The open fuse is
    ceramic, 5 x 20 mm, and marked "T3.15AH250V", with no obvious spacing
    between the various markings. I understand the speed rating (T) and the
    voltage (250V), but I'm unclear on the amperage. Is it 3.15 amps, .15
    amps, or 15 amps? Did some searching, but all I can find are pages
    trying to sell fuses, not any giving how to read the ratings.


  2. mike

    mike Guest

    Google doesn't seem to have any trouble finding it.
    First hit.
    Assume you fixed what blew it.
  3. Dan

    Dan Guest

    No schmuck, the 1st hit is AMAZON selling the fuses. The next 99 on
    that page are ALSO simply sellers.

    Thanks so much for taking the time to be HELPFUL.
  4. Dan

    Dan Guest

    Arfa - Thanks for all the great info. What happened was my daughter
    accidentally knocked the monitor on the floor when it was on, badly
    banging the screen area. Oddly, it blew the ground fault interrupter
    on that circuit, but also the internal fuse. The monitor is older,
    probably not worth the more expensive repairs you suggest, but thanks
    for passing on all the possibilities, I appreciate your help!

  5. mike

    mike Guest

    Ok, let me quote the amazon page for you.

    QUALITY USA brand fuses!! Set of 5 pieces, T3.15AH250V, T3.15A 250V,
    T3.15 H250V, T3.15A 250V, T3.15H250V cartridge CERAMIC fuses 5X20mm
    (3/16" X 3/4"), 3.15A 250V, SLOW-blow (Time Delay)

    And again with focus on the rating:
    3.15A 250V

    And again with just the amperage
    The A is for Amps.
    That helpful enough for you?
  6. Phil Allison

    Phil Allison Guest

    "Arfa Daily"
    ** Fuses cost money and I don't like to see them blow at switch on.

    So my standard practice is to fit a new fuse and then connect the AC lead to
    a Variac via a current meter ( true rms) and slowly wind up the voltage.
    With a SMPS, the current reading should be small until it suddenly strikes
    and runs at some voltage between 80V and 160V as shown on the Variac's dial.

    If a current near to the fuse's rating appears at a low setting on the
    Variac, then its game over.

    In cases where the PCB has to be exposed to get at the fuse, visual
    inspection for damage is the first step and then an old fashioned moving
    coil multimeter on the low ohms range to check PSU diodes and the switching
    transistor for shorts.


    Guitar amps regularly arrive with either no fuse or a glaringly wrong size
    or type fuse - ie fast fuses in the AC.

    However, the ones I really hate arrive with NO fuse cap in the holder.


    ..... Phil
  7. Phil Allison

    Phil Allison Guest

    ** Got any idea what the letter "T" stands for ?

    Hint - it ain't "time".

    .... Phil
  8. Shaun

    Shaun Guest

    "Phil Allison" wrote in message

    ** Got any idea what the letter "T" stands for ?

    Hint - it ain't "time".

    .... Phil

    How about T for tiny... like your dick!
  9. Dan

    Dan Guest

    Ok, let me quote the amazon page for you.
    Yeah, I saw that. It also says "You will receive 5 fuses and they can
    be either of these marking: T3.15AH250V, T3.15A 250V, T3.15 H250V,
    T3.15A 250V, T3.15H250V." Not being a fuse expert, I don't know that
    these are all in fact interchangeable, and I thought perhaps it might be
    better to clarify the rating numbers with someone who's NOT trying to
    'sell me something'.
    That's what I was looking for. That wasn't so hard, now was it? I
    guess I'll leave it to you and your shrink to determine why you didn't
    simply give that info to begin with, and instead felt the need to make a
    total stranger feel stupid for asking a legitimate question in an
    appropriate forum. My guess would be low self esteem, but check Amazon,
    I bet they have a self-help book on it.
  10. Guest

    A lot of nerve calling names when YOU'RE the who one can't read a fuse or even look it up. And what creative name will you have for me?

  11. JW

    JW Guest

    Better yet if you can find one cheap, a Sencore PR570.
    It has the capability to automatically trip and shut down it's AC output
    if the current exceeds whatever setting you dial in. It doubles as an
    isolation transformer and variable AC power supply as well. Got one of
    these for $150 a year ago on Ebay, I can't count how many times I've used
  12. Phil Allison

    Phil Allison Guest


    ** A Variac alone provides no protection.

    But the method I outlined is vastly superior to the old light bulb trick.

    Variac + true rms current meter + fuse (which can be a fast blow type).

    There are far too many anomalies when a series lamp is fitted to the AC

    .... Phil
  13. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"
    ** A bulb that passes 0.5A rms when lit will pass 8A peak at switch on.

    That is no soft start.

    ** Then you have to remove it and all protection is lost.

    ** A valve guitar amp of over 100 watts will not warm up and run with a 0.5A
    bulb in the AC.

    ** A"soft start" is always provided when using a Variac.

    ** You can stick with what you like and are used to.

    .... Phil
  14. Bob F

    Bob F Guest

    Is this the way YOU say thank you. Remind me no to respond to your posts.
  15. mike

    mike Guest

    My bad, I forced the issue.
    Blame me for my inciteful response.
  16. Phil Allison

    Phil Allison Guest

    "Arfa Daily"

    ** OK, my test set up is used with ALL items for repair.

    The AC current meter uses a Hall effect transducer ( totally isolates the
    mains supply) feeding a true rms converter IC and then a 3.5 digit LED
    display. There are two ranges, 2A and 20A with 1mA and 10mA resolution

    A standard 3 pin outlet is fitted to the front and all jobs get plugged in

    A BNC socket on the back of the unit provides a waveform for the scope that
    is either 1V or 100mV per amp of AC current. Signal bandwidth is DC to 100
    kHz and max output is +/- 10V, so current surges up to 100 amps peak can be
    seen on the scope.

    The current meter's electronics has its own AC lead so the Variac does not
    affect it.

    The set up easily caters for the smallest plug pak (wall wart) right up to
    2.5kW/ch stage lighting dimmers and the largest power amps. Been in
    continuous use now since 1996.

    ..... Phil
  17. Guest

    I see Australian musicians have discovered the "universal replacement"
    fuse, too - it's the fuse in a particular physical size with the biggest
    number preceding the "A" printed on the box. It is guaranteed not to
    blow - something else will usually pop before the fuse does.

    I used to work on trucks where AGC 10 A fuses were routinely replaced
    with 30 A fuses in an attempt to stop the fuse from blowing. Usually
    the actual problem was that an added wire had been run through a hole
    with no grommet, sleeving, etc and was grounding out.

    For a while in the 70s, some US cars had "SFE" fuses that were all 0.25"
    diameter but different lengths - lower ratings were physically shorter -
    to help prevent overfusing. An SFE 4 was a little over 0.5", and an
    SFE 20 was the same length (1.25") as an AGC fuse. I think it was a
    good idea, but it went away with the changeover to the flat blade fuses.
    At a previous employer, one of our products was in a 6U 19" rackmount
    chassis with two holders for 5 x 20 mm fuses on the back. We shipped
    this in its own cardboard box, and it always showed up with everything
    intact. Our customer then bolted that chassis into their cabinet, along
    with other equipment, and shipped the whole shebang on a flatbed truck
    to the site. When it got to site, there was a pretty good chance that
    the fuse holder caps would be missing. We told them a) don't ship it
    that way and b) put a piece of tape over the caps before shipping. They
    ignored a), did b), and asked us for a case lot of replacement caps,
    which we gave them.

    In the previous generation of that product, the fuses were in the power
    inlet module - one where you have to remove the main power connector to
    open the little flap that lets you change the fuses. I don't know
    whether that inlet module was eliminated from the design for cost or
    customer reasons. (It wasn't quite an IEC inlet module, because the
    incoming power was DC with a unique connector, but it was the same

    Matt Roberds
  18. Phil Allison

    Phil Allison Guest

    ** I have a box full of 20A, 25A 30A and 35A automotive 3AG fuses pulled
    from guitar and power amps.

    Not to mention all the blown ones wrapped in foil paper from a cigarette

    ** In my example, the fuse caps were removed by the owners and thrown away

    They all wrongly imagine techs can easily get spare ones to suit.

    With many USA built guitar amps, it means drilling or punching a new hole in
    the chassis to fit a new holder.

    ** That level sophistication of stops most of them in their tracks.

    ..... Phil
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